Wool crimp is the natural, regular wave along the length of each wool fibre, which shows in a fleece as the bands of waves running across every staple. It grows into the fibre in the follicle: the fibre’s cortex is built from two kinds of cell, the orthocortex and the paracortex, lying side by side, and the cells on the outside of each bend are longer than those on the inside, so the fibre curves. Finer wool generally has more crimps, about 20 to the inch (8 per centimetre) in fine 20-micron wool against about 4 to the inch in coarse 40-micron wool. Crimp is counted as crimps per centimetre on the staple or measured by instrument as fibre curvature in degrees per millimetre. In processing it gives wool its bulk and springiness and helps hold slivers together before they are twisted, but its effect on spinning is small beside fibre diameter and length, and very high crimp slightly lowers the evenness of fine yarns.
Last verified: 11 October 2026. Every figure is linked to the research report, university guide, journal paper or standards body it comes from.
Wool crimp at a glance
| Question | Short answer | Source |
|---|---|---|
| What it is | The natural wave of the fibre itself; neighbouring fibres curve in phase, so the staple shows regular bands | Woolmark; CSIRO (Lamb, 1997) |
| What causes it | A two-sided cortex: orthocortical cells on the outside of each bend are longer than paracortical cells on the inside | Harland et al., 2018 |
| How much | About 20 crimps per inch at 20 µm, 10 at 31 µm, 4 at 40 µm; one fine-wool Merino flock ranged from 3 to 10 per cm | NMSU Guide B-409; Fish, Mahar and Crook, 1999 |
| Measured as | Crimps per cm or inch; crimp % (extra length when straightened); fibre curvature in °/mm on Laserscan or OFDA; resistance to compression in kPa; bulk in cm³/g | IS 6124; IWTO-12 and IWTO-47; AS 3535; NZS 8716 |
| Curvature of commercial combing blends (16–25 µm) | 74 to 120 °/mm, mean 94, across 643 commercial topmaking consignments averaging 20.1 µm | TEAM-3 final report, 2004 |
| What it does | Bulk, springiness and warmth; cohesion in slivers; bulkier woollen yarns; thicker, more resilient fabrics | UNE course notes; CSIRO (Lamb, 1997); Fish, Mahar and Crook, 1999 |
| What it does not do | Set how fine a yarn can be spun (diameter does that), or measurably change top length and noil in commercial blends | CSIRO (Lamb, 1997); TEAM-3 |
What is crimp in wool?
Crimp is the waviness of the fibre, not of the lock. Woolmark calls it “the natural waviness of a wool fibre”, a three-dimensional shape that traps small pockets of air (Woolmark). The fibres in a staple grow side by side and curve in phase, so their waves line up as bands across the staple. CSIRO’s wool scientists describe a staple’s crimp by its frequency (crimps per centimetre), its definition (how clearly and regularly the waves are formed, a measure of whether the fibres curve in phase) and the depth of its loops (Purvis; Lamb, CSIRO Finewool Newsletter, October 1997).
The shape is set permanently as the fibre forms in the follicle. Moderate stretching or squashing removes it only for a time: relaxed in water, a fibre recovers most of its crimp unless it has been set into a new shape, for example by dyeing, or strained by more than about a quarter of its length (Fish, Mahar and Crook, IWTO report CTF 01, 1999; Lamb, 1997). That is why Woolmark credits crimp with helping wool fabrics recover their shape after stretching or compression.
What causes crimp: a two-sided cortex
The cortex makes up the bulk of a wool fibre. Its two cell types, the orthocortex and the paracortex, “grow side by side, but at different rates, to create a natural bend in the fibre known as crimp” (Woolmark). University of New England course notes call it “bilateral differentiation in the internal cell structure of the wool fibre”, most pronounced in Merino (Wood, WOOL422/522 notes, 2012).
