yep! It's very easy. First knit a watch, and calculate your stitches per inch. The easiest way to do this (IMHO) is to cast on 20 stitches, knit 10-15 rows, then cast off but don't cut your yarn*. Lay it flat, and measure how many inches across your swatch is, go to the closest 1/4th inch. So let's pretend your swatch is 4.5 inches across.... divide 20 stitches by 4.5 inches and you get your guage -- in this case it's 4.44444 stitches per inch, but we'll round up to 4.5 stitches per inch.
Then, measure DH's shorts... around the leg opening, length from leg hem to waist band, length from leg to crotch, top of the leg to waist, and waist around. write it all down and draw a diagram if you want to. (hildie is anal retentively into math). You could start at the waist and knit down, or do a leg, put it on holder, do the other leg, join them together and knit up. Either way, it's the same to figure out how many stitches to ask on.
Let's pretend you're starting from the waist down.... My DHs waist is 34 inches, so we'll go with that. to get 34 inches you have to cast on 34 inches x 4.5 sts (your pretend guage) per inch = 153 stitches. If you do the waist in 2x2 ribbing, to make it more elastic, you'll have to round up or down to the closent # divisible by 4, in this case 152.
SO, cast on 152 stitches, and knit until it's as long as the top of leg to waist measurement, then divide for the legs.... this will require some more math, which I will help you with if you need it when you get to that point with your actualy guage and measurements. Cool?
*i said don't cut the yarn, but you CAN if you want to, to keep your swatch. If you don't cut it, you can just unravel it, and use the yarn you did your swatch with to start your project. Either way, it doesn't matter.
ALSO, you don't HAVE to cast on 20 stitches. You can cast on 10 or 30 or 17 or 57 or whatever. Just remember what you cast on, then divide however wide your swatch is into that # of stitches[/size]