Packaging and Logistics
Can Seam Overlap: The Measurement That Decides Whether a Can Holds
A double seam is five layers of metal folded into a hook, and whether it holds comes down to how much the two hooks overlap. The formula is simple and the measurement discipline is not.
The double seam is the only thing standing between a filled can and its contents, and its integrity is governed by geometry rather than by anything you can see from outside. Two hooks, one formed from the can body and one from the end, interlock and are compressed with sealing compound between them. Overlap is the length over which those two hooks actually engage, and it is calculated from a teardown rather than observed directly.
The formula, worked
Overlap equals body hook plus cover hook plus end plate thickness, minus seam length. Take a teardown measuring a body hook of 2 mm, a cover hook of 1.95 mm, a seam length of 2.9 mm, and end plate thickness of 0.21 mm. Overlap is 1.26 mm. Expressed as a percentage of the available internal seam length, which is seam length minus 1.1 times the plate thickness, that is roughly 47%. The percentage rather than the absolute figure is what acceptance criteria are written against, because it normalizes across can sizes and material gauges.
- Calculated overlap: 1.26 mm
- Overlap percentage: 47%
- Seam length measured: 2.9 mm
Why percentage and not millimetres
A given absolute overlap means something different on a thin-gauge beverage can than on a heavy-gauge food can, because the available seam length differs. Industry practice, reflected in seam evaluation guidance from can makers and packaging associations, is therefore to specify a minimum overlap percentage. Consult the specification for your specific can and end combination rather than adopting a general figure, because the acceptance threshold is a property of the can design and the supplier who made it.
Overlap alone does not qualify a seam
A complete evaluation includes all of these
- Body hook and cover hook lengths individually, since a good overlap can be produced by a long hook compensating for a short one, which is not a robust seam.
- Tightness, assessed from wrinkle rating on the cover hook. A loose seam can measure acceptable overlap and still leak.
- Countersink depth, which relates the seam to the end profile and drifts as tooling wears.
- Thickness and width, which detect over-compression that thins the metal and creates a fracture risk.
A seam that measures well on overlap and badly on tightness will pass your teardown and fail in the warehouse. Both belong on the record.
Sampling frequency is the real control
Seam parameters drift as first and second operation rolls wear, and the drift is gradual, which means the value of a teardown lies in its trend rather than in any single result. Establish frequency by seaming head and by shift, chart the parameters rather than merely recording pass or fail, and the tooling change becomes a scheduled event rather than a response to a leaker. A seam programme that records only pass or fail throws away the information that would have predicted the failure.
Use the seam check rate calculator to set inspection frequency against line speed and head count. Check your seam programme
Published 2026-08-08.