Understanding Heat Seal Strength Units: N/15 mm, N/25.4 mm, and lbf/in

Despite looking like a fraction, the slash in a heat seal strength unit is not a division sign. It is a label: force measured on a specimen of that width. A reading of 6.0 N/15 mm means the tensile tester recorded 6.0 newtons of peak peel force when pulling apart a 15 mm wide strip of sealed film.

Seal strength scales roughly in proportion to specimen width. A 25.4 mm strip of the same seal will resist about 1.69 times more force than a 15 mm strip, simply because there is more bonded area under load. That ratio (25.4 divided by 15) is the source of most conversion errors.

ASTM F88/F88M allows three specimen widths: 25 mm, 15 mm, or 1.00 inch (25.4 mm). The choice of width determines the unit label on your test report and affects the raw force number, even though the underlying seal quality is identical.

Two points that catch people off guard. First, 25 mm and 25.4 mm are not the same width. The difference is small (1.6%), but ASTM F88 treats them as separate systems and states that values from each system shall be used independently. Second, the standard records peak force as measured, not normalized to a per-millimeter basis. That means you cannot divide an N/25.4 mm result by 25.4 to get "N per mm" and then multiply by 15 to get N/15 mm unless you accept a simplifying assumption that seal strength is perfectly linear with width.

For a detailed walkthrough of the ASTM F88 test procedure itself, see the step-by-step heat seal strength test method guide on this site.

Conversion requires two separate steps: a force-unit conversion (newtons to pound-force or the reverse) and a width-scaling step (adjusting for different specimen widths). Skipping either step produces a wrong number.

Step 1: Force-unit conversion

One pound-force equals 4.448 newtons. This factor handles the switch between the imperial and metric sides:

• N to lbf: divide by 4.448
• lbf to N: multiply by 4.448

Step 2: Width scaling

If the source and target units use different specimen widths, scale the force in proportion to the width ratio:

• 15 mm to 25.4 mm: multiply by 25.4 / 15 = 1.693
• 25.4 mm to 15 mm: multiply by 15 / 25.4 = 0.591
• 15 mm to 25 mm: multiply by 25 / 15 = 1.667
• 25 mm to 25.4 mm: multiply by 25.4 / 25 = 1.016

Combined conversion factors

Use this combined conversion table to go directly from one heat seal strength unit to another. Find your starting unit in the left column and your target across the top, then multiply.

Worked example

A European medical packaging supplier reports seal strength as 7.5 N/15 mm. The customer in the United States requests the value in lbf/in.

1. Width scaling: 7.5 N on 15 mm needs to become force on 25.4 mm. Multiply by 1.693 to get 12.70 N/25.4 mm.
2. Force conversion: 12.70 N divided by 4.448 equals 2.86 lbf.
3. Result: 7.5 N/15 mm equals 2.86 lbf/in.

Or use the shortcut: 7.5 multiplied by 0.381 equals 2.86 lbf/in.

Ignoring the specimen width. Comparing 5.0 N/15 mm directly to 5.0 N/25.4 mm and calling them equal is the single most frequent error. The 15 mm reading, when scaled to the same width, is actually 8.5 N/25.4 mm, nearly 70% stronger.

Converting force but not width. Some conversion guides simply multiply lbf by 4.448 and relabel the result as N/15 mm. That gives 1 lbf/in = 4.4 N/15 mm, which overstates the correct figure of 2.63 N/15 mm by about two thirds. The error comes from swapping the width label without adjusting the force for the different specimen size.

Mixing ASTM and ISO test conditions. Even after correct unit conversion, an N/15 mm result from an ISO 11607 test and an lbf/in result from an ASTM F88 test may not be directly comparable. The two standards allow different peel techniques, crosshead speeds, and conditioning requirements. Unit conversion makes the numbers look like the same language, but the underlying test method differences remain.

Rounding before converting. Rounding 0.591 to 0.6 introduces a 1.5% error. Over a batch of results near an acceptance threshold, that rounding can flip a pass/fail verdict. Carry at least three decimal places through the calculation and round only at the final reported value.

Your report unit is not a preference. It is set by the standard your customer or regulator requires. If no standard is specified, match the unit your customer uses in their own specification sheet so the numbers can be compared without conversion.

When a customer specification gives an acceptance limit like "minimum 1.5 lbf/in" and your lab tests at 15 mm width, convert your result to lbf/in before comparing it to the limit. Do not convert the limit to your unit and then compare, because the rounding direction can differ and audit trails become harder to follow.