Bursting Strength Unit Conversions for kPa, psi and kgf/cm2

The bursting strength unit in ISO test methods is the kilopascal (kPa), while many North American reports still quote psi and older reports use kgf/cm2. All three measure the same thing: the peak hydraulic pressure that ruptures the paper or board. This guide gives the exact conversion factors and explains burst index.

What unit is bursting strength reported in?

Bursting strength is a pressure. ISO 2759 for board and ISO 2758 for paper both define it as the maximum pressure the hydraulic system develops in forcing an elastic diaphragm through a circular area of the test piece.

Both ISO methods state their ranges in kilopascals and ask for the mean result in kilopascals, to the nearest kilopascal. The pascal is the SI unit of pressure, equal to one newton per square metre.

TAPPI T 810, the North American method for corrugated board, states its range as 690 kPa (100 psi) to 4,825 kPa (700 psi). So a US report may show either unit, or both.

The kgf/cm2 is an older, non-SI unit. NIST notes that one technical atmosphere equals one kilogram-force per square centimetre. A report that writes kg/cm2 normally means kgf/cm2, because the kilogram on its own is a unit of mass, not force.

Questions about a model or a test? Email Amy Zhao, Export Manager, at [email protected].

Converting kPa, psi and kgf/cm2

The factors below come from NIST Special Publication 811, Appendix B.8. The first three rows are the NIST values; the last three are derived from them and rounded.

As a worked example, a board that bursts at 1,000 kPa reads about 145.0 psi or 10.20 kgf/cm2. Going the other way, the 100 psi lower limit of TAPPI T 810 is about 689.5 kPa, which T 810 states as 690 kPa.

Convert the unrounded mean, then round once at the end. NIST SP 811 advises rounding a converted value to the number of significant digits that matches the precision of the original value. A reading taken to the nearest kilopascal does not become more precise because it was converted to psi.

If you are checking a supplier report, our step by step burst test guide explains how the reading itself is taken and which readings should be rejected.

Burst index and why grammage matters

Heavier board usually bursts at a higher pressure, so the raw kPa value does not tell you whether the fibre is being used well. Burst index takes grammage out of the picture.

ISO 2759 defines burst index as bursting strength divided by grammage, with grammage measured to ISO 536. The formula is x = p / g, where p is the mean bursting strength in kPa and g is the grammage in g/m2.

The unit of burst index is kPa·m2/g, reported to three significant digits. For example, a hypothetical 400 g/m2 board that bursts at 1,000 kPa has a burst index of 2.50 kPa·m2/g.

Because burst index is a ratio with its own unit, do not convert it with the psi or kgf/cm2 factors alone. Compare burst index values only when both are in kPa·m2/g.

Our NPD-2D burst strength tester is one of the burst testers we supply. Tell us which units and report format your buyer expects and we can talk it through with you.

Units in TAPPI and ISO reports

ISO 2759 and ISO 2758 reports give the mean bursting strength to the nearest 1 kPa, the standard deviation of each mean and, if required, the burst index. They also state the conditioning atmosphere and the instrument used.

TAPPI T 810 lists its working range in kPa with psi alongside. When a buyer quotes a minimum in psi and your lab reports in kPa, agree on one unit and one rounding rule before the first shipment.

Units do not make two methods equivalent. ISO 2758 and ISO 2759 both note that tests on the same material will not necessarily give the same results, because the apparatus differs.

Frequently asked questions

Is psi the same as lbf/in2?

Yes. NIST SP 811 lists psi and lbf/in2 as the same unit, pound-force per square inch, with the same factor of 6.894 757 kPa.

Is bursting strength the same as edge crush strength?

No. Bursting strength is a pressure measured with a diaphragm, in kPa or psi. Edge crush is an edgewise compression test on a short column of corrugated board, such as TAPPI T 839, and is reported as a force per unit length.

Talk to our factory team

We sell direct from our factory, so you work directly with our factory team. Questions about a model or a test? Email Amy Zhao, Export Manager, at [email protected].