How many millimeters is a ⅝ inch seam allowance?

A ⅝ inch seam allowance is 15.9 mm, or about 16 mm. In centimeters that is 1.59 cm, so a 1.5 cm allowance on a metric pattern is almost the same.

Is 1 cm the same as ⅜ inch?

Almost. ⅜ inch is 9.5 mm, so 1 cm is half a millimeter bigger. For a single seam that is too small to matter, but use one system for the whole project.

What is the standard seam allowance?

It depends on the project. Traditional US dressmaking patterns use ⅝ inch, many modern indie patterns use ⅜ inch, bags and cushions often use ½ inch, and quilting uses ¼ inch. Always use the allowance your pattern states.

Can I use a different seam allowance than the pattern says?

Only if you change the cut size to match. If a pattern includes ⅝ inch and you sew ½ inch, each seam adds ⅛ inch to the finished width, and on a fitted garment that adds up.

Seam Allowance Converter: Inches to mm and cm (Free)

Seam allowance converter

How the seam allowance converter works

One inch is exactly 25.4 millimeters, so converting is a single multiplication or division. The converter does it as you type, then rounds the inch result to the nearest ⅛ in, because that is the finest mark on most sewing machine plates and seam gauges.

Millimeters = inches × 25.4Centimeters = millimeters ÷ 10Inches = millimeters ÷ 25.4, shown as the nearest ⅛ in

Worked example. A ⅝ in seam allowance is ⅝ in × 25.4 = 15.875 mm, which is 15.9 mm or 1.59 cm. Going the other way, a pattern that says 1.5 cm is 15 mm ÷ 25.4 = ½ in, and the nearest ⅛ in mark is ⅝ in.

If you are working from a pattern in the other system, convert once and use the same allowance for every seam. The difference between 1.5 cm and ⅝ in is under a millimeter per seam, but mixing different allowances on one project is what makes pieces fail to match. The seam allowance guide shows how to sew an even seam, and which allowance suits which project.

Common seam allowances in inches and millimeters

Common seam allowances
Inches	Millimeters	Centimeters	Often used for
¼ in	6.4 mm	0.64 cm	Quilt piecing, small toys, ornaments, tight curves
⅜ in	9.5 mm	0.95 cm	Many modern PDF and indie patterns, baby clothes
½ in	12.7 mm	1.27 cm	Bags, cushions, many home projects
⅝ in	15.9 mm	1.59 cm	Traditional dressmaking patterns in the US
1 in	25.4 mm	2.54 cm	Hems, bag tops, extra room to adjust a fit

Metric patterns often use 1 cm or 1.5 cm instead. 1 cm is a little over ⅜ in, and 1.5 cm is a little under ⅝ in. Use the amount your pattern gives and keep it the same on every seam.

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