How to Calculate Bale Density

Written by Andrew Morrison


straw bale hooksMost codes require specific measurements for the density of a construction grade bale. The code I work with calls for a bale density of at least 7 pounds per cubic foot of material. The bales I use measure 18″ wide x 14″ tall x 45″ long and weigh roughly 50 pounds. Many bales measure 18″ x 14″ x 36″ and weigh between 40 and 45 pounds. These numbers are for two string bales. To calculate the density of the bales I use, follow the formula below:

Multiply 18*14*45 = 11,340 cubic inches

Divide 11,340/1728 = 6.5625 cubic feet

Divide 50 pounds by 6.5625 cubic feet = 7.62 pounds per cubic foot

You can substitute your bale measurements and weight into this equation to calculate the density of your bales no matter what size they are.

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6 Responses

  1. this might be a little off topic but, i calculated the density of a a woven tatami straw mat. its like 11.61 lb per cubic foot. seems like it would make a killer straw bale wall. tatami is a type of traditional japanese flooring used in sleeping areas etc. they are about 2.36″ thick 33.85″ wide, 68.89″ long, and weigh about 37lbs. the mats are rice straw and hemp string. i am living in japan and people throw away tatami mats and replace them when the mats get too old, (straw can harbor mites). they are pretty ridged as well. i dont know the exact amount of deflection but they handle like a sheet of ply wood. you could probably cut it with a jigsaw, circular saw, or even drill a hole in it. I am sure there are ways to use it to make a straw bale wall. but, i dont know what would be the best way, or if its even a good idea.

  2. Sounds like a great idea to me. If you can cut it like you say and it is as rigid as you say, it would be a great building block. I don’t know what the R-value would be as much of that insulation value comes from the air space trapped within the straw and the bale itself. It would certainly be worth investigating if you can readily find the material to work with and as a waste product no less! I’d love to see some small, experimental buildings made from this to see what the possibilities are.

  3. I don’t know metric well enough to say. A great rule of thumb is that if you can pick the bale up by one string, shake it up and down, and place it back on the ground without it deforming at all, then you have a good bale. Hope that helps.

  4. Thanks for the video. That is very helpful. The bale looks okay to me. It could be tighter, but I think it will be fine. If you want it tighter, I suggest you stand it on end with new baling twine in position and temporarily tied off with the Miller’s Knot (not fully tightened yet). Then lean your body weight onto the bale so that the strings become loose on the bale. Tighten the new knots as far as they will go “bumping” your body onto the bales to help get the knots tight. Lock the knots off (make sure they don’t slip by using a half hitch or some other locking knot). Now you have a really tight bale. Cut off the old strings and put it in the wall. This takes a while to do if you plan to retie all of your bales, but it can be done. Good luck.

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