How Many Cinder Blocks for a 10x20 Shed?

How that number is worked out

These are the figures this page was built with and every line the calculator produces from them. Change anything above and all of it moves.

Stem wall height compared

The same 60 ft of wall from two courses to six. This is perimeter only, so whatever the middle of the shed needs is on top of every row here.

What else matters here

One hundred and forty two block, for a mortared stem wall around the 60 ft perimeter. The largest shed on this list, and the one where the perimeter stops being the whole foundation.

The floor of a shed spans between its supports, and on a 10 by 20 the long direction is twenty feet of unsupported span if the only bearing is the outside wall. No sensible floor joist crosses twenty feet under a shed, and no skid does either. So a shed this long carries a bearing line down the middle, which is either a beam on piers or a second run of block, and it is not in the 142 above, because the calculator has been given a perimeter and knows nothing about what is inside it.

What that means in practice is that the perimeter figure is a floor rather than a total. Work out the floor framing first, decide where the intermediate bearing lands and how it is held up, and add that material to this list. It is a smaller quantity than the perimeter, but leaving it out is how a shed floor ends up with a dip down the middle of it within a couple of seasons.

What this calculator assumes

• Every block here is 8 by 16 nominal on the face, which is the standard unit whatever its width. That works out to 1.125 blocks per square foot of wall, and the figure already contains the 3/8 mortar joint.
• Mortar is the bed and head joints across the face shells, multiplied by 3.6. The multiplier lands the bag count on the coverage printed on the bag, where an 80 pound bag of mortar mix yields about 0.6 cubic feet and lays roughly 13 standard block.
• Grout volume uses the cell section for the block width: about 27 square inches for an 8 inch block, less for a 4 or 6 and more for a 10 or 12. Fully grouting an 8 inch wall works out near 0.28 cubic feet per square foot of wall, which is the figure the block industry publishes.
• Vertical bars are one per grouted cell. On a fully grouted wall they are figured at 48 inches on center instead, because grouting every cell does not mean putting a bar in every cell.
• Horizontal joint reinforcement is every other course, which is 16 inches.

Questions people ask

Other common sizes