How many bags of mortar per 100 blocks?

Where 1.125 blocks per square foot comes from

You will see "1.125 blocks per square foot" quoted everywhere with no explanation, which makes it look like a magic number. It isn't. A concrete masonry unit sold as 8 × 8 × 16 in is manufactured at 15-5/8 × 7-5/8 in. Add one 3/8 in mortar joint to each dimension and you get exactly 16 × 8 in — 128 square inches, or 0.889 sq ft of wall. So:

144 ÷ (16 × 8) = 1.125 blocks per square foot

The joint is not decoration; the block is dimensioned around it. That is also why the figure is fragile: lay the same block with 1/2 in joints and coverage drops to 1.10 per sq ft, because each unit now claims 16.125 × 8.125 in. A page that gives you 1.125 without telling you the joint width has given you a number you cannot check. This calculator takes both as inputs so you can.

The mortar figure most calculators get badly wrong

The published yield is on the bag. The QUIKRETE Mortar Mix (No. 1102, Type N) and Mason Mix (No. 1136, Type S) data sheets carry the identical sentence: each 80 lb bag "will lay up to 37 standard bricks or 13 standard (8 in by 8 in by 16 in) blocks."

Thirteen. So 100 blocks needs about 7.7 bags, call it 8 before waste and 9 with. Some calculators tell you 3 bags per 100 blocks. That is a two-and-a-half times underestimate, and you find out about it on a Saturday afternoon with a half-built wall and a mixer full of mortar going off.

Note the words "up to". That yield assumes face-shell bedding and a clean 3/8 in joint. Buttering full beds, or a first course levelled on a thick bed, uses more. The data sheet's own application note calls for a bed 1/2 to 3/4 in thick.

Bricks and blocks are not interchangeable in this sum — the same bag lays 37 bricks or 13 blocks, because a brick has far less joint area per unit. Anyone quoting one figure for both has not read the bag.

Count courses before you buy

Two of the outputs above matter more than the total. Blocks per course is your wall length divided by 16 in. Come out at a fraction and every single course needs a half block or a cut — a 10 ft wall is 120 in, which is 7.5 blocks, so you need a half block on alternate ends all the way up. A 20 ft wall is 15 blocks exactly and needs none. Courses high is the wall height divided by 8 in; 8 ft is 12 courses exactly.

If you have any freedom over the dimensions, snapping them to multiples of 16 in and 8 in is the single cheapest improvement you can make to the job.

Two things this does not include

• Grouted cores. If the wall is reinforced, the cores get filled, and the volume depends on your specific unit's core size. Get that from the block manufacturer's data sheet — we would rather leave it out than guess it.
• "Cinder" blocks. Genuine cinder block, made with coal cinders, is not in general production. What you are buying is a concrete masonry unit. The names are used interchangeably but only one of them is on sale.