In 2018 Harland and colleagues measured the two sides directly, by confocal microscopy of Merino fibre snippets of known curvature. In every case the orthocortical cells near the outside of the bend were longer than the paracortical cells near the inside. They found evidence against the older idea that the outer side simply has more cells, and curvature did not follow the proportions of the two cell types: it appears that only the difference in their length matters (Harland et al., Journal of Experimental Biology, 2018).
Crimps per inch by wool type and fibre diameter
Across breeds, finer wool carries more crimps. The table sets New Mexico State University’s grading guide (crimps per inch at each grade, and the usual diameter range of each breed) beside the fibre curvature that a 1997 study of about 2,000 New Zealand wool samples found at each diameter.
| Mean fibre diameter | Grade (NMSU) | Crimps per inch (NMSU) | Crimps per cm | Breeds whose usual range includes it (NMSU) | OFDA curvature predicted, °/mm (NZ, 1997) |
|---|---|---|---|---|---|
| 20 µm | Fine | 20 | 7.9 | Merino (fine), Rambouillet, Delaine-Merino | 103–105 |
| 24 µm | 1/2 blood | 14 | 5.5 | Columbia, Southdown, Corriedale | 87–90 |
| 27 µm | 3/8 blood | 12 | 4.7 | Columbia, Southdown, Corriedale, Dorset, Hampshire, Suffolk | 75–80 |
| 31 µm | 1/4 blood | 10 | 3.9 | Corriedale, Dorset, Hampshire, Suffolk | 62–66 |
| 34 µm | Low 1/4 blood | 8 | 3.1 | Lincoln (34–41 µm) | 54–56 |
| 40 µm | Common and braid | 4 | 1.6 | Lincoln | 36–42 |
Grades and crimps per inch: figure 1 of NMSU Guide B-409, Understanding Wool Grades (December 2025); breeds from its table 4, leaving out the double-coated Navajo Churro, listed at 22–47 µm. Crimps per cm = crimps per inch ÷ 2.54. Curvature: the straight-line fit (170.64 − 3.376 × diameter, R² 0.93) and the quadratic fit (R² 0.95) of OFDA curvature against Airflow diameter published by Greatorex, Knowles and Barker (IWTO report 18, 1997), worked out at each diameter; they note the relationship levels off between 30 and 35 µm. The column shows the trend, not a target: curvature had no calibration standard then, and a later recalibration moved high readings (see measurement).
Crimp is a poor guide to micron within a flock
Before diameter was measured routinely, the “quality number” system sorted fleeces into lines of similar crimp so that buyers could guess their fineness. Within a flock, a bloodline or a strain of Merino, though, CSIRO found the association between crimp frequency and diameter “essentially zero”, in the animals and in their genes, so selecting for finer wool alone does not change crimp. Crimp frequency is moderately heritable, about 0.3 (Purvis, CSIRO, 1997). Across breeds it is another matter: in trials in South Africa, New Zealand and Australia covering everything from carpet wool to fine Merino, curvature and crimp frequency moved closely together (R² 0.80 to 0.91), but within the CSIRO fine-wool flock, where crimp ran from 3 to 10 per cm, R² was only 0.29 (Fish, Mahar and Crook, 1999). The table is a guide to wool types, not a way to estimate one fleece’s micron.
