Don't forget your inseam.
Men's pants sizes rely on inseams more than women's pants sizes, but knowing your inseam as a woman can save you a lot of hassle when you're shopping online. To find it, measure your inner leg from your crotch to where you want your pants to fit on your ankle. Add 1/2" to 1" if you plan on mostly wearing heels with them.
Or take a pair of pants you currently wear, lay them flat, and measure the inseam—the visible inner leg seam—from the top to the hem.
If possible, use this measurement to select the pant length you need, rather than relying on words like "short," "regular," and "long." Those descriptive words aren't helpful if you're a short woman with really long legs, for instance, or a tall woman with short ones.
Check if the pants are for juniors.
Before you select your pants, see if the word "junior" is listed anywhere. Junior cuts are designed for tweens and teens, and thus, are narrower and slimmer than women's (AKA misses) cuts. They'll often be labeled with odd-numbered sizes, whereas misses cuts have even-numbered sizes.