# 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.

Source: https://fabricmeasure.com/calculators/seam-allowance-calculator/
Last updated: 2026-08-20 (method v1)
Published by FabricMeasure.

---

## The formula

```
cutWidth  = finishedWidth  + seamedSideEdges x seamAllowance
cutLength = finishedLength + seamedEndEdges  x seamAllowance + hemAllowance
```

Only the edges that are actually joined take an allowance. A folded edge, a selvedge edge or
a hemmed edge takes none - adding twice the allowance to both dimensions by reflex is the
commonest way a cut piece comes out too big.

## The reverse, which is the one people get wrong

```
finishedDrift = (patternAllowance - yourAllowance) x seamedEdges
```

The cut size is fixed by the pattern, so sewing at a different allowance changes the FINISHED
size, not the cut size. Sew shallower and the piece comes out bigger, by the difference on
every seamed edge. A bodice with ten seamed edges sewn at 10 mm instead of 15 mm finishes
5 cm wider - consistently, which is why it never looks like a mistake.

## Common allowances, exactly

| Allowance | Millimetres | 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. |

5/8 in is 15.88 mm and 1.5 cm is 15 mm. They are not the same allowance.

## Worked example

A 50 x 60 cm finished panel, sewn at 1.5 cm, seamed on all four edges, cut twice.

| 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 |

## Assumptions

- Straight seams. A curve sewn at the wrong allowance changes shape as well as size.
- A hem allowance is ALL the fabric the hem turns under: a 2 cm double hem uses 4 cm.
- Ease is whatever the pattern already includes; none is added here.
- Fabric take-up across a thick seam is not modelled.

## 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](https://fabricmeasure.com/charts/seam-allowance-chart/): Every common allowance in millimetres and inches, and which projects use which.
- [Cushion fabric calculator](https://fabricmeasure.com/calculators/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](https://fabricmeasure.com/calculators/tablecloth-fabric-calculator/): Table plus drop, round or rectangular — and whether turning the cloth loses a seam.
- [Fabric yardage calculator](https://fabricmeasure.com/calculators/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](https://fabricmeasure.com/conversions/fabric-gsm-calculator/): g/m², oz/yd², running-yard ounces and momme — plus the weight of an unlabelled fabric from a swatch.