Free tool, no account
Seam Allowance Calculator: Cut and Finished Sizes
Work from a finished rectangle to its cut size, or start with a cut piece and find the finished dimensions. Give each edge its own allowance, choose your units and count identical pieces. The outline and cutting list keep the seam line separate from the cutting edge.
The pattern margin desk
Results stay on this device. Starting values are fictional examples.
Worked example
| Quantity | Value | Unit |
|---|---|---|
| cut_length | 34.5 | cm |
| cut_width | 22 | cm |
| finished_length | 30 | cm |
| finished_width | 20 | cm |
| piece_count | 2 | pieces |
| total_cut_area | 0.1518 | m2 |
How to use it
Replace the example with your values and check units. Results update as you edit. Keep a scenario to compare, then export a file; this tool’s JSON can restore your inputs.
Worked example
A finished rectangle of 30 × 20 cm with 1 cm at the top, 3 cm at the bottom and 2 cm at each side needs a 34 × 24 cm cut piece. Two identical pieces use 0.1632 m² before layout waste.
Method and sources
Cut length adds the top and bottom allowances to finished length. Cut width adds left and right allowances. Reverse mode subtracts those edges instead. Total cut area multiplies cut length, cut width and the number of pieces. The diagram is proportional; it is not a printed pattern at actual scale. Inch conversion uses exactly 2.54 cm.
Sources checked October 28, 2026.
Starting values are a fictional example. The method and limits below describe what is actually calculated.
Who it is for, and limits
For rectangular pattern pieces, pockets, panels and test swatches. Curved seams, darts, fold edges, stretch, fabric shrinkage and grain placement need their own pattern work. Enter zero for an edge cut on a fold if your pattern calls for it. If allowances leave a nonpositive finished side, the result is refused.
Common questions
Can each edge use a different allowance?
Each edge has its own field. A hem can be deeper than a joining seam, and a fold edge can use zero when the pattern specifies it.
Can I enter fractions of an inch?
Simple fractions such as 3/8 are accepted. Select inches first; every field uses the selected length unit.
Does the area equal fabric to buy?
Area does not account for usable bolt width, grain or nesting. Use the cutting list to plan a layout before buying fabric.
Where do my inputs go?
They stay on this page. An explicit export keeps data in your chosen file; nothing is saved automatically.
Can I compare and restore a scenario?
Compare keeps up to three scenarios during this session. The JSON download lets you restore a scenario after closing.