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Knits fail biaxially, so a single-direction tensile figure never predicts burst on its own.
Predicted Bursting Strength
—kPa
Thin-walled sphere approximation
Equivalent Units
Bursting Strength
—kgf/cm2
Bursting Strength
—psi
Biaxial Tension per Metre
—N/m
Burst Index
—kPa per g/m²
The sphere model assumes uniform biaxial loading and ignores the bending stiffness of the fabric, so treat it as a design estimate and confirm on a diaphragm tester.
Using this calculator
About the Knitted Fabric Bursting Strength Modeler
The formula
This is the expression the tool evaluates. Every term is named underneath, with the unit it must be supplied in.
Biaxial tension per metre of fabric widthmeanTensile = sqrt( (tensileStrengthWales / 0.05) x (tensileStrengthCourses / 0.05) )
0.05 is the 50 mm strip width expressed in metres, so dividing by it converts N/5cm into N/m: 400 N/5cm is 8,000 N/m. The geometric mean is used rather than the arithmetic mean because it leans toward the weaker direction — for 8,000 and 6,000 N/m it returns 6,928 N/m where a straight average would return 7,000.
P = 2T/r is Laplace's law for a thin-walled sphere — pressure is balanced by tension carried around a curved membrane. testDiameter / 2000 does two conversions at once: divide by 2 for radius, by 1000 for mm to metres, giving 0.01525 m for the standard 30.5 mm head. The bracket yields pascals, so the trailing /1000 puts it in kPa.
The same pressure in kgf/cm2 and psiburstPressureKgf = burstPressure x 1000 / 98066.5 and burstPressurePsi = burstPressure x 1000 / 6894.757
98066.5 Pa is one kgf/cm2 — 9.80665 N acting on 1 cm2, which is 9.80665 x 10^4 Pa — and 6894.757 Pa is one psi. Older Mullen-type testers and a lot of buyer specifications are still written in these units, so keep the chain 1 kgf/cm2 = 98.07 kPa = 14.22 psi to hand when reading an old spec sheet.
Burst pressure per unit fabric weightburstIndex = burstPressure / gsm
Units are kPa per g/m2. Fabric weight does not enter the pressure model at all, so this is a bookkeeping ratio rather than a physical one. Its purpose is to compare two constructions of different weight on how efficiently the yarn in them is carrying load.
Symbols used above
Symbol
Stands for
Unit
tensileStrengthWales
Strength, Wale Direction
N/5cm
tensileStrengthCourses
Strength, Course Direction
N/5cm
testDiameter
Test Aperture Diameter
mm
gsm
Fabric Weight
g/m²
burstPressure
Predicted Bursting Strength
kPa
burstPressureKgf
Bursting Strength
kgf/cm2
burstPressurePsi
Bursting Strength
psi
meanTensile
Biaxial Tension per Metre
N/m
burstIndex
Burst Index
kPa per g/m²
How the result is derived
Step by step, from the values you type to the figure on screen.
Both strip strengths are converted to tension per metre of fabric width. The N/5cm unit comes from a strip test on a 50 mm specimen, and the membrane model needs newtons per metre, so each figure is multiplied by 20.
The two directional tensions are collapsed into a single biaxial tension using the geometric mean. Under diaphragm pressure the fabric domes and loads wales and courses at the same time, so neither directional figure on its own describes the stress state at failure.
The aperture diameter is halved and converted to metres to give the radius the model assigns to the dome. This is the most influential input on the page: predicted pressure is inversely proportional to it, so a 50 cm2 head and a 7.3 cm2 head cannot produce the same number.
Laplace's law for a thin-walled sphere, P = 2T/r, turns tension into pressure. The relation assumes a membrane with no bending stiffness, equal tension in every direction and constant curvature — three things a knitted fabric only approximates, which is why the output is a design estimate.
The pascal result is divided out into kPa, kgf/cm2 and psi, because burst specifications are still written in all three depending on the buyer and the vintage of the tester.
