sewing.tools

Sewing

Seam Allowance Calculator

Convert between finished size and cut size for any seam allowance — 1/4", 3/8", 1/2", 5/8", 1", or a scant quarter — in either direction.

  • Updates as you type
  • Inches or centimetres
  • Copy or print your results

Calculator

in
in
in

Presets

1/4" quilting · 1/2" home decor · 5/8" garment

Finished → Cut

Enter your measurements above

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What size do I cut a 3" finished square?

For a 3" finished square with a 1/4" seam allowance on each edge:

You enter

Finished measurement
3" (7.6 cm)
Seam allowance
1/4" (0.6 cm)

You get

Cut measurement
3-1/2" (8.9 cm)
Conversion
3" finished + 2 × 1/4" = 3-1/2" cut

The formula: cut = finished + 2 × seam allowance; finished = cut − 2 × seam allowance.

Frequently asked questions

  1. How do I add seam allowance to a measurement?

    Add the seam allowance twice — once for each edge that gets sewn. A 3" finished square with a 1/4" seam allowance is cut at 3-1/2". To go the other way, subtract it twice: 4" cut − 2 × 5/8" = 2-3/4" finished.

  2. What seam allowance should I use?

    Quilting uses 1/4", home decor usually 1/2", and commercial garment patterns 5/8" (many indie and European patterns use 3/8" or 1 cm — check the pattern). Use 1" where you may need to let a seam out later.

  3. What is a scant quarter-inch seam?

    A seam a thread or two narrower than 1/4" — about 0.23" — so that after pressing, the fold takes up no extra and blocks finish at the right size. Enter 0.23 as the seam allowance to see the exact arithmetic: 3" finished + 2 × 0.23" = 3.46", so cut at 3-1/2".

  4. Is seam allowance included in pattern pieces?

    In most commercial patterns, yes — the cutting line already includes it, and the pattern states the allowance. Some vintage, European, and magazine patterns don't, so add it before cutting.

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