Concrete block calculator

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, and why it is easy to 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. Three bags per 100 blocks is also a published answer to the same question, and the difference is the product, not the masonry: that figure belongs to a 70 lb bag of masonry cement, which is binder only and does nothing without 240 lb of sand. Cost an 80 lb preblended job at the masonry cement rate and you are short by a factor of two and a half, which you find out on a Saturday afternoon with a half-built wall and a mixer full of mortar going off.

One thing to be careful of in the other direction: the mortar figure here is sized off the blocks that go into the wall, not the blocks you buy. Your waste allowance covers units that arrive cracked or get cut in half, and neither of those consumes a full block's worth of joint. Applying the waste percentage to the mortar as well — which is what you get if you simply divide the purchase count by 13 — adds a bag to a typical wall for no reason.

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.

The wall this assumes you are building

Both figures — 1.125 per square foot and 13 blocks a bag — describe a plain stretcher wall in running bond. The further your wall is from that, the further the count is from what you will need:

• Every unit is treated as the same unit. Corner blocks, sash blocks, bond-beam and lintel units, half-high and half-length block all occupy different face areas and are ordered separately at the yard. A wall with returns, a bond beam at the top and a lintel over the door needs a count by unit type, not one total. This page gives you the total.
• Openings come off as bare area. Deducting a 3 × 7 ft door assumes the opening falls on the 16 in × 8 in module. It rarely does, so the courses beside and above it get cut, and the head needs lintel or bond-beam units that are not in the deduction. That is what the waste percentage is standing in for — and on a wall that is mostly openings, 5% will not cover it.
• The course figures are exact division, on purpose. 7.5 blocks per course is a real answer: it means a cut on alternate ends, every course. The block total does not add anything for those cuts beyond your waste allowance.
• The mortar figure is one product in one bag size. Thirteen blocks per bag is the 80 lb preblended mortar mix, sand included. Buy masonry cement instead — a 70 lb bag of binder that needs 240 lb of sand with it — and the published yield is 46 blocks per bag, so the count above is three and a half times too high for the sack in your hand. Only the 80 lb yield is published for the preblended product, so nothing here scales a yield to another bag size by weight. The mortar calculator carries both products and the sand that goes with the second.
• Nothing here is a structural check. Vertical reinforcement, ties, control joint spacing and whether a wall of this height needs an engineer are all outside what a quantity page can answer, and they change what you buy.
• A wall holding back soil is a different calculation. Earth pressure is not in any of these numbers — see the retaining wall block page instead.