How it works
Squares per fat quarter = ⌊18 ÷ size⌋ × ⌊21 ÷ size⌋
Fat quarters to buy = round up( pieces needed ÷ pieces per fat quarter )
1 yard = 4 fat quarters = 8 fat eighths (by area)
A US fat quarter is a quarter yard cut "fat": half a yard long and half the width of the fabric, so about 18" x 22". After you trim the selvage and straighten the edge, 18" x 21" is a safer working size, and that is the default here. A fat eighth is half of that, about 9" x 21". UK shops cut fat quarters in metric, usually 50 x 55 cm.
For squares, the count is simple: how many fit across the 18" side times how many fit along the 21" side. Rectangles are more interesting, because they can be turned. The calculator tries every way of cutting some rows one way round and filling the leftover strip with pieces turned the other way, and keeps the layout that gives the most pieces. It never counts a piece that would have to be pieced together.
Fat quarters needed is your total divided by the pieces per fat quarter, rounded up. One yard is four fat quarters by area, so the converter multiplies yards by four.
Worked example
You need 42 squares cut at 5". Three fit across 18" and four fit along 21", so one fat quarter gives 12. 42 / 12 rounds up to 4 fat quarters, with 6 squares spare. For 64 rectangles at 2 1/2" x 4 1/2", a plain grid gives 28 or 32, but mixing directions gives 33 per fat quarter, so 2 fat quarters do it.
Common mistakes
- Planning on the full 22". Selvages and a crooked cut can take an inch, so plan on 21" unless you have measured your fat quarters.
- Treating a fat quarter like a long quarter yard. A long quarter is 9" x 42"; it gives different counts and is the only one that works for long strips.
- Forgetting that square sizes on a pattern are cut sizes. Use the size you will cut, including seam allowance, not the finished size.
- Mixing US and UK fat quarters in one plan. A UK 50 x 55 cm fat quarter is a little bigger than a US one.
Last checked October 2026 against standard quilting and knitting math. Found a mistake? Tell us and we’ll fix it.