How to use the quilt block size calculator
- Choose whether you know the finished size or the cut size.
- Enter the block size in inches.
- Open More options to change the seam allowance, or to split the block into a grid of patches.
- Read the size to cut or the finished size, with the nearest 1/16 inch fraction for your ruler.
Formula
cut size (unfinished) = finished size + 2 x seam allowance
finished size = cut size - 2 x seam allowance
patch finished size = block finished size / patches across
The seam allowance is added on both sides of a piece, which is why 1/4 inch seams add 1/2 inch in total.
A 6 inch block
A 6 inch finished block with 1/4 inch seams is cut at 6 + 2 x 0.25 = 6.5 inches. In the other direction, a block cut at 12.5 inches finishes at 12 inches.
For a nine patch with a 9 inch finished block, each patch finishes at 9 / 3 = 3 inches and is cut at 3.5 inches. An 8 inch block split three ways gives patches that finish at 2.6667 inches, so each is cut at 3.1667 inches, which is close to 3 3/16.
Finished versus unfinished
Patterns and rulers use two kinds of numbers. The finished size is what the block measures once it is sewn into the quilt. The unfinished size is what you cut and what the block measures before it is joined. Mixing them up is the most common reason a quilt top ends up too small.
If your seams are wider or narrower than the allowance, the real finished size shifts a little for every seam. Sew one test block and measure it before cutting the whole project.
Related quilting tools
Use the result to plan a layout with the Quilt Sashing and Border Calculator. For triangle units, try the Half Square Triangle Calculator, and for the back and edge, the Quilt Backing and Binding Calculator. The Fat Quarter Cutting Calculator shows how many patches fit in a fat quarter.
Assumptions and limits
- Every cut piece has the same seam allowance on all four sides.
- The seam allowance starts at 1/4 inch, which is an example you can change.
- Sizes are in inches. For metric work, enter the same numbers in your unit and read the answer in that unit.
- Fractions are rounded to the nearest 1/16 inch for reading on a ruler.
Frequently asked questions
What is the difference between finished and unfinished quilt block size?
The finished size is the block as it appears in the quilt after sewing. The unfinished size is the cut size, which adds the seam allowance on every side, so a 6 inch finished block is 6.5 inches unfinished.
How do I convert a finished block size to a cut size?
Add two seam allowances to the finished size. With 1/4 inch seams, add 1/2 inch. A 12 inch finished block is cut at 12.5 inches.
How big are the patches in a nine patch block?
Divide the finished block size by 3 and add two seam allowances to get the cut size. A 9 inch block gives 3 inch finished patches that are cut at 3.5 inches.
Can I use a different seam allowance?
Yes. Open More options and enter yours, for example 0.375 for a 3/8 inch seam. The cut sizes update right away.