Curved Hem Facing Calculator
A curved hem is part of a circle. If that circle has a 60 cm radius and the hem arc is 200 cm, turning a 5 cm facing up puts its free edge on a 55 cm radius. Arc length scales with radius, so the free edge only needs 200 × 55 ÷ 60 = 183.3 cm. Apply a straight 200 cm strip and 16.7 cm has nowhere to go but into ripples on the inside. The strip itself is cut at 202 cm, with 1 cm at each end to join it.
Your numbers
Facing strip length
202cm
Inner edge length
183.3cm
Difference
16.7cm
Formula
Inner edge length = Hem edge length × (Radius − Facing width) ÷ Radius, Facing strip length = Hem edge length + 2 × Seam allowance
There are three fixes. Cut the facing as the same curve — a doughnut, not a straight strip — and it fits exactly. Use bias tape and you can shrink its inner edge under the iron as you go, which is the fastest route at narrow widths. Or make the facing narrower: the excess scales with the width, so the 16.7 cm at 5 cm becomes 3.3 cm at 1 cm, which is exactly why A-line skirt hems are traditionally turned under 1 cm or less. Get the radius from circumference ÷ 2π on a circle skirt, or by fitting a circle against the hem curve.
What to enter
| Input | Default | Accepted range |
|---|---|---|
| Hem edge length (cm)Length measured along the hem curve, not straight across it. | 200 | 1 and up |
| Radius (cm)Centre to edge — half the diameter. | 60 | 1 and up |
| Facing width (cm)How far the facing reaches up; the wider it is, the more a curve leaves over. | 5 | 0.5 and up |
| Seam allowance (cm)Allowance on one edge; it is added on both, so twice over. | 1 | 0 ~ 5 |
Step by step
Quick reference table
Results when only Hem edge length (cm) changes and everything else stays put.
| Hem edge length (cm) | Facing strip length (cm) | Inner edge length (cm) | Difference (cm) |
|---|---|---|---|
| 100 | 102 | 91.7 | 8.3 |
| 150 | 152 | 137.5 | 12.5 |
| 200 | 202 | 183.3 | 16.7 |
| 300 | 302 | 275 | 25 |
| 400 | 402 | 366.7 | 33.3 |
What each result means
| Result | At default values |
|---|---|
| Facing strip length (cm)Length of strip to prepare for the facing, joining allowances included. | 202 |
| Inner edge length (cm)Length the free inner edge sits at once turned up — shorter than the hem. | 183.3 |
| Difference (cm)The gap between two values — a quantity, not a rate. | 16.7 |
Common mistakes
There are three fixes. Cut the facing as the same curve — a doughnut, not a straight strip — and it fits exactly. Use bias tape and you can shrink its inner edge under the iron as you go, which is the fastest route at narrow widths. Or make the facing narrower: the excess scales with the width, so the 16.7 cm at 5 cm becomes 3.3 cm at 1 cm, which is exactly why A-line skirt hems are traditionally turned under 1 cm or less. Get the radius from circumference ÷ 2π on a circle skirt, or by fitting a circle against the hem curve.
Glossary
- Hem edge length
- Length measured along the hem curve, not straight across it.
- Radius
- Centre to edge — half the diameter.
- Facing width
- How far the facing reaches up; the wider it is, the more a curve leaves over.
- Seam allowance
- Allowance on one edge; it is added on both, so twice over.
- Facing strip length
- Length of strip to prepare for the facing, joining allowances included.
- Inner edge length
- Length the free inner edge sits at once turned up — shorter than the hem.
- Difference
- The gap between two values — a quantity, not a rate.
Frequently asked questions
Q. How is Curved Hem Facing Calculator calculated?
Inner edge length = Hem edge length × (Radius − Facing width) ÷ Radius, Facing strip length = Hem edge length + 2 × Seam allowance — A curved hem is part of a circle. If that circle has a 60 cm radius and the hem arc is 200 cm, turning a 5 cm facing up puts its free edge on a 55 cm radius. Arc length scales with radius, so the free edge only needs 200 × 55 ÷ 60 = 183.3 cm. Apply a straight 200 cm strip and 16.7 cm has nowhere to go but into ripples on the inside. The strip itself is cut at 202 cm, with 1 cm at each end to join it.
Q. Can you walk through an example?
With Hem edge length 200cm, Radius 60cm, Facing width 5cm, Seam allowance 1cm, the answer is Facing strip length 202cm.
Q. What do I need to enter?
Enter Hem edge length, Radius, Facing width, Seam allowance. The result recalculates as you type, and an empty box counts as zero.
Q. How much does the answer move if I change a number?
Changing only Hem edge length (cm) moves the answer to Hem edge length (cm) 100 → Facing strip length (cm) 102 and Hem edge length (cm) 400 → Facing strip length (cm) 402. The table below lays out five steps.
Q. How are the numbers rounded?
Money is shown to the nearest whole unit, percentages to one decimal place and everything else to two. What you see is rounded; the calculation itself carries the unrounded value forward.
Q. Anything to watch out for?
There are three fixes. Cut the facing as the same curve — a doughnut, not a straight strip — and it fits exactly. Use bias tape and you can shrink its inner edge under the iron as you go, which is the fastest route at narrow widths. Or make the facing narrower: the excess scales with the width, so the 16.7 cm at 5 cm becomes 3.3 cm at 1 cm, which is exactly why A-line skirt hems are traditionally turned under 1 cm or less. Get the radius from circumference ÷ 2π on a circle skirt, or by fitting a circle against the hem curve.
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