Hem & Seam Calculator
Calculate material allowance for various sheet metal hem and seam types including open, closed, teardrop, and rolled hems.
About this calculator
Hem Allowance per Edge is the flat-pattern length one hemmed edge consumes: the hem return leg you specify, plus a bend allowance. For a closed (flattened) hem the crushed fold adds only about 43% of material thickness on top of the return leg; open, teardrop and rolled hems keep a real bend radius, so their allowance adds a 180-degree arc length of pi times (inside radius + 0.43 × thickness) instead. Industry practice sets a minimum hem return of 4 times material thickness for every hem type -- open hems included -- and the calculator raises any shorter entry to that floor. Seam Allowance per Edge is set by the forming tooling rather than by material thickness: a Pittsburgh lock consumes twice the lockformer pocket depth plus 3/16 inch on the pocketed edge, and the pocket depth minus 1/16 inch on the flanged edge, which is why a Pittsburgh on 26-gauge and on 16-gauge stock both take roughly an inch of flat material even though the gauges differ threefold.
A lap seam simply consumes the overlap width you specify, independent of thickness entirely. Hem Length only feeds into Extra Blank Area -- the additional blank area one hemmed edge consumes, equal to the per-edge allowance multiplied by the edge length -- and has no effect on the per-edge allowance figures themselves. Hem Type has no bearing on the seam figures and Seam Type has no bearing on the hem figures; the two allowances are calculated completely independently even though they appear together in the results. The Formability Note flags what the raw thickness number does not: hems are routinely formed from about 0.040 in to 0.125 in (1.0 to 3.2 mm), above which the fold becomes prone to cracking regardless of hem type, and closed (crushed) hems are not recommended in aluminium at any thickness -- which is exactly why the teardrop hem exists, keeping an inside diameter equal to material thickness so crack-prone alloys are never folded flat on themselves.
Inputs
Results
Hem Allowance per Edge
4.43 mm
Thickness Check
Out of range
How to Use This Calculator
- Enter Material Thickness and select the Hem Type: open, closed/flattened, teardrop, or rolled/rope.
- Optionally enter a Hem Return Length; leave it at 0 and the calculator applies the industry minimum of 4x material thickness, which applies to every hem type.
- Enter Hem Length to get the extra blank area one hemmed edge consumes.
- Select the Seam Type and, for a Pittsburgh lock or double seam, the Lockformer Pocket Depth -- seam allowances are set by the tooling, not by the gauge.
- Add Hem Allowance per Edge and Seam Allowance per Edge to your blank before cutting, and check the Formability Note: closed hems are not recommended in aluminium, and hems above 0.125 in (3.18 mm) are prone to cracking.
How the result changes with Material Thickness
| Material Thickness | Hem Allowance per Edge | Thickness Check |
|---|---|---|
| 0.5 | 2.22 mm | Out of range |
| 0.75 | 3.32 mm | Out of range |
| 1.5 | 6.65 mm | Within range |
| 2.5 | 11.08 mm | Within range |
What each input means
- Material Thickness
- Sheet metal thickness.
- Hem Type
- Type of edge hem to apply.
- Hem Return Length
- Length of the folded-back leg. Leave at 0 to use the industry minimum of 4x material thickness, which applies to every hem type.
- Material Class
- Closed (crushed) hems are not recommended in aluminium at any thickness -- use a teardrop hem instead.
- Hem Length
- Length of the edge to be hemmed.
- Seam Type
- Type of joint seam if applicable.
- Lockformer Pocket Depth
- Only used for Pittsburgh Lock and Double Seam. Set by the lockformer tooling, not by material gauge; 7.94mm (5/16 in) is a common standard pocket depth.
- Lap Overlap Width
- Only used for Lap Seam. The overlap width you specify -- lap seam allowance does not scale with material thickness.
- Standing Seam Height
- Only used for Standing Seam. Height of each of the two upright folds.
How this is calculated
Worked example, using the default values
- Identify Input Parameters4 parametersMaterial Thickness = 1, Hem Type = 2, Hem Length = 500, Seam Type = 1 = 4 input(s) provided
- Calculate Hem Allowance per EdgeHem Allowance per Edge4.43 = 4.43
- Calculate Seam Allowance per EdgeSeam Allowance per Edge25.4 = 25.4
- Calculate Both EdgesBoth Edges8.86 = 8.86
Engine last updated . Checked against 3 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
Why doesn't increasing Hem Length change the Hem Allowance per Edge figure?
Hem Allowance per Edge is a per-edge fold specification driven by Hem Return Length, Material Thickness and Hem Type -- it describes how much extra material one edge needs to fold over, independent of how long that edge is. Hem Length only multiplies into Extra Blank Area, the additional square centimetres of blank the full edge consumes.
Which hem type needs the most extra material for a given return length?
A rolled hem, because it wraps a full circumference rather than a single 180-degree fold. Among the folded types, open and teardrop hems need more than a closed hem for the same return leg, since each keeps a real inside radius while a closed hem is crushed flat and only adds the ~43% of material thickness rule of thumb. The 4x-thickness minimum return length applies to every hem type, not just closed hems, so it is never the thing that distinguishes them.
How much flat material does a Pittsburgh lock need compared with a lap seam?
A Pittsburgh lock formed on a standard 5/16-inch lockformer consumes about 1 inch (roughly 25 mm) of flat material across both edges -- 2 x pocket depth plus 3/16 inch on the pocketed edge, plus pocket depth minus 1/16 inch on the flanged edge. A simple lap seam consumes only whatever overlap width you specify, commonly 1 inch total. Neither figure scales with material gauge: the lockformer pocket depth is fixed by the tooling, so a Pittsburgh on 26-gauge and on 16-gauge stock take nearly the same flat allowance.
Is there a maximum material thickness for hemming?
Hems are routinely formed in stock from about 0.040 in to 0.125 in (1.0 to 3.2 mm); above 0.125 in the outer fibre at the fold becomes prone to cracking regardless of hem type, and you should switch to a teardrop hem or open the inside radius. The bigger constraint is alloy, not gauge: closed (crushed) hems are not recommended in aluminium at any thickness, which is exactly why the teardrop hem exists -- it keeps an inside diameter equal to material thickness so crack-prone alloys are not folded flat on themselves.
What should I check before cutting the blank?
Add Hem Allowance per Edge and Seam Allowance per Edge to your nominal part size to get the flat-pattern blank size, then check the Formability Note before you commit to a hem type -- it will tell you if the selected thickness and alloy combination is outside the normal hemming range.
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