How wool crimp is measured
| Measurement | Unit | How it is made | Standard | Mainly used for |
|---|---|---|---|---|
| Crimp frequency | crimps per cm (or per inch) | Waves counted along a staple or a fibre | ASTM D3937 counts it, but for manufactured staple fibres, not wool | Breeding, classing, research |
| Crimp percentage | % of the crimped length | A single fibre is measured lying relaxed, then straightened without stretching, and the extra length is taken as a percentage of the relaxed length | IS 6124:2025, Textiles — Crimp in wool fibres — Methods of test (BIS; first revision of IS 6124:1971) | Wools without well-defined waves |
| Fibre curvature | °/mm | Angle a short fibre snippet turns through per millimetre, read during the diameter test | Reported by the Laserscan (IWTO-12) and OFDA (IWTO-47) instruments; AWTA calls the results uncertified | Sheep selection; some sale lots |
| Resistance to compression | kPa | Force to squeeze a standard mass of cleaned, carded wool into a small fixed volume | AS 3535-2004, Wool — Method for the measurement of resistance to compression (Standards Australia; withdrawn) | Worsted apparel wools |
| Bulk | cm³/g | Volume of a standard mass under a small standard pressure, in a bulkometer | NZS 8716:1994, Method for the measurement of the bulk of raw wool (Standards New Zealand) | Carpet and knitwear wools for woollen processing |
Resistance to compression is “a high-compression test which is appropriate for assessing wools for worsted apparel products”, and it rises with both fibre diameter and crimp frequency; bulk is mainly used for wools going to woollen carpets and knitwear (Wood, 2012). IWTO-12 and IWTO-47 are diameter methods, titled in IWTO’s committee papers “Measurement of the Mean and Distribution of Fibre Diameter Using the Sirolan-Laserscan Fibre Diameter Analyser” and “Measurement of the Mean and Distribution of Fibre Diameter of wool Using an Optical Fibre Diameter Analyser” (Greatorex et al., 1997); curvature is read from the same snippets. The American Laserscan method is ASTM D6466. India’s standard measures crimp as a length, not a count: the first edition, IS 6124:1971, notes that crimp is assessed either by counting waves per unit length or by the difference between a fibre’s relaxed and straightened length, and adopts the second because, it says, Indian wools do not have well-defined waves or curls; at least 300 single fibres are measured. The 2025 catalogue entry does not say whether the revision adds a wave count.
Reading a curvature figure. IWTO adopted degrees per millimetre as the unit in 1998: a 1 mm length of fibre that bends through 80° has a curvature of 80 °/mm. Within one laboratory, Laserscan and OFDA results agree closely over a wide range of wools, but curvature is easily disturbed. In AWTA’s trials, carding samples on a Shirley Analyser raised it by 3.5 °/mm on average, breaking the fibres of high-curvature staples lowered it by about 10 °/mm, and instruments of the same type differed by around ±6 °/mm (Fish, Mahar and Crook, 1999). Calibration tops now carry assigned curvatures, and when AWTA recalibrated in 2018 its results above about 85 °/mm rose by typically 25 °/mm (AWTA, 1 May 2018). Compare curvature results from one laboratory and one period.
Worked example: is this curvature high or low for its micron?
A 20.0 µm sale lot tests at a mean fibre curvature of 85 °/mm. The New Zealand line in the table above predicts 170.64 − 3.376 × 20.0 = 103 °/mm at that diameter, and the TEAM-3 trial’s commercial topmaking blends averaged 94 °/mm at a mean of 20.1 µm (TEAM-3 final report, IWTO, 2004). So 85 °/mm is below both. How far below matters less than it seems. The TEAM-3 blends spread over only about 10 to 15 °/mm at each micron level, but they mix many lots, and TEAM-3 itself notes that single sale lots and fleeces range more widely. Among 213 Australian sale lots, curvature scattered broadly at every diameter, and Fish, Mahar and Crook warned of a “high probability of making an incorrect judgment” about a wool from a curvature-for-diameter line (1999). Read the lot as lower in crimp than usual for its micron only if the same laboratory, on the same calibration, puts most 20 µm lots higher. Trials comparing extreme fleeces found that low-curvature wool gave longer tops, less combing waste and more even yarn; across commercial blends TEAM-3 could not detect the effect (see carding and combing).
