Seam Allowance Calculator

Work out seam allowance instantly with clear inputs, formula shown and shareable results.

About Seam Allowance Calculator

Each dimension gains twice the seam allowance because there is a seam on both edges, so the cut size is the finished size plus two allowances. On small pieces the seam allowance dominates: a 5 cm finished square at 1.5 cm allowance needs an 8 cm cut, using more than twice the fabric area.

Formula

Cut size

cut dimension = finished dimension + 2 x seam allowance

Worked example

With the default values below, Seam Allowance Calculator returns Cut width of 53.00. Change any input above and every figure here is recalculated in your browser.

Inputs used

Finished piece widthminimum 0.1 cm: 50 cm
Finished piece heightminimum 0.1 cm: 60 cm
Seam allowance per edgeminimum 0 cm: 1.5 cm

Results

Cut width: 53.00 cm
Cut height: 63.00 cm
Extra fabric area for the seams: 11.30%
Length of seam to sew: 232.00 cm

Step by step

1Values used
2Cut size
3Cut width= 53.00 cm
4Cut height= 63.00 cm
5Extra fabric area for the seams= 11.30
6Length of seam to sew= 232.00 cm

Assumptions and limits

The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

How is the Seam Allowance figure worked out?

cut dimension = finished dimension + 2 x seam allowance. Each dimension gains twice the seam allowance because there is a seam on both edges, so the cut size is the finished size plus two allowances.

Why does this matter?

On small pieces the seam allowance dominates: a 5 cm finished square at 1.5 cm allowance needs an 8 cm cut, using more than twice the fabric area.

What do I need to enter?

Enter finished piece width, finished piece height, seam allowance per edge. The defaults shown are a realistic worked example — swap in your own figures to get a result you can use.