Seam Allowance & Hem Calculator
Sewing an eighth narrow adds two inches over eight seams. And a 2in hem on a circle skirt has 12.6in of surplus, whatever size the skirt.
What the wrong seam allowance does
The error is doubled, then multiplied
Every seam has two allowances, one on each piece. So sewing an eighth of an inch narrow puts a quarter of an inch back into the garment at that seam — and then the garment has several seams. A fitted bodice has four; a tailored jacket has six.
| Pattern says 5/8″, you sew… | Per seam | 2 seams | 4 seams | 6 seams |
|---|---|---|---|---|
| 0.25″ | +0.750″ | +1.50″ | +3.00″ | +4.50″ |
| 0.375″ | +0.500″ | +1.00″ | +2.00″ | +3.00″ |
| 0.5″ | +0.250″ | +0.50″ | +1.00″ | +1.50″ |
| 0.75″ | -0.250″ | -0.50″ | -1.00″ | -1.50″ |
Sewing 3/8″ where the pattern drafted 5/8″ — a difference you would not notice on a single seam — puts two full inches on a four-seam garment. And note the direction, which people reliably get backwards: sewing narrower makes the garment bigger, because less fabric disappears into the seam.
This is usually the answer when a garment comes out a size large with every piece cut correctly. It is also why a machine's seam guide is worth setting up properly once rather than eyeballing the plate markings each time.
Why curves have to be clipped
The raw edge of a curved seam sits on a different radius from the seamline, so it is a different length. Turn the allowance over and it now has to reach around the other side. The mismatch is twice the allowance times the angle in radians — and it does not depend on the radius at all.
| Seam allowance | Over 45° | Over 90° | Over 180° |
|---|---|---|---|
| 0.25″ | 0.39″ | 0.79″ | 1.57″ |
| 0.375″ | 0.59″ | 1.18″ | 2.36″ |
| 0.5″ | 0.79″ | 1.57″ | 3.14″ |
| 0.625″ | 0.98″ | 1.96″ | 3.93″ |
A 5/8″ allowance on a neckline turning through 180° is out by nearly four inches. At 1/4″ it is one and a half. That is the entire argument for trimming curved allowances back before turning them, and for drafting narrow ones on necklines and armholes to begin with.
- Concave curves come up short, so you CLIP — cut into the allowance so the edge can spread.
- Convex curves come out long, so you NOTCH — take small wedges out so the edge can compress.
- Same formula, opposite sign. Getting the two confused is why a collar ripples or a neckline puckers.
Hems on a flared skirt
Turning up a hem on a flared skirt means fitting the raw edge, which sits on the larger radius, against fabric on a smaller one. There is always surplus, and there is more of it than anyone expects:
| Hem depth | Full circle | Half circle | A-line (30°) |
|---|---|---|---|
| 0.125″ | 0.79″ | 0.39″ | 0.07″ |
| 0.25″ | 1.57″ | 0.79″ | 0.13″ |
| 0.5″ | 3.14″ | 1.57″ | 0.26″ |
| 1″ | 6.28″ | 3.14″ | 0.52″ |
| 2″ | 12.57″ | 6.28″ | 1.05″ |
| 3″ | 18.85″ | 9.42″ | 1.57″ |
And none of those figures depend on the size of the skirt. A 2″ hem on a full circle skirt has 12.57″ of surplus to lose whether it is on a child or sweeping the floor — the radius cancels out of the arithmetic completely. That is why circle skirts get narrow rolled hems and straight skirts can have three-inch ones.
The mirror of that fact is also exact and more useful when sewing: the surplus per inch of hem is the depth divided by the radius, and this time the sweep cancels instead. A full circle skirt has four times the surplus of a quarter circle one, but four times the hem to lose it along, so it is no harder. What decides that is how far the hem is from the centre.
