Crotch depth and crotch length are two different measurements that a trouser pattern needs both of. Crotch depth is vertical: the distance from your waist to the chair seat when you sit, and it sets the rise, how far below the waist the crotch of the trousers hangs. Crotch length is a curve: the distance from the front waist, down between the legs and up to the back waist, and it sets the length of the crotch seam, how far forward and back the curve reaches. A trouser with the right depth and the wrong length pulls or droops at the crotch seam; one with the right length and the wrong depth hangs too low or drags down the waistband.
| Crotch depth | Crotch length | |
|---|---|---|
| Also called | Rise, body rise, seat depth | Total crotch, crotch seam length |
| Direction | Vertical, waist to seat | Curved, front waist to back waist through the legs |
| How to take it | Seated on a hard chair, tape down the side from waist to chair | Standing, tape between the legs from waist elastic to waist elastic |
| What it sets | The height of the crotch line below the waist | The length and shape of the crotch curve |
| Ease | 1.5–2.5 cm (5/8–1 in) fitted; 2.5–4 cm (1–1 1/2 in) pull-on | 2.5–5 cm (1–2 in) in total |
| Fixes | Trousers that bind when you sit, or a crotch that hangs low | Pulling, smile lines and wedgies, or droop and vertical folds |
How do you measure crotch depth?
Sit on a hard, flat chair with your feet on the floor and your back straight. Tie elastic around your natural waist. Hold a ruler flat on the chair seat beside your hip and measure vertically from the waist elastic to the ruler. Keep the tape straight down, not following the curve of the hip.
A typical result is 25–30 cm (10–12 in). Add the ease your style needs: 1.5–2.5 cm (5/8–1 in) for a fitted trouser, 2.5–4 cm (1–1 1/2 in) for a pull-on trouser or jogger, where the crotch is meant to sit a little lower and the elastic waist is held up by the body rather than a fitted band. The sum is the rise the pattern should have, measured on the side seam from the waist stitching line to the crotch line. The glossary entry on rise covers the difference between the front and the back rise, which are not the same length because the back curve is longer.
How do you measure crotch length?
Stand with the waist elastic still in place. Hold one end of the tape at the center front waist, pass it down between the legs, not pulled tight, and up to the center back waist. Read the total; typical values are 60–80 cm (23 1/2–31 1/2 in). The tape should sit where the seam will: against the body but without pressing.
Measure the front and the back separately as well. Mark the point directly below the crotch, where the inseams would meet, with a string tied around the tape or a pencil mark, and read the front and back portions. The back is usually longer, about 55–60 percent of the total, because the seat curves farther out than the stomach. That split is what decides whether the extra length belongs on the front or the back pattern piece. How to measure yourself for trousers goes through the whole set, with a tip for doing it alone.
On the pattern, measure the crotch curve between the stitching lines with the tape on its edge, from the front waist seam around to the back waist seam, and add the front and back together. The pattern should exceed your measurement by 2.5–5 cm (1–2 in), depending on how close a fit you want and how much the fabric stretches.
What goes wrong when the crotch depth is wrong?
- Depth too short: the trousers are fine standing but pull down when you sit, the waistband drops below the waist, and the crotch cuts in. On a pull-on trouser the elastic slides down the hip.
- Depth too long: the crotch hangs between the legs, the thighs chafe, and the trousers look like they belong to someone taller. Vertical folds may appear at the top of the inner thigh.
Both are fixed on the lengthen/shorten line that crosses the rise, between the crotch line and the hip, by spreading or tucking the pattern the full amount. Lengthening or shortening trousers shows the steps and how the knee line comes into it.
What goes wrong when the crotch length is wrong?
- Length too short: horizontal pulls at the front, "smile lines" radiating from the crotch toward the hips, and the back seam riding up between the seat. The waistband dips at the back when you bend. If only the back is short, you feel it when you sit.
- Length too long: a droopy crotch with loose fabric under the seat, folds that point from the crotch down toward the inner thigh, and a seam that twists.
A length change does not touch the rise. It is made at the crotch point, where the inseam meets the crotch curve: extend the point outward along the crotch line to lengthen, or trim it to shorten, then blend the inseam into the new point over 15–20 cm (6–8 in). Put the change on the piece that needs it: for a full seat, most of it goes on the back point, 1–2.5 cm (3/8–1 in); for a full stomach, some goes on the front. The second way to add back crotch length is to raise the center back waist by 1–2 cm (3/8–3/4 in), tapering to nothing at the side seam, which lengthens the seam from the top instead of the bottom and helps when the waistband dips at the back.
Warning: Adding length by scooping the crotch curve deeper also makes the trousers tighter across the hip at that point, because the fabric that is scooped away was wrapped around the body. Scoop 6 mm (1/4 in) at a time, and only after the crotch point has been extended.
How do the two work together?
The crotch depth decides where the crotch line sits; the crotch length decides how much seam there is to get from the front waist to the back waist via that line. A trouser with a long rise and a short crotch seam sits low but still pulls between the legs; one with a short rise and a long seam cuts in when you sit and bags under the seat. That is why trousers you have tried on in a shop can be wrong in two opposite ways at once, and why fitting them from a single "rise" number never quite works.
The sequence that works on a toile: set the depth first, because it moves the crotch line and everything below it; then fit the length at the crotch points; and only then look at the shape of the curve, a deeper J for a flatter seat, a shallower one for a fuller seat. Fitting trousers: the five most common problems walks through what each pull means once the two lengths are right.
Does a made-to-measure pattern take both?
A standard size gives one rise and one crotch length per hip size, which assumes that everyone with a 100 cm (39 1/2 in) hip has the same seat and the same height of waist. A pattern drafted from your own measurements uses your numbers instead. In Sewly the measuring assistant explains where to put the tape for each measurement, and the trousers are drawn around them, so the depth and length are set before the pattern is printed. The shape of the curve between the two, how much it scoops, is still the one thing to check on a toile, because two lengths describe the distance but not the road.