Four wools measured side by side
Crimp, not diameter alone, decides how springy a wool is. Swan and Mahar chose four wools for an extreme range of bulk, and measured diameter, resistance to compression (to AS 3535-1988) and curvature on 0.5 mm snippets under the microscope.
| Wool | Mean fibre diameter | Resistance to compression | Mean curvature, rad/mm | Same, °/mm |
|---|---|---|---|---|
| Superfine Merino | 17.2 µm | 12.2 kPa | 2.50 | 143 |
| Medium Merino | 21.7 µm | 9.1 kPa | 1.77 | 101 |
| Lustre-mutant Merino | 22.4 µm | 6.6 kPa | 0.86 | 49 |
| Downs wool | 29.6 µm | 10.9 kPa | 1.31 | 75 |
Source: tables 1 and 2 of Swan and Mahar, IWTO report 18, 1998; degrees = radians × 57.3. Their microscope method is not the OFDA’s, so these curvatures do not compare directly with the first table. At almost the same diameter, the medium Merino has twice the curvature of the lustre mutant and 38% more resistance to compression. The Downs wool, 8 µm coarser, resists compression more than the medium Merino despite its lower curvature, because resistance to compression rises with diameter as well as with crimp.
What crimp does in processing and spinning
Bulk, handle and springiness
Crimp is “the fibre characteristic that largely determines the bulk, resistance to compression and lustre of wool”. Bulk is a wool’s ability to fill space and have a springy handle, and because air is trapped between crimped fibres, products made from high-bulk wools are warm. Lustre runs the other way: generally, the higher the lustre, the lower the bulk (Wood, 2012). Crimp also lets the fibre stretch and recover without breaking (Woolmark).
Cohesion: what holds a sliver together
In the early processing steps wool travels as slivers with no twist. “Crimp promotes cohesion between fibres so that higher bulk wools are generally easier to process” (Wood, 2012). Peter Lamb of CSIRO put it more strongly: some crimp is desirable “because it is the crimp that holds the slivers together in the earlier processing steps”, which is why synthetic fibres processed on the worsted and cotton systems, along with mohair, usually have to be artificially crimped or twisted (Lamb, 1997).
Carding and combing: top length and noil
Combing removes the short fibres (noil) to leave a top, whose mean fibre length is its hauteur; romaine is the combing waste. Trials in South Africa and Australia comparing wools of extreme crimp found that low crimp frequency, low curvature or low resistance to compression went with less fibre entanglement, longer hauteur, lower romaine and fewer neps (Fish, Mahar and Crook, 1999; TEAM-3, 2004). Two later results put that in proportion. In CSIRO’s sire-progeny trials, over 200 batches processed to top, higher-crimp batches gave shorter tops but also had shorter staples, and within classes of similar staple length there was no association between crimp and hauteur (Purvis, 1997). And in TEAM-3, 643 commercial consignments from 34 mills, adding mean fibre curvature to the TEAM formulae did not improve the prediction of hauteur, its variation or romaine (TEAM-3 final report, 2004). The TEAM-3 committee found the established predictors (staple length, staple strength, diameter, mid-breaks and vegetable matter) still applicable.
Processing changes the crimp too. Fibres lose crimp to the strains of worsted processing, the loss being largely irreversible in high-speed carding and spinning, and are set into helical shapes in worsted yarn; even so, the curvature left in a top is directly related to the crimp of the greasy wool (Fish, Mahar and Crook, 1999; Lamb, 1997).
Drafting and spinning
Fibre diameter controls how fine a yarn can be spun. How often the yarn breaks on the frame is governed first by the number of fibres in its cross-section, which for a given count is set by diameter, and then by mean fibre length: a typical fine weaving yarn from 17 µm wool weighs about 12.5 g per km and has only about 40 fibres in its cross-section. Beside that, crimp is a small effect. Higher fibre curvature goes with poorer yarn evenness, plausibly because crimped fibres interact more in drafting, but the clear evidence came from superfine wool of more than about 7 crimps per cm, and Lamb knew of none for most Australian wools. Very high crimp, he added, can give a double penalty, a less even yarn and a shorter top, both meaning a weaker yarn and more ends down (Lamb, 1997; Wood, 2012). Count, draft and twist arithmetic is in the site’s yarn spinning formulas.
