Is a 5/8 inch seam allowance the same as 1.5 cm?

Not quite. Five-eighths of an inch is 15.875 mm and 1.5 cm is 15 mm, a difference of 0.875 mm per seam. On one seam it is invisible. On a bodice with eight seamed edges it is 7 mm of finished width, which is most of a size grade.

Why is patchwork sewn at a quarter inch?

Because a quilt block has dozens of seams meeting at its corners, and a wide allowance would create a lump where they cross. A quarter inch is 6.35 mm — enough to hold, small enough that six of them can meet without the block distorting. Many quilters sew a scant quarter inch, a thread narrower still, to allow for the fabric that turns the fold.

Does every edge take an allowance?

No, and treating all four edges the same is the usual mistake. A seamed edge takes one 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 at all, and neither does an edge left on the selvedge and used as a finished edge.

Why do curtains use 2 cm when dressmaking uses 1.5?

Because curtains may need letting out. A generous allowance leaves cloth to work with if a hem has to drop or a width has to be re-seamed, and the extra bulk does not matter in a heavy furnishing fabric hanging in folds. The same reasoning applies to loose covers.

Seam allowance chart

Seam allowances cluster around seven values: a quarter inch for patchwork, three-eighths for overlocked knits, 1 cm for European patterns and bag making, half an inch for general home sewing, 1.5 cm as the metric dressmaking standard, five-eighths of an inch as the imperial one, and 2 cm for curtains and loose covers. Five-eighths is 15.875 mm rather than 15 mm, so sewing a metric 1.5 cm on an imperial pattern finishes wider on every seam.

Every common allowance, by size

Seven values cover almost everything a pattern will ask for. They are listed here smallest first rather than in imperial and metric groups, because the labels do not sort the way people expect: 1 cm falls between three-eighths and a half of an inch, and 1.5 cm is smaller than five-eighths rather than equal to it.

Common seam allowances, converted exactly rather than approximately.
Allowance	Millimetres	Inches	Centimetres	Where it is used
¼ in	6.35 mm	¼ in	0.64 cm	Patchwork and quilting, where every seam is pieced.
⅜ in	9.52 mm	⅜ in	0.95 cm	Overlocked knits and fine seams.
1 cm	10 mm	⅜ in	1 cm	European patterns, bag making, home furnishing.
½ in	12.7 mm	½ in	1.27 cm	General home sewing, US patterns for straight seams.
1.5 cm	15 mm	9/16 in	1.5 cm	The metric dressmaking standard.
⅝ in	15.88 mm	⅝ in	1.59 cm	The imperial dressmaking standard. Not the same as 1.5 cm.
2 cm	20 mm	13/16 in	2 cm	Curtains, loose covers, anything that may be let out.

The seam allowance is the strip between the stitching line and the raw edge. It is not decoration and it is not waste: it is what holds the seam, what lets a seam be pressed open, and what a garment is let out into when it no longer fits. A pattern that quotes one figure and a sewist who works to another produce different finished garments from the same pieces.

The pairs that look equivalent and are not

Three substitutions get made constantly, all of them because the two figures look close enough on paper. Each one shifts the finished size, and the shift compounds with the number of seams.

What substituting one allowance for another costs in finished size.
Swapped for	Per edge	Across two seamed edges	Across eight
⅝ in → 1.5 cm	0.875 mm	1.75 mm	7 mm
½ in → 1 cm	2.7 mm	5.4 mm	21.6 mm
¼ in → 1 cm	3.65 mm	7.3 mm	29.2 mm

Five-eighths of an inch against 1.5 cm is the one that matters most, because it is the difference between the imperial dressmaking standard and the metric one, and the two are printed on patterns that otherwise look identical. Under a millimetre per edge is invisible. Across a fitted bodice with eight seamed edges it is most of a size grade, and it always runs the same way: sewing metric on an imperial pattern finishes wider than the designer intended.

Quarter-inch against 1 cm is the largest of the three and the least often noticed, because it turns up where a European bag pattern meets a quilter's habits. At 3.65 mm per edge it will not survive a piece that has to fit something else.

Which edges actually take one

Counting every edge the same is the usual mistake, and it inflates a cutting list on every piece. Four cases, and only the first takes a plain allowance:

A seamed edge takes one allowance — the strip on this piece only. The piece it joins carries its own.
A hemmed edge takes the whole hem, and twice its depth if it is turned twice. A 2 cm double hem consumes 4 cm.
A folded edge takes nothing. Where a piece is cut on a fold and opens out, there is no edge to finish.
A selvedge used as a finished edge takes nothing either — a common trick on curtain linings and tote bags, and one that quietly saves cloth on every piece.

Working from the finished size outwards is the only order that keeps this straight. Start from what you want to end up holding, add what each edge consumes, and the cut size falls out. Working backwards from a cut size to a finished one requires knowing what was already added, which is exactly the information a pattern tends not to print.

Why patchwork is different

A quarter inch is 6.35 mm, and it is the smallest allowance on the chart for a structural reason rather than a thrifty one. A quilt block has dozens of seams meeting at its corners; at any wider allowance the layers pile up where they cross and the block will not lie flat. Many quilters sew a scant quarter inch, a thread narrower still, to allow for the fabric consumed by the fold itself when the seam is pressed to one side.

That take-up is real on every fabric and negligible on most. A thin cotton loses a fraction of a millimetre as it turns the corner of the allowance; a heavy wool coating loses enough to matter, and a lofty quilted panel loses a great deal. None of the figures on this page account for it, because it depends on the cloth rather than the pattern.

To turn any of this into cut dimensions for a specific piece, the seam allowance calculator works from the finished size, counts only the edges that are really seamed, and shows the drift if you sew at a different allowance from the one the pattern assumed.

Common questions

Is a 5/8 inch seam allowance the same as 1.5 cm?: Not quite. Five-eighths of an inch is 15.875 mm and 1.5 cm is 15 mm, a difference of 0.875 mm per seam. On one seam it is invisible. On a bodice with eight seamed edges it is 7 mm of finished width, which is most of a size grade.
Why is patchwork sewn at a quarter inch?: Because a quilt block has dozens of seams meeting at its corners, and a wide allowance would create a lump where they cross. A quarter inch is 6.35 mm — enough to hold, small enough that six of them can meet without the block distorting. Many quilters sew a scant quarter inch, a thread narrower still, to allow for the fabric that turns the fold.
Does every edge take an allowance?: No, and treating all four edges the same is the usual mistake. A seamed edge takes one 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 at all, and neither does an edge left on the selvedge and used as a finished edge.
Why do curtains use 2 cm when dressmaking uses 1.5?: Because curtains may need letting out. A generous allowance leaves cloth to work with if a hem has to drop or a width has to be re-seamed, and the extra bulk does not matter in a heavy furnishing fabric hanging in folds. The same reasoning applies to loose covers.

Seam allowance chart

Every common allowance, by size

The pairs that look equivalent and are not

Which edges actually take one

Why patchwork is different

Common questions

Related tools

Seam allowance calculator

Cushion fabric calculator

Fabric yardage calculator

Fabric width chart