Seam Allowance Calculator
Turn a finished measurement into a cut size (or back again) without the mental math at the cutting mat.
📖 How it works & FAQWhat seam allowance actually does
Seam allowance is the strip of fabric between the stitching line and the raw cut edge. Every pattern assumes one, and if you cut to the finished size by mistake, your project ends up small on every side. Because a seam runs along two opposite edges of each dimension, you add the allowance twice per width and twice per height. This calculator does that doubling for you so you cut once and stitch with confidence.
Add vs. subtract
Use Finished → Cut (add) when you know the size you want the piece to be after sewing and need to know how big to cut the fabric: cut = finished + 2 × allowance. Use Cut → Finished (subtract) when you already have a cut piece and want to know how big it will be once seamed: finished = cut − 2 × allowance. The presets cover the two most common defaults — 1/4" for quilting patchwork and 5/8" for garment sewing — plus 1/2", 3/8", and a custom field for anything else.
How to use it
- Pick a direction: Finished → Cut to size fabric before cutting, or Cut → Finished to check a piece you already have.
- Type your width and height in inches.
- Choose a seam allowance preset, or select Custom… and enter your own.
- Read the result — the big number is your answer, and the line below shows exactly how much was added or removed per dimension.
FAQ
- Why multiply the allowance by two?
- Each dimension has two edges that get seamed, so a 1/4" allowance adds 1/2" total to the width and 1/2" to the height.
- What allowance should I use?
- Quilting almost always uses 1/4". Home garment patterns typically use 5/8". Always follow your specific pattern if it states one.
- Can I work in centimeters?
- The tool is unit-agnostic math — if you enter all values in the same unit (say cm), the result is in that unit too. Just ignore the inch mark.
- Is my data uploaded anywhere?
- No. Everything runs in your browser and nothing you type ever leaves your device.
Results are estimates to guide cutting — always test on scrap and follow your pattern's stated allowance.