How is wood measured?

Answer 1

I'm guessing you are using either plywood or particle board. Doesn't matter which is Length or Width. Your only concern as how many smaller squares can be made from the larger one. Make a pattern or template out of cardboard and place this onto the larger sheet to determine best way to utilize the sheet. Keep in mind what you are using to cut the board with since the width of the cutting tool will take away some of the material.

For simple picture frames I've used corrugated material and cut with a utility knife. Used a framing square for a straight edge. Then used postal accepted tape to secure the backing to the frame.