If you are new to buying pants sight unfitted, buying pants online can be a frustrating experience.
A saavy online buyer considers 4 things when buying new or used pants on the internet:
First, clothing manufactures vary in the way they fit like shoes. Some pants are made to fit smaller or larger than what is printed on the tag size. It could be the style of how the manufacturer intended them to be worn (loose, relaxed, slim, regular) or just that particular company's forte.
Second, the cut of the pants may play a role in the roominess or lack thereof in the legs and rise/crotch. Vintage pants or slacks 20-50+ years old will fit very different from pants of today. These were meant originally meant to ride on your hips right beneath your belly button. Nowadays you'll find that the rise or crotch depth has shortened to be worn far beneath that generally being manufactured with a low rise. Leg cuts have varied back and forth over time. A boot cut, like that found on a Levi's 517, usually flares the legs out at the ankle so they rest over your shoe but also if your pants are too long, the heel will drag. Straight leg means they are the same leg width from thigh to ankle. Skinny or slim pants are meant to be worn very snugly and usually the leg tapers in at the ankle so that it stops right at your shoe.
Third, used pants could possibly be stretched out by their previous owner. There is no telling how tight or loose, rugged or relaxed the last guy or gal wore his or her pants. They could have stretched over time from the owner's growth or the material deteriorating from usage and washing.
Fourth, pants depending on the material can and do shrink. Consider wool versus cotton twill. Wool requires much more delicate laundering care but once pressed, creases seam to be sharper and hold much longer than their cotton competition. There are other comparision like the feel that we'll hold for another conversation. Also, even though a brand new pair of pants claims to be preshrunk you are still subject to thicker twills shrinking about 2-3% after that first wash.
The moral of this general advice is do not trust the tag size when buying pants online used or new. The safest approach to get a better fit than not is to compare 5 points of measurement using your best fitting pair of pants and the item's physical measurements.
Remember, buying sight unfitted can be fun when you do it right!