Concrete Block Calculator

Blocks = ceil( area × (1 + wastage ÷ 100) ÷ ((length + joint) × (height + joint)) )

L

Length of one block plus one mortar joint — a 16 inch module for a US block, 400 mm for a 390 mm block

H

Height of one block plus the joint — 8 inches, or 200 mm

A

Face area of the wall, in square feet

w

Wastage allowance, as a percentage of the net count

mortar

Wall volume minus the volume of the blocks in it — which for a hollow block is exact, because the cores cancel out of both terms

Use it once the wall's face area is known and before you order. The block count is the same for a 90 mm block and a 190 mm block of the same face size — thickness only enters the mortar, which is why the table's mortar column differs between them while the block column does not. For filling the cores of a block wall with concrete, use the block fill calculator; for a brick wall, use the brick calculator, which does have a thickness field because brick walls come in four.

1. 100 square feet of wall in US concrete block, 5% allowed

   1. One block plus its joint: (15.625 + 0.375) × (7.625 + 0.375) = 16 × 8 = 128 in²
   2. 128 in² = 0.8889 ft², so one square foot holds 1.125 blocks
   3. Area with 5% allowed: 100 × 1.05 = 105 ft²
   4. Blocks: 105 × 1.125 = 118.1, rounded up to 119
   5. Wall thickness: one leaf = 7.625 in = 0.1937 m
   6. Mortar: (0.1937 − 12.11 × 0.014887) × 9.755 m² = 0.1308 m³ = 4.62 ft³

   1.125 blocks per square foot is the figure on every American block chart, and it comes from the nominal 8 × 16 inch module rather than from the actual block. The mortar is only 6.9% of the wall's volume here against 19% for a brick wall: the block is 8 inches thick and the joints are the same ⅜ inch, so mortar is a much smaller share of the wall. That ratio is also why block walls are fast — far fewer units to lay than brick.

2. 100 square feet in 390 × 190 mm block, 5% allowed

   1. One block plus its 10 mm joint: 400 × 200 mm = 0.08 m²
   2. One square metre holds 1 ÷ 0.08 = 12.5 blocks
   3. Area with 5% allowed: 100 ft² = 9.290 m², × 1.05 = 9.755 m²
   4. Blocks: 9.755 × 12.5 = 121.9, rounded up to 122
   5. Wall thickness: 190 mm
   6. Mortar: (0.190 − 12.5 × 0.014079) × 9.755 m² = 0.1367 m³ = 4.83 ft³

   The metric block is slightly smaller than the US one in both directions and the joint is thicker, so it takes 122 blocks where the US block takes 119 — a 2.5% difference from a nominal size nobody would guess by looking. Both are 12.5 or 12.1 blocks per square metre, which is the number to carry in your head: roughly twelve and a half blocks to the square metre, not the nine or ten that gets quoted for larger blocks.

3. 250 square feet of 90 mm partition, 10% allowed

   1. The face is the same 390 × 190 mm, so one square metre still holds 12.5 blocks
   2. Area with 10% allowed: 250 ft² = 23.226 m², × 1.1 = 25.549 m²
   3. Blocks: 25.549 × 12.5 = 319.4, rounded up to 320
   4. Wall thickness: 90 mm — the same count, less than half the thickness
   5. Mortar: (0.090 − 12.5 × 0.006669) × 25.549 m² = 0.1696 m³ = 5.99 ft³

   Swapping the 190 mm block for the 90 mm one leaves the block count untouched and halves the mortar, because the face of the two blocks is identical — only the width differs. This is the cleanest proof that the page's count comes from the face module and its mortar from the wall's volume. It is also the trade: a 90 mm partition weighs about 8 kg a block instead of 17, but its sound insulation is a good deal worse.

The core ratio — 50% of the block's volume — is an order of magnitude, not a specification: manufacturers run between 40% and 60% depending on the block and the plant, and it is what the void and core-fill figures on the related page rest on. Block dimensions move too: a nominal 8 × 8 × 16 inch block is 7⅝ inches actual on every face, but a split-face, slump or scored block of the same nominal size has a different actual face area and will not lay to the same 1.125 per square foot. The count assumes whole blocks everywhere, so corners, bond beams, control joints, openings, pilasters and any wall under about a square metre need more than this returns — that is what the wastage field is for, and 5% is a floor for a plain wall rather than a safe figure for a wall with openings. The mortar is exact for the wall's gross volume but assumes every bed and head joint is filled; a wall with raked or struck joints uses less, and one that is grouted uses a great deal more. And this page counts blocks: it does not add reinforcement, grout, wall ties, lintels, damp course or the concrete for a footing.

How many concrete blocks are in a square foot?

1.125 for a standard 8 × 16 inch block, which is what every American block chart prints: 16 × 8 inches is 128 square inches and 144 ÷ 128 = 1.125. A 390 × 190 mm block laid with a 10 mm joint is 12.5 to the square metre, or 1.16 to the square foot. Both are the same kind of number — the face module divided into the unit area — and neither depends on how thick the block is.

Is a CMU the same as a cinder block?

The trade term is concrete masonry unit, or CMU, and it is the accurate one: the block is made of Portland cement, sand and aggregate. Cinder block is the older name from when the aggregate was cinders from furnaces, and it is still what most people say. The page treats them as one thing, because they are one thing — an 8 × 8 × 16 inch hollow unit. Lightweight and aerated blocks are a different product and much lighter, at around a third of the density.

How much mortar does a block wall use?

About 4.6 cubic feet per 100 square feet of 8 inch block, which is 6.9% of the wall's volume. A brick wall of the same area uses three times as much as a share of its volume, because the units are smaller and the joints more numerous. The figure is the wall's volume minus the blocks in it, which happens to be exact for a hollow block: the cores are counted in both terms and cancel out.

Why is there no wall thickness on this page?

Because a concrete block wall is one block thick. A 390 mm block is 190 mm wide, so a single leaf is already half its own length and there is no second way to lay it — unlike brick, where four thicknesses are standard. A thicker wall is two leaves with a gap or insulation between them, which is two walls and two counts. For the brick version, use the brick calculator.

How much should I allow for wastage on block?

5% for a plain rectangular wall, 10% once there are corners, openings, a bond beam or any cutting. Block is more forgiving than brick because a block can be cut and the offcut still used as a half, so the allowance is usually smaller. The field takes anything from 0 to 30, and the mortar grows with it because the wastage is applied to the area before either is calculated.

How heavy is a concrete block?

A 390 × 190 × 190 mm block with 50% cores comes out at about 17 kg, which matches what the same block weighs on a builder's merchant's pallet — one of the checks that the 50% core ratio is the right order of magnitude. A 90 mm block of the same face is about 8 kg. US blocks are more variable: a normal-weight 8 inch CMU runs 30 to 40 lb and a lightweight one can be under 25, which is why the table gives the weight only on the metric side. Lightweight and aerated are different products and appreciably lighter again, and this page models the normal-weight unit throughout.

• Approximate Conversions from U.S. Customary Measures to Metric — the chart behind the inch, foot and square foot figures used here — National Institute of Standards and Technology
• The International System of Units (SI) brochure — the metre and its multiples, the units the block and joint dimensions are defined in — Bureau International des Poids et Mesures (BIPM)