Bursting Strength and Burst Factor: Formula, Units (kg/cm², kPa, psi) and Worked Examples

Bursting strength is the hydraulic pressure needed to rupture a clamped circle of paper or corrugated board by pushing a rubber diaphragm through it. It is measured on a Mullen tester and expressed in kg/cm² in India, kilopascals in the ISO methods and psi in North America, where 1 kg/cm² = 98.07 kPa = 14.22 psi. Burst factor (BF) is the same measurement divided by the paper's grammage, so that a 120 GSM and a 180 GSM sheet from the same furnish can be compared: BF = bursting strength in g/cm² ÷ GSM. A 150 GSM kraft liner at BF 20 bursts at 20 × 150 ÷ 1,000 = 3.0 kg/cm².

Every Indian box plant owns a Mullen tester and every kraft mill sells by BF grade, so bursting strength is the number that dominates Indian carton specifications. It is a genuine measure of puncture and rough-handling resistance. It is not a measure of stacking strength, and the second half of this guide explains why a specification written only in BS routinely buys the wrong box.

How the Mullen test works

The sample is clamped between two annular plates over a circular aperture 30.5 mm in diameter. Beneath it is a rubber diaphragm backed by glycerine. A piston pushes the glycerine at a fixed rate, the diaphragm bulges through the aperture into the sample, and the pressure rises until the sample ruptures. The peak gauge pressure is the bursting strength. The method for paper is IS 1060 (Part 1) in India, equivalent to ISO 2758 and TAPPI T 403; for corrugated board it is ISO 2759 and TAPPI T 810, which use a stiffer diaphragm and a higher pressure range. Samples are conditioned at 23 °C and 50 % RH; ten readings, five from each side of the sheet, are averaged.

What is being measured is a combination of the tensile strength and stretch of the fibres in both directions, since the diaphragm loads the sheet biaxially. That is why long-fibre virgin softwood kraft gives high BF and short-fibre recycled furnish gives low BF at the same grammage, and why moisture — which lets fibres slip — cuts bursting strength sharply. A board tested at 85 % RH reads 20–30 % below the same board at 50 % RH.

Units and conversions

The American "200# test" single-wall box is 200 psi, which is 14.1 kg/cm². "275#" double wall is 19.3 kg/cm². Indian buyers who receive a US specification in psi should convert before comparing with a local plant's certificate in kg/cm² — a 200 psi requirement is a heavy 3-ply or light 5-ply here, not a standard 3-ply.

Burst factor and burst index: the formulas

Bursting strength rises with paper weight, so it cannot be used to compare fibre quality between grammages. Burst factor removes the grammage:

Burst factor (BF) = bursting strength (g/cm²) ÷ grammage (g/m²), which is the same as BF = bursting strength (kg/cm²) × 1,000 ÷ GSM.

Rearranged for the number a specification needs: bursting strength (kg/cm²) = BF × GSM ÷ 1,000.

The ISO equivalent is burst index, in kPa·m²/g: burst index = bursting strength (kPa) ÷ grammage. Because 1 kg/cm² is 98.07 kPa, burst index = BF × 0.0981. A BF 18 kraft has a burst index of 1.77 kPa·m²/g; a BF 24 kraft, 2.35. Indian mills grade kraft at BF 16, 18, 20, 22, 24, 28 and 35, with BF 16–18 the recycled and semi-kraft grades, 20–24 the common virgin-blend liner grades and 28–35 premium virgin kraft for export-grade liners. The kraft paper glossary entry covers how the grades are made.

Worked examples: paper

The table shows why BF and GSM have to be specified together. A 120 GSM BF 24 sheet (2.88 kg/cm²) is weaker in burst than a 180 GSM BF 18 sheet (3.24 kg/cm²), even though its fibre is better. If the specification says "BF 24 kraft" and the box plant supplies 120 GSM, it has complied and you have a weaker box. Specify "180 GSM, BF ≥ 22" — both numbers — and the bursting strength follows. The GSM entry explains how grammage is measured and how the board total is built up.