The kPa figure is divided by fabric weight to give burst index, which is the figure to quote when comparing constructions that do not weigh the same.
What each input means
Where to read each value on the floor, the unit it must be in, and the range the tool accepts.
Input
Unit
Accepted range
Default
What it means
Strength, Wale Direction
N/5cm
1 to 20000 N/5cm
400
Strength, Course Direction
N/5cm
1 to 20000 N/5cm
300
Test Aperture Diameter
mm
5 to 200 mm
30.5
Fabric Weight
g/m²
10 to 1000 g/m²
180
What the tool returns
The headline figure and every supporting value it is built from.
Read 908.62 kPa as an upper bound, not a prediction of what the tester will read. A 180 g/m2 knit with 400 and 300 N/5cm strip strengths would commonly measure 400 to 600 kPa on the same 7.3 cm2 head, because the sphere model sets the dome radius equal to the aperture radius when the real dome at failure is shallower and therefore less curved. Note also how little the fabric drives the answer compared with the geometry: halving both strengths halves the pressure, and doubling the aperture halves it too.
How to use it
Work through the input groups in order — Tensile Strength and Test Geometry. The defaults are a realistic case, so you can change one value at a time and watch what moves.
There is no calculate button. Every figure recalculates as you type or drag, which is what makes this usable for a what-if sweep rather than a single answer.
Read Predicted Bursting Strength in the dark results panel — that is the headline figure, expressed in kPa.
Check the supporting rows underneath (Bursting Strength, Bursting Strength, Biaxial Tension per Metre and Burst Index) before acting on the headline — they are where an implausible input usually shows itself first.
Reset to defaults returns every field to the reference case, which is the quickest way to check whether a surprising result came from the tool or from an input you had changed earlier.
Where this is used
Knit development — converting the tensile figures from a similar running quality into an expected burst before the fabric is knitted, so a construction that will not clear a 200 kPa buyer minimum is caught at the specification stage rather than after dyeing and finishing.
Test-area translation — a buyer quotes burst on the 7.3 cm2 area and the mill lab is fitted with a 50 cm2 head. Running both apertures through the model shows the direction and rough scale of the gap, which stops a lab reporting against the wrong basis without realising it.
Failure investigation — a lot fails burst but passes tensile. Feeding the measured wale and course strengths back through the model shows whether the burst result is consistent with the tensile result; when the measured burst is far below what the strengths support, look for a local defect such as a needle line, a drop stitch or a slack course rather than a strength problem.
Weight reduction — burst index compares two constructions on burst per gram, so yarn can be taken out of a fabric that is over-engineered against its spec, provided the comparison stays within one fibre and one knit structure.
Base-cloth selection for coating and lamination, where a burst minimum is set on the substrate and the coating is not allowed to be the load-carrying layer.
Reading the result
Typical bands and what each one is telling you.
Value
What it indicates
Below 200 kPa (29 psi)
Under the floor most knitted apparel programmes set. Buyer minimums for T-shirt and underwear jersey commonly sit around 200 kPa on the 7.3 cm2 area, and since the model runs optimistic, a prediction this low leaves no margin at all on the tester.
200 to 400 kPa
Fine-gauge lightweight single jersey territory, roughly 110 to 160 g/m2. Workable for linings and light tees, but check extension as well: thin jersey often fails the diaphragm by stretching into the burst rather than by lacking strength, and the specification may cap distension.
400 to 900 kPa
Where mainstream interlock, rib and mid-weight jersey at 170 to 240 g/m2 actually measure. A predicted figure in this band usually corresponds to a fabric that measures comfortably inside typical apparel specs.
900 to 1500 kPa
Predicted for heavier double knit, fleece and filament sportswear constructions. The worked example lands here at 908.62 kPa from a 180 g/m2 input, which is the model showing its optimism — the same fabric on a tester would be expected one band lower.
