Depends on the brand.. Some of em stray pretty far from the standard. If you have the size of something like Levis, someone ought to be able to help.
women's clothes can vary quite a bit by brand.
men's clothes are usually standardized. example men's pants/jeans 29X32 is 29 inches around belt loop 32 inches inseams. inseam is from the crotch part to the bottom. conversion to MM, inches times 25.4 conversion from MM to inches divide by 25.4
in men's jeans you can measure yourself naked around where the belt sits. then measure the inside of your leg down to how far you want it. (usually just below the ankle.) take the number you get in millimeters divide by 25.4 will give you the inches. for off whole numbers round it up. for a 41.3 inches around the belt go up to 42.
oh and you won't find sizes in the waist like 39, 41, 43. just round it up. at a certain point they just use even numbers and go up by 2 inches.
it is usually easy to order men's jeans, the main thing is the cut. that is the shape of the jeans that allows for the shape of your body.
as for suits they are best to be Tailor made to fit you. otherwise they won't sit right and look not so good.