Seam allowance calculator
To find the cut size, add the seam allowance once for every edge that is actually seamed: a 50 cm finished width sewn at 1.5 cm on both side edges is cut at 53 cm, while an edge that is folded or left on the selvedge adds nothing. The reverse matters more and is easier to miss — because the cut size is fixed by the pattern, sewing at a different allowance from the one the pattern assumed changes the FINISHED size instead, by twice the difference on every seamed edge. The calculator below shows both, in metric or imperial.
Your piece and your seams
53 cm × 63 cm
Finished 50 cm × 60 cm once the seams are sewn at 15 mm.
- Finished size
- 50 cm × 60 cmwhat you want to end up with
- Added by the allowances
- 30 mm × 30 mm2 side and 2 end edges seamed
- Extra cloth in 2 pieces
- 0.068 m²over the finished area
How we got this
- Cut width — 53 cm50 cm finished + 2 × 1.5 cm
- Cut length — 63 cm60 cm finished + 2 × 1.5 cm seams
- Extra cloth in the allowances — 0.068 m²2 × (53 cm × 63 cm − 50 cm × 60 cm)
Cut size and finished size are two different numbers
Every pattern deals in both and only labels one of them. The finished size is what you end up holding; the cut size is what you take the scissors to. Between them sit the allowances, and the only reliable way to keep them straight is to work in one direction: start from what you want, add what each edge consumes.
What each edge consumes is not the same for all four. A seamed edge takes one seam allowance. A hemmed edge takes the whole hem — twice its depth if it is turned twice. A folded edge, where the piece is cut on a fold and opens out, takes nothing. Nor does an edge left on the selvedge and used as a finished edge, which is a common trick on curtain linings and tote bags.
The calculator asks how many of the two side edges and how many of the two end edges are seamed for exactly that reason. Adding twice the allowance to both dimensions by reflex is the single commonest way a cut piece comes out too big.
A worked example
A 50 × 60 cm finished panel, sewn at 1.5 cm, seamed on all four edges, cut twice. The table is generated by the calculation.
| Step | Working | Result |
|---|---|---|
| Cut width | 50 cm finished + 2 × 1.5 cm | 53 cm |
| Cut length | 60 cm finished + 2 × 1.5 cm seams | 63 cm |
| Extra cloth in the allowances | 2 × (53 cm × 63 cm − 50 cm × 60 cm) | 0.068 m² |
Each piece is cut 53 cm × 63 cm, so the allowances add 3 cm across and 3 cm down. Over two pieces that is 0.068 m² of cloth that exists only to be sewn away — worth knowing when you are deciding how much to buy.
The mistake that has no visible cause
Here is the failure this calculator was built for, and it is invisible while it is happening.
The cut size is fixed by the pattern. If you then sew at a different allowance from the one the pattern assumed, the cut piece does not change — the finished piece does. Sew shallower and you leave more fabric inside the garment, so it comes out bigger. Sew deeper and it comes out smaller. Every seamed edge contributes the full difference, so the error scales with how many seams the thing has.
| You sew at | Difference per edge | Finished width | Change |
|---|---|---|---|
| 10 mm | 5.88 mm | 51.2 cm | +11.75 mm |
| 12 mm | 3.88 mm | 50.8 cm | +7.75 mm |
| 15 mm | 0.88 mm | 50.2 cm | +1.75 mm |
| 15.88 mm | 0 mm | 50 cm | as intended |
| 20 mm | -4.13 mm | 49.2 cm | −8.25 mm |
A centimetre on a cushion cover is a shrug. The same error on a fitted bodice with a centre back seam, two side seams and two princess seams — ten seamed edges — is five centimetres, which is a full size. It is also perfectly consistent, which is why it never looks like a mistake: every piece is wrong by the same amount, so everything still fits together beautifully and the finished garment is simply the wrong size.
Five-eighths of an inch is not one and a half centimetres
Half the sewing world treats them as the same thing. They are 15.88 mm and 15 mm respectively, which is 0.88 mm apart.
On one seam that difference is invisible and nobody should worry about it. What makes it worth knowing is that it compounds in exactly the way described above: a garment with ten seamed edges sewn metric from an imperial pattern finishes about 8.75 mm narrower than drafted. Add a machine guide that is itself a millimetre out and it starts to matter on a close-fitting bodice.
The practical rule is not to convert, but to commit. Sew the whole garment at whatever the pattern was drafted at, in the pattern's own units, using a guide you have checked. Mixed allowances within one project are what produce pieces that do not match at the notches.
