Google the "Barton Model Flying Club", a really good  Australian site that is also a forum.  Click on forum, click on general, look under 'sticky' for balsa weight calculator.  It really works.  I use to use it for balsa now I use it for poplar and cedar since I no longer use balsa.

I guess there are a lot of people not aware of the download area here:
http://stunthanger.com/smf/index.php?board=14.0

According to this , a little less than 4 pounds per cubic foot. That makes it Contest grade as most see it.

Robert

If math doesn't make your head spin, and if you don't want to drag a computer into the workshop, just divide the weight of the wood in pounds by its volume in cubic feet.  This is a bit of a pain because you'll weigh it in ounces and measure it in inches, but for your block it works like this:

(9.75 oz / (16 oz/lb)) = 0.6 lb
(30 in / (12 in/ft)) = 2.5 ft
(4.25 in / (12 in/ft)) = 0.35 ft
(2.125 in / (12 in/ft)) = 0.18 ft

(0.6 lb) / ((2.5 ft)(0.35 ft)(0.18 ft)) = 3.8 lb/cu ft

Or you can find the density in oz/cu in and convert:

(9.75 oz) / ((30 in)(4.25 in)(2.125 in)) = 0.036 oz/cu in
(0.036 oz/cu in)(108 (lb/cu ft)/(oz/cu in)) = 3.9 lb/cu ft

I get two different answers because I was rounding -- but that's close enough.

If you have 36 x 3 sheets, you can divide the weight by the thickness to get density in lbs/ft*3.

That's very clever.  Now if only I can remember it...