Woollen and worsted systems
Crimp matters more on the woollen system. The worsted route has the most steps and “produces a lean, smooth yarn”; most wool apparel is made this way, including about 90% of Australian Merino apparel wool. The woollen route has the fewest steps and produces “a relatively bulky and hairy yarn”; about 80% of New Zealand wool goes that way. In UNE’s ranking, bulk and resistance to compression are of secondary importance for worsted processing and of major importance for woollen apparel and carpet yarns, and woollen-spun yarns “tend to have a higher bulk and springy handle” (Wood, 2012). Some curvature is lost in woollen spinning, but wool of higher raw curvature still gave thicker yarns (Swan’s thesis, in Fish, Mahar and Crook, 1999). Both systems are described step by step in the wool fibre guide.
In the fabric
With diameter and yarn matched, fabrics from lower-crimp superfine wool were thinner and smoother with a preferred, softer handle, and fabrics from higher-crimp wool bulkier and more resilient (Fish, Mahar and Crook, 1999; Lamb, 1997). Lamb also noted the other side: low crimp may make less bulky knitwear, and “warmth-without-weight is a highly-prized attribute”. In 45 Merino mid-side samples (13.5–24.0 µm, 46–108 °/mm), fabric felt shrinkage was negatively correlated with curvature (r = −0.62), so the higher-curvature wools shrank less (Schlink et al., Animal Production Science, 2009).
Merino crimp compared with other wools
Merino has the most pronounced crimp, in blocky, rectangular staples; crossbred staples tend to taper to the tip, and staples of lower crimp frequency tend to be more lustrous (Wood, 2012). Merino crimp varies a good deal: from 3 to 10 per cm in the CSIRO fine-wool flock, and from 4 to 6 per cm across seven sites on a single fleece (Fish, Mahar and Crook, 1999), so fleeces should be sampled at the same place each time; NMSU recommends the shoulder (NMSU B-409). As curvature, Merino mid-side samples ran from 46 to 108 °/mm (Schlink et al., 2009). Breeding moves crimp slowly: CSIRO predicted that ten years of within-flock selection for fleece weight and fineness would lower crimp frequency by at most 4%, about 0.3 crimps per cm, and that bigger changes would need sires from a medium-wool bloodline. In 1997 most buyers of superfine wool paid more for high crimp frequency, while CSIRO’s processing results pointed the other way for very high crimp (Purvis; Lamb, 1997). Coarser breeds sit lower on the scale (table by wool type); measured figures for 23 British breeds are in the wool fibre guide, and the grading systems in grading of wool.
Crimp in textiles: fibre crimp and yarn crimp
“Crimp” has three meanings in textiles. Fibre crimp is the wave in a single fibre: natural in wool, given to synthetic staple fibres in manufacture, and counted for those under ASTM D3937. Acrylic, for example, is crimped and heat set before it is cut into staple (acrylic fibre). Textured yarn is crimped continuous filament, whose bulk and crimp contraction are tested under ASTM D4031. Yarn crimp in a woven fabric is the waviness a yarn takes as it passes over and under the yarns crossing it, so that every thread is longer than the cloth.
Yarn crimp is measured by taking threads out of a strip of fabric of known length, straightening each under a tension chosen for that kind of yarn and measuring it; the difference is expressed as a percentage of the length in the fabric (ISO 7211-3; in the United States, ASTM D3883). So crimp % = (straightened length − fabric length) ÷ fabric length × 100, and take-up states the same difference against the yarn length. A warp thread of 10.8 cm from a 10 cm strip has 8% crimp and 7.41% take-up; a weft thread of 10.5 cm has 5% crimp and 4.76% take-up. The site’s crimp percentage calculator, set to these threads, shows 8.00% warp crimp, 5.00% weft crimp, take-up of 7.41% and 4.76%, and a crimp ratio of 1.60. Reed width, fabric weight and warp requirement all depend on it; see weaving calculations. The same sum describes a wool staple: in sound wool the straightened fibres may be about 20% longer than the staple (Wood, 2012), so an 80 mm staple of 96 mm fibres has 20% crimp on the staple length, or 16.7% on the fibre length. IS 6124 measures crimp on single wool fibres the first way, against the relaxed length.
