Jeans sizes usually are in inches in waist. For example, if your waist is 27 inches, size would be 27.
The length of jeans is the second size number.
Due to waist is not the only measure, that may accuse jeans size, Sizeguide service use hips too. If hips and waist proportion differs from brands perfect fit, service may offer two sizes.
Other size variations are classic S, M, L, or imperial 10, 12, 14, or 36, 38, 40 (these are more like trousers).