The conventional unit of measurement for the density of balsa in the USA is pounds per cubic foot. Those who build rubber band powered models and other super lightweight planes like to use what’s known as “contest” balsa, which typically weighs 6 pounds or less per cubic foot. Wing spars and other load bearing members are made from hard balsa, with density as high as 18 to 20 pounds per cubic foot. Medium balsa is preferred for general construction of the rest of the plane. Balsa weighing 10 to 14 pounds per cubic foot is typically considered to be medium balsa.

If you like to build from scratch you can usually tailor your design to the wood you have on hand. Harder balsa can be used farther forward in the airplane, for instance when building a balsa engine cowling. The tail should be built from lighter wood. Spars can be made of medium balsa, as long as the cross section is large enough to handle the load. For a typical sport plane with sheeted wings, medium balsa is used because super light balsa can’t stand up to rough handling. I’ve put my thumb right through the wing when the sheeting was too light. On the other hand, hard balsa is just too heavy for big pieces of wing sheeting. You want your wing sheeting to be like Baby Bear’s soup: just right. But if you don’t have balsa sheet of the appropriate weight you can build a non-sheeted wing instead. With a little bit of experience you can judge a piece of balsa by picking it up, flexing it, and comparing it to other pieces of balsa.

Before you have experience, it’s nice to have some numbers to work with. A piece of sheet balsa weighs a lot less than a pound, and the volume is far less than a cubic foot, so if you want to calculate the density of a piece of balsa there’s a lot of math involved.

Let’s say you have a piece of 1/16 x 3 x 36 balsa that weighs 1.2 ounce. That’s your first problem right there. Density is commonly stated in pounds and cubic feet, but we measure the pieces in ounces and inches. That’s not a problem, because that’s what this article is for. 1.2 oz is 1.2/16 of a pound, or .075 pound. The sheet is .00528 ft x .25 ft x 3 feet, or .0039 cubic foot. Dividing .075 by .0039 tells us that your piece of balsa is 19.2 pounds per cubic foot.

Wow, that’s some pretty hard balsa. It’s probably best suited for bulkheads in a small plane, or you could cut it into sticks for a small rubber band model or something like that. If you want to use 1/16 x 3 x 36 balsa for wing sheeting you’re a lot better off with a sheet that weighs 3/4 ounce, which would have a density of 12 pounds per cubic foot. If you use 5 pieces of it (top and bottom, left and right at the leading edge, and one extra piece split into four 3/4 inch strips for the trailing edge) you’ll save a little over two ounces. If it’s a 5 pound airplane, that’s almost 3% of the overall weight.

Back when I was taking math classes in school, we were supposed to learn the concept, and then we would be given a formula to do it the easy way. Now that you know the concept, here’s the easy formula:

I find this formula really handy, because sometimes when you weigh a sheet it’s hard to relate to the number. For instance, I weighed a 1/16 x 4 x 48 sheet and got 1.2 oz, and I thought it sounded pretty heavy, like the example I gave above. But at that size, a 1.2 ounce sheet has a density of 10.8 pounds per cubic foot, which is a really nice piece of balsa for wing sheeting.