Above 1500 kPa
High-tenacity filament and industrial knits. Before quoting a figure this high, confirm the tester's pressure range and the rating of the diaphragm fitted, because heads are supplied in different pressure classes and the instrument, not the fabric, may become the limit.
Assumptions and limits
The sphere model assumes uniform biaxial loading and ignores the bending stiffness of the fabric, so treat it as a design estimate and confirm on a diaphragm tester.
Every input is bounded to the range normal practice occupies (Strength, Wale Direction 1 to 20000 N/5cm, Strength, Course Direction 1 to 20000 N/5cm and Test Aperture Diameter 5 to 200 mm, and so on for the rest). Those bounds are guard rails against typing errors, not a claim that the formula fails one unit outside them.
The calculation is deterministic: the same inputs always give the same result. It carries no allowance for machine condition, operator skill, ambient conditions or lot-to-lot material variation unless an input above explicitly represents one.
Nothing is sent anywhere. The maths runs in your browser, so the numbers you type never leave the page.
Standards and further reading
ISO 13938-1 (hydraulic) and ISO 13938-2 (pneumatic) — bursting strength by the diaphragm method. Both define test areas of 7.3, 10, 50 and 100 cm2; the 7.3 cm2 area is the 30.5 mm aperture used as the default here, and results from different areas are not interchangeable.
ASTM D3786 — diaphragm bursting strength tester method for knitted, nonwoven and woven fabrics. This is the mill instrument the predicted pressure should be checked against.
ISO 13934-1 — breaking force of fabric by the strip method on a 50 mm specimen, which is the test that produces the N/5cm inputs for the wale and course fields.
ASTM D3776 — mass per unit area of fabric, the measured input behind the burst index denominator. ISO 3801 covers the same determination but is written for woven fabrics; for knits, mill practice is the same gravimetric method on a cut die specimen, conditioned before weighing.
Questions people ask
Why does this predict a higher burst than my tester reads?
Two reasons, both geometric and both pushing the same way. The model puts the dome radius equal to the aperture radius, but a fabric bursts as a shallower cap whose radius of curvature is larger, and P = 2T/r falls as r grows. On top of that, a fabric loaded biaxially fails below its uniaxial strip strength, because the cross-direction yarns are already tensioned when the failing direction reaches its limit. Treat the output as an upper bound and calibrate a correction ratio against your own tester for each fabric family — a stable factor per quality is far more useful than the raw number.
Which aperture diameter should I enter?
The one your test method specifies, because bursting strength is not a material constant — it is a property of the fabric and the test area together. The 30.5 mm default is the 7.3 cm2 area; the 50 cm2 head is 79.8 mm across, so entering it drops the predicted pressure to about 38 percent of the 7.3 cm2 figure — 908.62 kPa becomes roughly 347 kPa on the same strengths. The model gets the direction right but overstates the size of the change: measured burst does fall as the aperture grows, but by less than the 1/r the sphere relation implies, because the larger dome strains further before failing and its curvature does not scale with the aperture.
Should I enter strip or grab tensile figures?
Strip, on a 50 mm specimen — that is exactly what the N/5cm unit means. Grab results load a 100 mm specimen through 25 mm jaws, and the yarns outside the jaws share the load, so a grab force does not divide cleanly by a width to give N/5cm. Many knit labs run grab by preference because jersey curls and necks badly in the strip test; if grab is all you have, note the basis on the calculation rather than converting it with an assumed factor.
My wale and course strengths differ by a factor of three. Is the geometric mean still usable?
It is a compromise, and it degrades as the ratio widens. In a spherical membrane the tension is the same in every direction, so failure should really be governed by the weaker direction rather than by any mean of the two. At the default 400 and 300 N/5cm the geometric mean of 6,928 N/m sits 15% above the weak-direction tension of 6,000 N/m, which is tolerable. At 3:1 or worse, run the calculation again with the weaker strength entered in both boxes and use that lower figure as the working prediction.
