1) This measurement is actually easiest measured by taking a pair of trousers that you like in terms of length. Pick a pair of trousers that like in their length and lay them flat.
2) Fold the top leg above the waistband, exposing the crotch seams. Use your hands to gently extend and flatten the fabric.
3) Measure from the crotch seam downward to the end the trousers.