Worked examples: corrugated board

The bursting strength of a corrugated board is close to the sum of the bursting strengths of its liners. The fluted medium is under the diaphragm too, but it is corrugated and loosely bonded, so it adds little — typically 5–10 % — and most plants ignore it for estimating. That gives a quick check on any board quotation:

• 3-ply, 150 GSM BF 20 liners both sides, 120 GSM medium: 3.0 + 3.0 = 6.0 kg/cm², expected test result 6–6.5 kg/cm².
• 3-ply, 180 GSM BF 24 outer, 150 GSM BF 20 inner: 4.32 + 3.0 = 7.3 kg/cm², expected 7.5–8 kg/cm².
• 5-ply, three 150 GSM BF 22 liners, two 120 GSM media: 3.3 × 3 = 9.9 kg/cm², expected 10–11 kg/cm².
• 5-ply, 180 GSM BF 24 liners, 140 GSM media: 4.32 × 3 = 13.0 kg/cm², expected 13–14.5 kg/cm².
• 7-ply, four 180 GSM BF 24 liners: 4.32 × 4 = 17.3 kg/cm², expected 18–20 kg/cm².

Typical Indian market ranges by ply are 6–11 kg/cm² for 3-ply, 9–17 kg/cm² for 5-ply and 14–22 kg/cm² for 7-ply. When a 5-ply quotation claims 20 kg/cm² on 150 GSM BF 18 liners, the arithmetic — 2.7 × 3 = 8.1 kg/cm² — says the claim and the paper cannot both be true. That five-line check has saved more buyers than any test report.

What bursting strength does and does not predict

Bursting strength is a face property. It predicts how well the carton resists a corner of an adjacent crate punching through it, a forklift tine catching a panel, or a heavy loose item inside pushing outward on a drop. It correlates with the fibre quality and weight of the liners, so it also tracks the general robustness of the board under rough handling. It does not predict stacking. A carton in a pallet fails when its vertical panels buckle under the load above, and that is governed by the edge crush test and box compression, which depend on liner compression strength, medium GSM and flute height rather than diaphragm rupture.

Why Indian buyers over-specify BS and under-specify ECT

Three reasons, all historical. IS 2771 (Part 1), the Indian specification for corrugated fibreboard boxes, grades boxes by bursting strength and puncture resistance, so the tender template inherited BS. The Mullen tester is inexpensive and sits in every box plant and most QA departments, while an ECT fixture on a compression tester is rarer, so BS is the number both sides can verify. And kraft is traded by BF, so the paper cost and the BS figure move together and the buyer feels the specification is tied to what is being paid for.

The consequence is boxes with high-BF virgin liners on a light recycled medium: excellent BS, mediocre ECT, and a bottom tier that crushes in a container. The fix costs nothing: keep the BS line for puncture, add an ECT line for stacking, and let the box plant choose the paper combination that meets both. For a 5-ply export carton that usually means a modest liner and a heavier medium than the BS-only specification would have bought.

How to write bursting strength into a specification

1. State the board bursting strength in kg/cm² with the test method (ISO 2759 / TAPPI T 810) and the conditioning (23 °C, 50 % RH).
2. State liner GSM and BF together for every liner, and the medium GSM — "150 GSM BF 22 / 120 GSM medium / 150 GSM BF 22" for a 3-ply.
3. Check the arithmetic: sum of liner BS should be within 10 % of the board BS claimed.
4. Add an ECT or BCT line if the carton stacks, and a Cobb water-absorption limit if it is stored or shipped in the monsoon.
5. Ask for a per-lot test certificate with the unit written out — a certificate that says "12" with no unit is a transcription error waiting to happen.

Boxes specified from the paper up

We build custom corrugated boxes in 3-, 5- and 7-ply from stated liner grades, and test bursting strength, edge crush and box compression on every construction we quote, so the numbers on the certificate are the numbers the arithmetic predicts. Send us the product weight, the carton size, whether it stacks and how it travels, and we will propose a liner and medium combination with the BS and ECT figures written out — and tell you where a BS-only specification you have been given is buying the wrong paper.