Trousers have three separate lengths, and each is changed in its own place: the rise, above the crotch line, sets how far the crotch hangs below your waist; the thigh, between the crotch and the knee, sets where the knee of the trousers lands; and the lower leg, between the knee and the hem, sets the finished length. Measure your crotch depth, your crotch-to-knee and your inseam, compare each with the pattern between its stitching lines, and cut and spread or tuck on the printed line for that section. Adding everything at the hem is only right when the rise and the knee already sit where they should.
Which measurement goes with which part of the trousers?
Take three measurements, and read the trouser measuring post for the details of each:
| Your measurement | How to take it | What it sets on the pattern |
|---|---|---|
| Crotch depth | Sit on a hard chair; measure from the waist down the side to the seat | The rise, from waist to crotch line |
| Crotch to knee | From the crotch, down the inside of the leg to the middle of the kneecap | The thigh section, where the knee line sits |
| Inseam | From the crotch down the inside of the leg to where you want the hem | The whole leg, crotch line to hem |
On the pattern, measure the same lengths along the side seam or inseam between the stitching lines: from the waist seam to the crotch line for the rise, from the crotch line to the knee line (usually marked, or the narrowest point of a tapered leg), and from the crotch line to the hem fold. Leave out the hem allowance and, on a pull-on style, the waistband casing.
A fitted trouser carries 1.5–2.5 cm (5/8–1 in) of ease in the rise over your seated crotch depth, so you can sit without the waistband dropping. A pull-on trouser with an elastic waist carries 2.5–4 cm (1–1 1/2 in). The inseam needs no ease, only the length you want plus the hem.
Where do you cut a trouser pattern to change its length?
Most trouser patterns print two lengthen/shorten lines: one across the rise, between the crotch line and the hip, and one across the lower leg, between the knee and the hem. Some add a third through the thigh. If a line is missing, draw it square to the grainline:
- Rise line: about halfway between the crotch line and the waist, below the hip level and above the crotch curve, where it changes nothing in the shape of the curve.
- Thigh line: halfway between the crotch line and the knee.
- Lower leg line: halfway between the knee and the hem.
Mark the same lines at the same heights on the front and back, on pocket pieces that extend that far, and on any lining or facing.
How do you decide how much to add at each line?
Work from the top down, because each change moves the lines below it.
- Rise. Your crotch depth plus ease, minus the pattern rise. If you measure 27 cm (10 5/8 in) seated and want 2 cm (3/4 in) of ease, you need a 29 cm (11 3/8 in) rise. A pattern with 26.5 cm (10 3/8 in) needs 2.5 cm (1 in) more.
- Thigh. Your crotch-to-knee minus the pattern's crotch line to knee line. This keeps the knee of the trousers at your knee, which matters on a tapered or jogger leg where the narrowest point is meant to sit there.
- Lower leg. Your inseam minus the pattern inseam, minus whatever you added at the thigh. If you need 4 cm (1 5/8 in) more inseam and added 1.5 cm (5/8 in) at the thigh, the remaining 2.5 cm (1 in) goes below the knee.
On a straight or wide leg, where no knee shaping exists, you can skip the thigh line and put the whole inseam change below the knee. On a wide leg the hem is parallel to the grain and the change can even go at the hem.
How do you lengthen trousers on the pattern?
- Cut along the line, after drawing a vertical guideline across it parallel to the grainline.
- Tape paper behind the upper piece and slide the lower piece down by the amount, keeping the vertical guideline straight.
- True the side seam and inseam. Both seams jog at the cut, because the leg is narrowing or widening. Redraw each from a point 10 cm (4 in) above the cut to a point 10 cm below it. On a tapered leg, the new line is a touch straighter than before; that is correct.
- Repeat on the back with the same amount, and on any piece that crosses the line.
- Walk the seams. Lay front on back, inseam to inseam, and check the two are the same length from crotch to hem. Then do the same along the side seam. A difference of up to 6 mm (1/4 in) can be eased on the back thigh; more means a piece was spread unevenly.
Spreading at the rise line adds length to the crotch seam and to the side seam equally, so the crotch curve keeps its shape and the pockets, fly or elastic casing move down with the waist.
How do you shorten trousers on the pattern?
Fold a tuck. Draw the line and a second line above it at the amount to remove; fold on the lower line and bring the fold up to the upper line, which takes out the full amount. Tape and true the seams as before.
If you shorten the rise, check that the crotch curve still clears your body: a rise that is too short makes the trousers pull down at the back and bind at the front. Crotch depth vs crotch length explains the two numbers that fit the crotch and why shortening the rise is not the same as shortening the crotch seam.
Note: Shortening at the hem alone moves the knee line up your leg. On a tapered leg the narrowest point then sits above your knee and the trousers pull every time you bend it. On a wide leg nobody will notice.
What else changes with the length?
- Hem allowance. Check the pattern's hem depth, often 3–4 cm (1 1/4–1 5/8 in). Keep it after the change.
- Pockets. An in-seam pocket bag that crosses the rise line gets the same spread. A slanted front pocket sits above the line and does not change.
- Fly or zip length and the waistband width do not change with the rise, but the position of the fly's bottom bar tack moves down with the crotch.
- Elastic length does not change; the casing moves with the waist.
Cut a toile in a cheap fabric with the same stretch as your final fabric, sit, squat and climb a step in it. Fitting trousers: the five most common problems tells you what to look for once the lengths are right.
Do made-to-measure trousers still need length adjustments?
A standard size assumes an average height and an average split between rise, thigh and leg. A trouser drafted from your own measurements takes those lengths from you. Sewly draws its pull-on trousers, wide, straight, tapered or jogger, from your measurements, so the rise and the inseam come out of your numbers rather than off a size chart, and changing a measurement redraws the pattern instantly. A toile still earns its place for the shape of the crotch curve, which two lengths cannot fully describe.