Which allowance for which job
| Allowance | In millimetres | In inches | Used for |
|---|---|---|---|
| ¼ in | 6.35 mm | ¼ in | Patchwork and quilting, where every seam is pieced. |
| ⅜ in | 9.52 mm | ⅜ in | Overlocked knits and fine seams. |
| 1 cm | 10 mm | ⅜ in | European patterns, bag making, home furnishing. |
| ½ in | 12.7 mm | ½ in | General home sewing, US patterns for straight seams. |
| 1.5 cm | 15 mm | 9/16 in | The metric dressmaking standard. |
| ⅝ in | 15.88 mm | ⅝ in | The imperial dressmaking standard. Not the same as 1.5 cm. |
| 2 cm | 20 mm | 13/16 in | Curtains, loose covers, anything that may be let out. |
Two principles sit behind that list. Narrow allowances reduce bulk, which is why patchwork uses a quarter inch — a quilt block has dozens of seams meeting at its corners, and anything wider would build a lump. Wide allowances buy options, which is why dressmaking uses 15 mm and curtains 20 mm: a seam that can be let out is worth more than the few centimetres of cloth it costs, especially on something fitted or something that may be rehung in a different window.
How to use this
- Start from the finished size. The size you want after sewing — the width of the cushion cover, the length of the panel, the measurement on the schematic.
- Count the edges that are really seamed. A joined edge takes an allowance. A folded edge, a selvedge edge or a hemmed edge does not, and treating all four the same is the usual mistake.
- Use the allowance you will actually sew at. Not the one you think you should use. If your machine guide is at 1 cm and you sew to the guide, that is your allowance.
- Say what the pattern assumed. If the pattern is imperial and you sew metric, or the other way round, this is where the difference shows up — in the finished size, not the cut size.
- Add the hem separately. A hem allowance is all the fabric the hem turns under, which for a double hem is twice its depth.
What this does not cover
- Curved and shaped seams. The arithmetic is the same, but a curve seamed at the wrong allowance changes shape as well as size, and no calculator can tell you how that will hang.
- Ease. A pattern's finished measurements already include whatever ease the designer intended. This tool works from the finished size you give it and does not add any.
- Fabric take-up. A thick or lofty fabric loses a little across every seam as it turns the corner of the seam allowance. On quilting cotton it is negligible; on a heavy wool coating it is not.
- How much to buy. That is a layout question — the fabric yardage calculator packs cut pieces onto a roll, and the cushion calculator does it for covers specifically.
Common questions
- Is 5/8 inch the same as 1.5 cm?
- No. Five-eighths of an inch is 15.875 mm and 1.5 cm is 15 mm, so they differ by 0.875 mm per edge. On one seam that is nothing; on a garment with eight seamed edges it is 7 mm of finished width, and on a fitted bodice that is a size you did not order.
- Do I add the allowance to every edge?
- Only to the edges that are actually joined to something. A hemmed edge takes its hem allowance instead, a folded edge takes nothing at all, and a piece cut on a selvedge may need nothing on that side. Adding twice the allowance to every dimension by reflex is the commonest way a cut piece comes out too big.
- What happens if I sew at a different allowance from the pattern?
- The finished piece changes size, not the cut piece. Sewing at 1 cm where the pattern assumed 1.5 cm leaves 5 mm more in the garment at every seamed edge, so a piece with two side seams comes out 1 cm wider. It is a consistent error, which is why it is so hard to spot on the table.
- How much fabric does a hem use?
- All of what it turns under, which for a double hem is twice its depth: a 2 cm double hem consumes 4 cm of cloth. Enter the total the hem eats rather than the depth you can see, or the piece will finish short by exactly one turn.
- What allowance should I use?
- A quarter inch (6.35 mm) for patchwork, where every seam is pieced and bulk matters. Ten millimetres for bags and home furnishing. Fifteen millimetres or five-eighths of an inch for dressmaking, because a wider allowance can be let out later. Two centimetres on curtains and loose covers for the same reason.
Related tools
Seam allowance chart
Every common allowance in millimetres and inches, and which projects use which.
Cushion fabric calculator
Cover size, back opening and piping in; the length to buy and a row-by-row cutting plan out.
Tablecloth fabric calculator
Table plus drop, round or rectangular — and whether turning the cloth loses a seam.
Fabric yardage calculator
Piece size, how many, roll width — the length to buy, and the strip you pay for but never cut.
Fabric GSM calculator
g/m², oz/yd², running-yard ounces and momme — plus the weight of an unlabelled fabric from a swatch.