Choose a seam allowance for a garment and fabric, see the cloth a seam actually consumes, and convert a pattern from one allowance to another — with fractions reduced and metric paired the way patterns print it.
Change the inputs that matter to your decision. Optional assumptions stay collapsed until you need them.
5/8" (1.5 cm) per edge on a plain seam, consuming 5/8" in total.
5/8 inch converts to 1.5875 cm, but every pattern company prints 1.5 cm and drafts to it. The paired figure is the one to use; treating the exact conversion as the standard puts a 4 mm error into every seam on the garment.
| Recommended allowance | 5/8" · 1.5 cmThe metric figure is the one pattern companies actually print, not an exact conversion |
|---|---|
| Total cloth per edge | 5/8" · 1.5 cmPlain seam, including the edge finish |
| Note | 5/8" is the drafting standard for commercial shirt patterns |
| Note | Overlocking trims a little of the allowance as it finishes it |
| Commercial pattern standard | 5/8" · 1.5 cmMost Big Four patterns |
| Industrial / RTW | 1/2" · 1.3 cmSaves cloth at scale |
| Knits on an overlocker | 1/4" · 6 mmTrimmed as it is sewn |
| Quilting | 1/4" · 6 mmScant, measured to the fold |
| French seam total | 5/8" · 1.5 cm1/4" then 3/8" |
| Flat-fell seam | 5/8" · 1.5 cmOne side trimmed to 1/4" |
| Single-fold hem | 1"–2" · 2.5–5 cmDepends on the hem's curve |
| Double-fold hem | 1/4" + 1" · 6 mm + 2.5 cmEncases the raw edge |
A French seam is two passes that together consume 5/8 inch, not twice whatever the base allowance happened to be. Flat-fell trims one side back to a quarter inch and folds the other over it. Both are modelled as their real passes here.
Practical notes about the inputs, assumptions, and result.
For patterns, 1.5 cm. The exact conversion is 1.5875 cm, but every commercial pattern company prints and drafts to 1.5 cm, and a metric pattern is not redrafted from an imperial one. Using the exact conversion puts a 4 mm error into every seam on the garment, which compounds badly across a fitted bodice.
About 5/8 inch in total, split across two passes — commonly 1/4 inch wrong sides together, trimmed, then 3/8 inch right sides together. It is not double the base allowance. Doubling a 5/8 inch base would leave 1 1/4 inch encased in a seam that is meant to be narrow and neat.
Trim or add the difference on every cut edge that carries a seam allowance. Going from 5/8 inch to 1/4 inch means trimming 3/8 inch from each of those edges. Hems, folds, and any edge cut on the fold are not seam allowances and stay as they are.
Because a knit seam is usually overlocked, which trims as it sews, and a wide allowance inside a stretch garment adds bulk without adding strength. A quarter to a half inch is normal. A woven garment that will be pressed open needs more room.
When you may need to let the garment out. An inch at the inseam, side seams, and centre back costs almost nothing in cloth and makes a real alteration possible later. Couture keeps generous allowances for exactly that reason.
More than people expect. Four 24 inch seams at 5/8 inch consume 120 square inches of cloth — most of a fat quarter — because both sides of every seam carry the allowance. The cloth-used mode adds that up so it can be planned for rather than discovered at the cutting table.
Other tools in tailoring & sewing.
Estimate cloth for a garment at your fabric's actual width, with pattern repeat matching computed from the repeat length, nap layouts, shrinkage, lining, and a quantity rounded to what the shop will cut.
Grade a pattern between any two sizes from your own base measurements, with per-measurement grade rules, a wider grade above L, and the amount to slash and spread at each pattern quarter.
Convert the units a workroom actually uses: fractional inches, yards and metres, fabric weight between gsm and ounces per square yard, and iron temperatures by fibre.