Dani's completely right. While you can fudge it a bit with elastic waist pants, with jeans and most pants there is no real way to be sure without trying it on. Clothing manufacturers do not adhere strictly to standards or even between a single brand's different lines. And, while I can appreciate Alaskan Danielle's effort to come up with a waist size guide, I've got a 32 inch waist and have never managed to fit into a size 8 pant.
My best advice is to either go to the store and try them on or order from one of the online dealers that offer free returns. Here are several good US sellers with free returns, BTW: Asos, Gap, Old Navy, Banana Republic, Macy's, Zappo's and Foot Locker. Zappo's and Macy's are my favs so far. Macy's seems to always have a special on either jeans or shipping.