Concrete block calculator

Enter the wall and this gives you the block count, the blocks per course, and the bags of mortar. The blocks-per-square-foot figure is calculated from the actual unit size and your joint width, so you can see where 1.125 comes from.

Wall length Wall height Doors and windows to deduct Total area of any openings. A 3 x 7 ft door is 21 sq ft; a 3 x 4 ft window is 12 sq ft. Deducting them is worth 20+ blocks on a garage wall. Actual block length The measured length, not the nominal name. A block sold as "16 in" is made at 15-5/8 in. A half block is 7-5/8 in. Actual block height A standard block is 7-5/8 in high. Half-high (4 in nominal) block is 3-5/8 in. Mortar joint width 3/8 in is the standard joint and the number the block is dimensioned around. Going to 1/2 in drops coverage from 1.125 to 1.10 blocks per sq ft. Extra for waste % 5% covers breakage in handling and the blocks you cut. Go to 10% if the wall has lots of corners or openings, where cut-offs pile up.

Blocks to buy 119 pieces Includes the waste allowance.

Blocks per square foot 1.125

Blocks per course 7.5 A fraction here means every course needs a half block or a cut.

Courses high 15

Mortar, 80 lb bags 10 bags At the published 13 blocks per 80 lb bag.

Mortar per 100 blocks 9 bags

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.

Common cases

Questions people ask

How many concrete blocks per square foot?

1.125 for a standard 8 x 8 x 16 nominal block with 3/8 in joints — the unit is really 15-5/8 x 7-5/8 in, so with the joint it occupies 16 x 8 in, and 144 ÷ 128 = 1.125. With 1/2 in joints it drops to 1.10.

How many bags of mortar per 100 blocks?

About 8 × 80 lb bags, or 9 with a waste allowance. The QUIKRETE Mortar Mix and Mason Mix data sheets both state that an 80 lb bag lays up to 13 standard blocks, so 100 ÷ 13 = 7.7 bags.

How many blocks does one bag of mortar lay?

Up to 13 standard 8 x 8 x 16 blocks per 80 lb bag, per the manufacturer's published yield. The same bag lays up to 37 standard bricks, because a brick has much less joint area per unit — do not use one figure for both.

Is a cinder block the same as a concrete block?

In conversation, yes; on the pallet, no. Cinder block used coal cinders as aggregate and is not in general production. Everything sold today is a concrete masonry unit, which is heavier and stronger. Buy by the unit dimensions and the ASTM grade, not the nickname.

Do I need to deduct doors and windows?

Yes. A single 3 x 7 ft door is 21 sq ft, which is 24 blocks. On a garage wall with a door and two windows that is 50 blocks you would otherwise pay to have delivered and stack in the garden.

How much does a concrete block weigh?

A standard 8 x 8 x 16 hollow unit runs roughly 30–38 lb depending on the aggregate, and lightweight units less. Product-specific weights vary enough that you should take them from the supplier, but plan for well over a tonne per 100 blocks when you arrange delivery.

Last reviewed 2026-07-30