How it works
Hat circumference = head × (1 − ease %)
Cast on = hat circumference × stitches per inch, to the nearest multiple of the rib
Crown depth = hat circumference ÷ (2 × 3.14)
A hat that fits is smaller than the head it goes on, so it stretches and stays put. That difference is negative ease. Ribbed and stretchy hats use about 10 to 15%; firm fabric such as colorwork or dense crochet needs less, around 5%. The hat circumference is the head measurement minus the ease.
Multiply that circumference by your stitches per inch (your gauge over 4" divided by 4) to get the stitch count. You cannot cast on a fraction, and ribbing needs a whole number of repeats, so the calculator rounds to the nearest multiple of 2 for 1x1 rib or 4 for 2x2 rib. It then shows the real hat size and ease for that count.
For height, we use typical total heights for each size. The crown, where the decreases go, is roughly a flat circle, so its depth is about the hat circumference divided by 2 x 3.14. Work the body until the hat measures the total height minus the crown depth, then start decreasing.
For crochet worked bottom up, chain the same number and join. Worked top down, increase until your flat circle is the hat circumference divided by 3.14 across, then work even.
Head sizes follow the Craft Yarn Council chart. Measure if you can.
Worked example
An adult medium head is 22". With 10% negative ease the hat should be 19 3/4" around. At 20 stitches per 4" (5 per inch) that is 99 stitches, and the nearest multiple of 4 for 2x2 rib is 100. The hat will measure 20" around, 9.1% smaller than the head. Knit to 5 3/8" and then work the crown decreases over about 3 1/8" for a total height of 8 1/2". In crochet, a top-down circle would be 6 3/8" across.
Common mistakes
- Measuring loosely. Wrap the tape around the widest part of the head, across the forehead and just above the ears, snug but not tight.
- Using the ball band gauge. Measure your own swatch in the stitch you will use, after washing it, because rib and stockinette give different gauges.
- Adding positive ease to a beanie. A hat the same size as the head slides down; it needs to be smaller to grip.
- Rounding to a multiple that does not match the rib. 2x2 rib in the round needs a multiple of 4, or the pattern will not line up at the end of the round.
Last checked October 2026 against standard quilting and knitting math. Found a mistake? Tell us and we’ll fix it.