Hem depths that behave
| Hem | Depth | Notes |
|---|---|---|
| Narrow rolled hem | 0.125″ | The only hem that behaves on a full circle skirt. Almost no excess to lose. |
| Narrow double-turned | 0.25″ | Shirts, linings, anything curved. Turned twice so no raw edge shows. |
| Blouse or shirt hem | 0.5″ | Standard on a gently shaped hem. |
| Skirt hem | 1.5″ | Wants a straight or nearly straight edge. |
| Trouser hem | 1.5″ | A straight edge, so the depth is free. |
| Coat or curtain hem | 3″ | The weight is the point — it makes the hem hang. Only on a straight edge. |
Let a flared skirt hang for a day or two before hemming it. The bias sections drop as the fabric relaxes, and a hem levelled straight after cutting will not be level afterwards.
How to use
- Enter what the pattern drafted and what you actually sewed.
- Set your machine guide once rather than eyeballing the plate.
- Trim curved allowances back before turning them.
- Let a flared skirt hang a day or two before hemming.
Frequently asked questions
Why is my garment bigger than the pattern says?
Almost always the seam allowance. Every seam has two allowances, one on each piece, so sewing an eighth of an inch narrow puts a quarter of an inch back into the garment at that seam. A fitted bodice has four vertical seams, so that eighth becomes a full inch of circumference — with every piece cut perfectly.
Does sewing narrower make it bigger or smaller?
Bigger, which is the direction people reliably get backwards. Taking less fabric into the seam leaves more of it in the garment. Sewing 3/8 inch where the pattern drafted 5/8 adds two full inches to a four-seam garment, and the same mistake on a tailored jacket with six seams adds three.
What is the formula?
The change in the finished measurement is two times the difference in allowance, times the number of seams. The two is because each seam consumes an allowance from both pieces. It is worth knowing because it also runs backwards: if a garment is an inch too big across four seams, sewing an eighth wider fixes it exactly.
Why do curved seams have to be clipped?
Because the raw edge sits on a different radius from the seamline, so it is a different length. Fold the allowance over and it has to reach around the other side. The mismatch is twice the allowance times the angle in radians — a 5/8 inch allowance on a neckline turning through 180 degrees is out by nearly four inches, which is why it will not lie flat.
What is the difference between clipping and notching?
Clipping cuts into the allowance so a concave edge can spread; notching removes small wedges so a convex edge can compress. Concave curves like necklines come up short and are clipped. Convex curves like collar edges come out long and are notched. Same arithmetic, opposite sign, and confusing the two is why a collar ripples or a neckline puckers.
Does the size of the curve matter?
Not for how much the allowance is out by — that depends only on the allowance and the angle, and is identical at every radius. The radius decides how long the seam is, and therefore how many cuts it takes to release. A tight curve is harder because there is less seam to distribute the same mismatch along.
Should I use narrower allowances on curves?
Yes, and many patterns already do. A quarter-inch allowance on a 180 degree curve is out by one and a half inches against nearly four for a 5/8 inch one. If your pattern drafts wide allowances everywhere, trimming the curved ones back before turning is the standard fix.
Why do circle skirts have such narrow hems?
Because turning up a hem means fitting the raw edge, which sits on the larger radius, against fabric on a smaller one. The surplus is the hem depth times the sweep in radians, so a 2 inch hem on a full circle skirt leaves 12.6 inches of fabric to dispose of. A narrow rolled hem leaves almost none, which is why it is the standard answer.
Does the skirt size change the hem surplus?
No, and this surprises people. The radius cancels out of the arithmetic completely — a 2 inch hem on a full circle skirt has 12.57 inches of surplus whether the skirt is on a child or sweeping the floor. What the radius does change is how much hem there is to spread that surplus along, which is what actually decides whether it eases flat.
How deep a hem can I get away with?
Roughly the hem radius divided by twelve, as a rule of thumb — about two inches on a 24 inch radius. Past that the surplus stops easing and starts rippling. Interestingly the sweep does not matter here: a full circle skirt has four times the surplus of a quarter circle, but four times the hem to lose it along.
Why let a skirt hang before hemming?
Because the sections cut on the bias drop as the fabric relaxes, and they do it over a day or two rather than immediately. A hem levelled straight after cutting will not be level afterwards. Hang the skirt, then level it from the floor up rather than measuring down from the waist.
Does this send anything anywhere?
No. Everything is calculated in your browser and nothing is stored or uploaded.
🔒 This tool runs entirely in your browser. Nothing you enter is uploaded, logged, or stored.