Something to keep in mind is how much is usable. Sometimes the selvedges can be and inch or so and you lose that. The 18" measurement should always be consistent. Most of the fabrics I buy are closer to 41-42 than 44-45, so the other dimension will be half of that width.
Afterthought - I have bought some in the past that were not cut straight. Could be wider at one end than the other. If you need absolutely every inch, it's best to measure before you buy.
Last edited by katier825; 04-24-2013 at 02:44 PM.