A couple of months ago somebody had a block of balsa 1x3x36 which weighed 7 ozs and was wondering what it was good for. Many people pointed out that the density of his wood was 7 lb/cu/ft. Leonard gave a better reply with a formula for mental calculations.
At the time I noticed that his 1x3x36 weighed 7ozs. This recalled a mental calculation I've always used for 40 years but never bothered to prove. On inspection, where any balsa sheet of 1x3x36 has a weight in ozs, the density of that wood will be the same number in lb/cu/ft, in this case, 7oz = 7lb density. In a 1x3x36 block there are 108 cu/in of balsa. 1x3x36=108. In a cu/ft of anything there are 12x12x12 cu/ins = 1728 cu/in. If you divide 1728 by 108 you get 16, the number of ozs in a lb. the relationship is valid. If you have a 1x3x36 that weighs 8ozs, it has a density of 8 lb/cu/ft. A block of balsa 3x3x36 is the same as 3 blocks of 1x3x36 so, if it weighs 18 oz, it is 18/3=6lb wood. If you find a 20oz, 3x4x36 at your hobby shop, buy it.
As for what it is good for, many suggested that it is a more efficient use of wood to mold where possible. I agree and mold about as much as anybody, but you can't mold balsa wing tips. Typically, they must be carved. What weight should we use for carved and hollowed airplane parts? Contest balsa is too valuable to carve. 10lb strip wood is too hard to work. Any wood of reasonable midweight can be used to achieve essentially the same finished weight. I buy misc block cutoffs from Lone Star. they are somewhat random in size but are about 3x3x6 and are sold 6 for....well, let's just say you wouldn't be interested. Too much activity would drive up the price or empty the cutoff bin. About every third block I pick up feels a lighter than most others and indenting it with a fingernail insures it is carvable. I don't think it is necessary to calculate the weight. After carving the outside of the tip, I break its temporary joint with the wing and split the tip into top and bottom halves. Half tips are much easier to hollow than trying to work down into a 2" hole. Hollowing is begun using a Moto-Tool with a drum sanding attachment. I hollow rather ruthlessly, feeling for vibration which tells me I am getting in the "ballpark". Then, as the carving/sanding continues, progress is checked by shining a flashlight on the outside of the carving. Light will begin to show through long before the wood gets too thin. I hollow until the tip is a uniform, fairly bright orange. With most woods, this will be about 3/32" thick. A dull red color is usually 1/8" or more. A bright yellow is a too thin 1/16" (OK in spots, but avoid these spots as you work farther). I sometimes work outside in the shade, but close enough to direct sunlight, that that I can reach out into the sunlight to check thickness.
Regardless of the actual weight of the balsa, my tips almost always come out about 6 grams. I figure the light is an indication of how many wood fibers there are in the way of the light beam and I hollow to a color which represents a desired number of wood fibers. If the wood is very light the same color of penetrating light will give me a thicker carved wall thickness than the same color with denser wood and a thinner wall. In all cases, I figure the strength of the finished parts, with woods of different density, are equivalent, as are their weight. If you carve 6 gram, or lighter, tips, you might not want to pull on them for pull tests. I stabilize the wing tip with one hand, but do my pulling with my other hand, over a wing/fuselage "hard spot".
My point is, that a good carving technique is more important than wood weight, and comparable results in strength and weights can be achieved simply and easily with woods of different density. There is no need to be anal about tip or top block carving to get good results.
Suggestion: Take a flashlight to your wood bin and shine it through a number of different wood thicknesses and densities.
Al