Circle Skirt Calculator
A half circle skirt needs TWICE the radius of a full circle, and cut in one piece it needs more fabric width, not less.
Every fullness, same waist
A half circle needs a bigger radius, not a smaller one
This catches almost everyone, because "half circle" sounds like half of everything. It is the other way round: a half circle skirt needs a radius twice as large as a full circle skirt, and a quarter circle needs four times.
The waist edge is an arc of the circle you cut, and it has to measure your waist. On a full circle that arc is the whole circumference, 2πr. On a half circle it is a semicircle, πr. Same waist, half the arc — so twice the radius. Less fullness means the same waist measurement has to be spread around a shorter arc, and a shorter arc of a given length comes from a bigger circle.
The fabric consequence is backwards too. Cut in one piece with a 28″ waist and a 24″ length, a full circle spans 56.9″ across and a half circle spans 65.8″. On a 60″ bolt the full circle goes through and the half circle does not. The skirt with less fabric in it is the one that will not fit the fabric.
The seam allowance goes inward
At the waist, the allowance sits outside the cut line all the way round, so the hole you cut has to be smaller than the finished waist — this page subtracts the allowance from the radius rather than adding it. Adding it is the classic error, and it gives a waist that will not close by roughly twice the allowance. The hem allowance works the normal way and is added to the length.
Cutting it out
- Fold and mark from the corner. Fold the fabric into quarters, put the compass point at the folded corner, and swing two arcs — one at the waist radius and one at radius plus length.
- Measure the arc before you cut the hem. A tape run round the marked waist circle should read your waist less the allowance. Two minutes there saves the whole piece.
- Hang it before hemming. The bias sections stretch under their own weight and will drop for a day or two. Hemming a fresh circle skirt gives a wavy hem.
- A narrow hem is easier. The hem edge is much longer than the fabric above it, so a deep hem has to be eased in. A quarter inch rolled or a bias facing avoids the fight entirely.
How to use
- Enter your waist and the finished length.
- Pick the fullness — the radius changes more than you expect.
- Check the width against your fabric before cutting.
- Measure the marked waist arc with a tape before you cut the hem.
Frequently asked questions
Why does a half circle skirt need a bigger radius?
Because the waist edge is an arc of the circle you cut, and it has to measure your waist. On a full circle that arc is the whole circumference, two pi r. On a half circle it is a semicircle, pi r. Same waist, half the arc, so twice the radius. Less fullness means the same measurement is spread around a shorter arc, and a shorter arc of a given length has to come from a bigger circle.
How much bigger exactly?
The radius is the reciprocal of the fraction. A three-quarter circle is 1.33 times the full-circle radius, a half circle is exactly 2 times, and a quarter circle is exactly 4 times. It is not an approximation or a rule of thumb — it falls straight out of the arc length.
Does a half circle skirt use less fabric?
Less area, but not necessarily less width, and width is what usually decides. Cut in one piece with a 28 inch waist and a 24 inch length, a full circle spans about 57 inches across and a half circle about 66. On a 60 inch bolt the full circle fits and the half circle does not, which is the opposite of what everyone expects.
Should the seam allowance be added or subtracted?
Subtracted, at the waist. The allowance sits outside the cut line all the way round, so the hole you cut must be smaller than the finished waist. Adding it is the classic mistake and leaves a waist that will not close by roughly twice the allowance. The hem allowance behaves normally and is added to the length.
How do I mark the circle without a huge compass?
Tie a length of string to a pencil, hold the free end at the folded corner with a pin, and swing an arc. Fold the fabric into quarters first so you only mark a quarter of the circle. A tape measure held at the corner works just as well if you mark points every few inches and join them.
Why does my hem come out wavy?
Because a circle skirt hangs partly on the bias, and bias stretches under its own weight. Hang the finished skirt for a day or two before hemming, then re-level it — anything hemmed straight off the cutting table will drop unevenly within a week and there is no fixing it afterwards.
Why is the hem so hard to sew?
Because the hem edge is much longer than the fabric just above it, so a deep hem has to be eased in around a curve that does not want to fold. A quarter inch rolled hem or a bias facing avoids the problem entirely, which is why nearly every commercially made circle skirt uses one or the other.
Can I cut a full circle from one piece?
Only if the fabric is wide enough for twice the radius plus twice the length, which for an adult skirt usually means 108 inch wide fabric. Otherwise cut it in halves or quarters with seams at the sides, which is what this page suggests when the piece will not fit. The seams are invisible in the drape.
What fullness should I choose?
A full circle gives the most swing and the smallest radius; a quarter circle is a gentle A-line. Three-quarter is a good compromise if a full circle will not fit your fabric — it keeps most of the movement and, being a smaller radius than a half circle, is often easier to lay out than the more obvious choice.
Does the waist measurement include ease?
This page uses whatever you type, so add ease yourself if you want it. A skirt sitting at the natural waist usually needs half an inch to an inch of ease to be comfortable; one sitting on the hip is measured at the hip instead. A zip does not change the measurement, only how you get into it.
Does this work in centimetres?
Yes. The arithmetic is a ratio, so any consistent unit works — put centimetres in every box and read centimetres out. The fabric widths offered are in inches because bolts are still sold that way in most places, so convert those if you are working metric.
Is anything sent anywhere?
No. Everything is calculated in your browser, nothing is uploaded, and nothing is kept between visits.
🔒 This tool runs entirely in your browser. Nothing you enter is uploaded, logged, or stored.