Skip to content
Randozi
All wheelsTools

Seam Allowance Calculator

A finished 12 × 18 in panel is not cut at 12 × 18 in. Enter the finished size and a margin for each side and get the exact rectangle to cut, drawn to scale so you can check it at a glance.

Cut size

13 × 19 in

Allowances add 1 in across and 1 in down.

How it works

  1. Pick the unit and enter the finished width and height.
  2. Enter the allowance for the left, right, top and bottom edges (the same value is fine for all four).
  3. Read the size to cut and look at the diagram.
  4. Copy the summary into your pattern notes or your message.

What people use it for

  • Cutting a cushion cover panel

    Turn the finished face size into the rectangle to cut, with a wider margin on the edge that carries the zip.

  • Patchwork pieces

    Check that an 8 × 8 in finished square needs an 8.5 × 8.5 in cut with a quarter-inch margin.

  • Fabric, leather or paper

    The same sum works for any flat material: wall hangings, bags, bookbinding covers.

Questions

What is the formula?
Cut width = finished width + left allowance + right allowance, and cut height = finished height + top allowance + bottom allowance. Nothing else is added.
Does it include hems or shrinkage?
No. A hem, a turn-under, a zip facing or the shrinkage of washed fabric are not guessed: add them to the matching allowance yourself, for example 1.5 in on the bottom edge for a hem.
Can I use different units?
Pick one unit for the whole form. Use the length converter first if your finished size is in centimetres and your ruler is in inches.
Is anything sent anywhere?
No. The calculation runs in your browser and nothing is stored.