Questions
What is crimp in wool?
Crimp is the natural wave along each wool fibre. Because the fibres in a staple wave in step, it shows as regular bands across the staple. It is described by its frequency (crimps per centimetre or inch), its definition (how regular the waves are) and its depth, and it gives wool its bulk and spring.
What causes crimp in wool fibres?
The cortex of a wool fibre has two kinds of cell, the orthocortex and the paracortex, side by side. The orthocortical cells on the outside of each bend are longer than the paracortical cells on the inside, and that difference curves the fibre (Harland et al., 2018). The shape is set as the fibre forms in the follicle.
How many crimps per inch does Merino wool have?
Fine wool of about 20 µm has about 20 crimps per inch, or 8 per cm (NMSU). Merino flocks vary widely: one CSIRO fine-wool flock ran from 3 to 10 crimps per cm, about 8 to 25 per inch, with a mean of 6.7 per cm. The 80 to 100 crimps per inch sometimes quoted for Merino is several times too high.
Does more crimp mean finer wool?
Across breeds, yes: fine Merino has many more crimps than a coarse longwool. Within one flock or bloodline, no: CSIRO found the link between crimp frequency and fibre diameter there to be essentially zero, which is why diameter is measured rather than judged from the crimp.
What is crimp in textiles?
Waviness, in a fibre or in a yarn. Fibre crimp is the wave in a single fibre, natural in wool and added to synthetic staple fibres. Yarn crimp is the waviness of a yarn as it interlaces in a woven fabric, given as the extra length of the straightened yarn over the fabric length, in per cent.
What is yarn crimp?
Yarn crimp is the extra length a yarn has in a woven fabric because it bends over and under the yarns crossing it: crimp % = (straightened yarn length − fabric length) ÷ fabric length × 100, tested to ISO 7211-3. A thread of 10.8 cm from a 10 cm strip has 8% crimp. Textured filament yarns are a different case, crimped in manufacture for bulk.
Sources
- Fibre structure: Woolmark, What is the wool fibre; Harland et al., Intrinsic curvature in wool fibres is determined by the relative length of orthocortical and paracortical cells, Journal of Experimental Biology 221 (2018), jeb172312.
- Wool-testing research hosted by AWTA: IWTO technical reports by Greatorex, Knowles and Barker, fibre curvature on New Zealand wools (1997); Swan and Mahar, measurement of fibre crimp curvature, part 1 (1998); Fish, Mahar and Crook, Fibre curvature morphometry and measurement (1999); TEAM-3 processing trial: final report (2004); and AWTA’s news item Update on fibre curvature measurement (2018).
- CSIRO and universities: Purvis and Lamb, CSIRO Finewool Newsletter, issue 10 (October 1997); Wood, Value and use of wool, WOOL422/522, University of New England (2012); NMSU Guide B-409 (2025); Schlink et al., Animal Production Science 49 (2009) 333–339.
- Standards: IS 6124 (first edition IS 6124:1971), NZS 8716, AS 3535-2004, ASTM D6466, ASTM D3937, ASTM D4031, ISO 7211-3, ASTM D3883; IWTO-12 and IWTO-47 are sold through IWTO’s wool testing resources.
How this page is kept
Last verified: 11 October 2026. Crimps per inch come from New Mexico State University’s grading guide; curvature figures from IWTO technical reports and the TEAM-3 trial; processing effects from CSIRO’s researchers and the University of New England’s course notes; the cause of crimp from Woolmark and the Journal of Experimental Biology. The predicted curvatures are worked out from a published 1997 equation and show the trend only, because curvature depends on the laboratory and its calibration. The prefilled calculator link was opened on this date to check that it shows the figures quoted. Spotted an error? Tell us through the contact page and it will be corrected.
