Burst Factor (BF)

The formula is BF = bursting strength (gf/cm²) ÷ grammage (g/m²). Because 1 kg/cm² equals 1,000 gf/cm², the working version is BF = BS in kg/cm² × 1,000 ÷ GSM. A 180 GSM liner that bursts at 4.3 kg/cm² gives 4,300 ÷ 180 = 23.9, sold as 24 BF. A 120 GSM paper bursting at 2.2 kg/cm² gives 18.3 BF. Reverse the formula to predict strength: a 150 GSM, 22 BF liner should burst at about 150 × 22 ÷ 1,000 = 3.3 kg/cm².

BF is a quality figure, not a weight figure. Two liners of identical 150 GSM can differ by 50% in burst because one is virgin long-fibre kraft and the other is recycled short-fibre test liner. Virgin Indian kraft normally sits at 24–35 BF, semi-kraft and recycled grades at 14–22 BF, and imported Scandinavian or North American kraft liner can exceed 35 BF. Mill pricing steps up with each BF grade, so BF is the single largest cost driver in a corrugated paper specification after GSM itself.

Paper BF and board bursting strength are different numbers. The board is tested as a whole on a Mullen tester (IS 1060 for paper, TAPPI T810 for board) and reported in kg/cm². As a rule of thumb the board bursts at close to the sum of its liner bursts — a tested board usually reads at or slightly above that sum — while the fluting medium contributes very little. A 3-ply board with 150 GSM 22 BF liners on both faces therefore lands around 3.3 + 3.3 = 6.6 kg/cm², in the lower half of the typical 6–11 kg/cm² 3-ply range. The bursting strength calculator runs the same sum for 3, 5 and 7-ply.

Indian purchase orders specify paper as GSM + BF for every layer, written outer / medium / inner — for example 180/120/150 at 22 BF for a 3-ply carton, or 180/120/150/120/180 for 5-ply. Samples must be conditioned at 27 ± 2 °C and 65 ± 2% RH before testing under IS 1060; a certificate showing unconditioned results in a humid Rajkot monsoon month will read low. Ask the converter for the mill test certificate per paper, not just a board burst figure.

The common over-specification is demanding 28–35 BF on every layer of a box whose real risk is warehouse stacking. Burst measures puncture and rough-handling resistance; stacking is governed by edge crush test and box compression, which respond more to medium GSM and flute choice than to liner BF. Where the failure mode is a heavy engineering part pressing outward or hooks and forklift tines, BF earns its cost; where cartons collapse in a stack, upgrade the ECT instead.

ISO 2758 reports the same property as burst index in kPa·m²/g. Since 1 kPa equals 10.2 gf/cm², BF is roughly burst index × 10.2. A European data sheet showing burst index 2.2 kPa·m²/g corresponds to about 22 BF, which is how to compare an imported kraft liner offer against a Gujarat mill quotation.