Crotch Curve Estimator
Get a starting-point crotch extension range and curve shape guidance based on your body shape. A muslin fitting is always required to dial in the final shape.
Describe your body shape
Select the options that best describe your proportions. You can use the Rise Calculator to determine your crotch depth category.
What is the crotch extension?
The crotch extension(also called the crotch fork or crotch point extension) is how far the crotch seam extends outward, away from the inseam. On the pattern, it's the distance from the inseam point to the tip of the crotch curve.
The front extension is small — typically around ⅝". The back extension is much larger — typically 1½–2" or more, because the back of the body has more volume and the crotch needs to travel further around it.
When the extension is too small, fabric pulls tight across the crotch and you get smile lines under the seat. When it's too large, fabric hangs loosely and the inseam swings forward.
Why this always requires a muslin
The crotch curve is shaped by geometry that no tape measure can capture: the angle of the pubic bone, the position and projection of the seat, and the depth of the perineum. Two people with identical measurements can need very different curves.
The ranges this tool gives you are informed starting points — better than the standard commercial assumption (which is drafted for an "average" body) but not exact. Cut your muslin with extra seam allowance at the crotch seam (1" instead of ⅝") so you can let it out in small increments during fitting.
Once you have a well-fitting crotch curve for your body, trace it onto card stock and keep it. Every future pair of pants starts from that curve.