The five trouser fit problems that come up most often are drag lines pointing at the front crotch, loose folds under the seat, a waistband that gaps at the back, horizontal pulls across the thighs, and legs that twist or wrinkle behind the knee. Each one shows as a distinct pattern of wrinkles, and each is fixed by a specific change to the crotch curve, the rise, the leg width or the grain. Fit in the order waist, crotch, hip and thigh, then leg, because an early fix often removes a wrinkle lower down.
How do you read the wrinkles on a pair of trousers?
Wrinkles are the fabric telling you where it is short or long. Drag lines that point toward a spot mean the fabric is being pulled toward that spot: it needs more length or width there. Soft horizontal folds mean there is too much length above them. Diagonal lines run between two points that are too far apart for the fabric in between. The post on how to read drag lines goes through the general method; here it is applied to trousers.
Put the toile on with the shoes you will wear, stand naturally, and look in a long mirror from the front, the side and the back. Then sit down: a seam that is fine standing can show its problem the moment you bend.
| What you see | Most likely cause | Fix |
|---|---|---|
| Lines fanning out from the front crotch ("smile lines") | Front crotch too short or too tight | Lengthen the front crotch extension or the front rise |
| Horizontal folds under the seat | Back crotch extension too long, or too much back rise | Shorten the back extension, or fold a wedge out of the back |
| Waistband stands away at center back | Hollow back: the back rise is too long at center back | Fold a wedge out of the back above the hip |
| Horizontal pulls across the thigh, pockets gaping | Thigh too narrow | Add width at the inseam, front and back |
| Leg twists around the ankle, wrinkles behind the knee | Off grain, or crotch-to-knee length wrong | Re-cut on grain; shorten or lengthen above the knee |
Problem 1: Why do my trousers pull at the front crotch?
Lines that fan out from the front crotch seam toward the hips, often with the fabric wedging uncomfortably when you sit, mean the front crotch is too short. Two measurements are involved. The crotch depth is the vertical distance from the waist to the chair when you sit; the crotch length is the curve from the front waist, between the legs, to the back waist. The post on crotch depth versus crotch length explains how to take both and what each one controls.
If the lines point toward the inseam, the front crotch extension, the hook that juts out from the inseam toward the back, is too short. Add 1 to 2 cm (3/8 to 3/4 in) at the tip of the front extension and redraw the inseam so the addition tapers to nothing about 15 cm (6 in) down the leg. If the whole crotch sits too high and the waistband is pulled down, the rise, the distance from crotch to waist, is too short: cut the front piece across between the crotch and the hip line, spread it 1 to 1.5 cm (3/8 to 5/8 in), and true the side seam and the center front.
Problem 2: Why is there loose fabric under my seat?
Horizontal folds that collect under the seat and above the thighs mean the back has too much length between the seat and the crotch. On a figure with a flat seat, the back crotch extension is too long: shorten it by 1 to 2 cm (3/8 to 3/4 in) at the tip, tapering down the inseam as before.
If the folds run across the whole back and the center back seam looks slack, fold a horizontal wedge out of the back piece instead: 1 cm (3/8 in) at the center back, tapering to nothing at the side seam, placed between the hip line and the crotch. This shortens the back crotch length without changing the hip width. Diagonal bagginess that collects on the inner thigh is different: take the back inseam in by 1 cm at the crotch, blending to the knee.
Problem 3: Why does the waistband gap at the back?
When the band stands away from the body at center back while the side seams fit, the back of the trousers is longer than your back. A hollow, curved lower back is the usual reason; it is called a swayback. Pinch the excess into a horizontal fold below the waistband on the toile and measure its depth, usually between 1 and 2.5 cm (3/8 and 1 in). Fold that amount out of the back pattern piece as a wedge from the center back to nothing at the side seam, above the hip line, and redraw the center back seam as a smooth line.
With an elastic waistband the gap disappears because the elastic draws the fabric in, but the surplus length is still there and shows as fabric pooling above the seat. The wedge is worth taking out on pull-on trousers too.
Problem 4: Why do my trousers pull across the thighs?
Horizontal strain lines across the front of the thigh, pocket openings that gape and a seat that feels tight when you sit are signs of a thigh that needs more room. Measure around the fullest part of your thigh, close to the crotch, and compare it with the pattern at the same level: add the front and back widths at the thigh line and subtract the seam allowances. Fitted trousers need about 5 cm (2 in) more than the body there; wide-leg trousers have plenty already.
Split the shortfall between the front and back inseams at the thigh level, tapering to the original line at the knee. Adding at the inseam rather than the side seam keeps the side seam straight on the body. If you need more than 3 cm (1 1/4 in) in total, lengthen the crotch extensions slightly as well, since a fuller thigh usually needs a longer crotch curve too.
Problem 5: Why do the legs twist or wrinkle at the knee?
A leg that spirals around the ankle, so the side seam drifts to the front or the back, was cut off grain, or has an inseam and an outseam of different lengths that were forced to match. Check the grainline on the pattern: it should run from the middle of the crotch level to the middle of the hem, and the fabric must be folded on grain before cutting. Wrinkles that gather behind the knee are a length problem: the piece is too long between crotch and knee, so the knee of the trousers sits below yours. Shorten at the lengthen-and-shorten line above the knee, usually by 1 to 2 cm, and check the hem length afterward.
Knock knees and bowed legs also produce diagonal lines at the knee. The fix is to shift the lower leg inward or outward by the amount of the drag, pivoting at the knee line, which keeps the hem level.
In what order should you fit trousers?
Start with the waist and the rise, because a crotch that sits at the wrong height changes every wrinkle below it. Then the crotch curve, then hip and thigh width, then leg length and width. Change one thing at a time, transfer it to the pattern and, if the change was large, cut a second toile. The guide on making a toile to test the fit explains how to mark the changes so they transfer accurately.
What changes when the pattern is drafted from your measurements?
A pattern drafted from your own waist, hip, crotch depth and inseam has the rise, the hip width and the length set before you cut, which removes the most common problems upstream. Sewly's pull-on trousers pattern works that way, with a wide, straight, tapered or jogger leg; its measuring assistant explains how to take the crotch depth sitting on a hard chair, which is the number most people get wrong. The post on how to measure for trousers covers the same measurements for any pattern.
What a draft cannot know from measurements alone is the shape of the body between them: a flat or full seat, a hollow back, a thigh that is full at the top but slim at the knee. Those are exactly problems 2, 3 and 4 on this list, and they are still checked on a toile. The difference is that you are refining a pattern that already fits in its main dimensions, rather than correcting a standard size in several places at once.