How many concrete blocks do I need per square metre?

For the standard metric 440 by 215 mm block with a 10 mm joint, 9.88 per square metre — usually rounded to 10 for ordering. The figure comes from the module rather than the block: 450 by 225 mm is what each block actually occupies once its joint is counted, and one divided by that area gives the coverage. Using the bare 440 by 215 gives 10.57 per square metre, which is about 7% too many.

How many blocks per square foot in US sizing?

1.125 for a standard 8 by 8 by 16 inch block, which is 112.5 per hundred square feet. That figure is clean because the sizing is designed to make it clean: the actual block measures 15 and five eighths by 7 and five eighths inches, and adding a three-eighths joint brings it to exactly 16 by 8. American block dimensions are quoted nominally, describing the module rather than the block, and that catches out anyone who measures a block and divides.

Why is the block smaller than its stated size?

So that block plus mortar joint comes to a round number. Building dimensions work in modules — a wall four feet long, a course eight inches high — and if the block itself were exactly 16 by 8, every joint would push the wall past its intended dimension and nothing would line up with anything else. Making the block undersize by the thickness of one joint means the wall grows in exact 16 inch steps regardless of how many blocks are in it. The same logic drives metric sizing, where 440 plus 10 gives a 450 mm module.

How much mortar do I need for blockwork?

Around 0.008 to 0.010 cubic metres per square metre of wall for solid bedding on a 100 mm block, once a realistic wastage allowance is included. The pure geometry gives less — about 0.0066 — because it assumes every joint is exactly 10 mm and perfectly filled, which never happens. Real bedding varies, blocks are not perfectly regular, and mortar is lost off the trowel and in the tub. That gap is why mortar carries a much higher waste allowance than blocks do.

What mortar mix should I use for blockwork?

One part cement to four parts soft sand for general external work, going to 1:3 below damp-proof course or anywhere exposed to severe weather. Internal walls can go leaner at 1:5 or 1:6. The instinct to add more cement for strength is usually wrong: mortar should be weaker than the blocks it joins, so that movement is taken up in the joints, which are repointable, rather than cracking the blocks, which are not.

How many courses will my wall be?

Divide the wall height by the course height, which is the block height plus one joint — 225 mm for a standard metric block, 8 inches for a US one. The answer is rarely a whole number, and that is worth resolving at the design stage rather than on site. Adjusting a wall height by two or three centimetres to land on a whole course costs nothing on a drawing; cutting a course of blocks lengthwise to make up the difference is slow, wasteful and visible in the finished wall.

What is running bond and why does it matter?

Running bond, or stretcher bond, is the arrangement where each course is offset by half a block from the one below, so vertical joints never line up between adjacent courses. It is not decorative. A wall with aligned vertical joints has a continuous plane of mortar running top to bottom, which is a line of weakness the wall will eventually crack along. Offsetting the courses means any load is transferred sideways into neighbouring blocks rather than straight down a single joint.

How much waste should I allow on blocks?

Five percent covers most jobs. Blocks are counted objects rather than a bulk material, so waste comes from breakage in transit and handling, cuts at openings and corners where a part block is needed and the offcut is unusable, and the occasional block that turns out to be damaged. Walls with many openings waste more, because every opening reveal produces cuts. Complex work with a lot of returns and corners can justify ten percent.

Do concrete blocks need to be wet before laying?

No, and unlike clay bricks they should not be. Concrete blocks are already cured and wetting them causes two problems: a wet block shrinks as it dries afterwards, which cracks the joints you just made, and a saturated block will not draw moisture from the mortar, so the mortar stays wet and the block floats on it rather than bedding. Keep blocks dry and covered on site. Clay bricks are the opposite case and often do want damping in hot weather.

Concrete Block Calculator

Almost every mistake in a block take-off comes from the same place: measuring the block instead of the space it occupies. A block does not sit against its neighbour, it sits against a bed of mortar, and the wall is built from that combination rather than from the block alone. A 440 by 215 millimetre...

THE WALL

WALL LENGTH (m)

WALL HEIGHT (m)

BLOCK SIZE

MORTAR JOINT (mm)

Module is 450 mm × 225 mm, giving 9.877 blocks per m² (0.9176 per ft²).

OPENINGS

DOORS

WIDTH (m)

HEIGHT (m)

WINDOWS

WIDTH (m)

HEIGHT (m)

BLOCKS TO ORDER

230

218.4 exact · 5% waste · 22.11 m² of wall

COURSES

PER COURSE

CEMENT

2 bags

The height does not land on a whole course. At 225 mm per course, 2.4 m gives 10 full courses and 150 mm left over. That means either a cut course, or adjusting the wall height to a multiple of 225 mm — 2.48 m would be the next whole one. Setting out to the module is far easier than cutting a course to fit.

Gross wall area

24.00 m²

Openings

1.89 m²

Net area

22.11 m²

Coverage

9.877 / m²

Blocks exact

218.4

With 5% waste

230

Courses × per course

11 × 23

Mortar volume

0.174 m³

Cement

65.2 kg / 2 bags

Sand

0.181 m³ (290 kg)

BUILDING IT

Order 230 blocks and 2 bags of cement with 0.181 m³ of soft sand.

Set out the first course dry along the full 10 m before mixing anything. Blockwork is unforgiving of a bad start, and a course that is out at one end stays out all the way up.

Lay 11 courses of 23, keeping the 10 mm joint consistent — the module only holds if the joint does.

Rake or tool the joints as you go, while the mortar is thumbprint-hard. Left until it has set, the joint has to be ground out instead.

THE WALL

Coverage comes from the module, not the block. The block measures 440 mm × 215 mm, but each one occupies 450 mm × 225 mm of wall once its joint is counted. Working from the bare block gives 10.57 per m² instead of 9.88 — about 7.0% too many, which on this wall is 15 extra blocks.

Mortar comes to 0.174 m³ at 1:4. The wall itself will weigh roughly 4.4 tonnes, which is worth knowing before deciding what it stands on.

Live calculation · updates as you type

Created with❤️byeaglecalculator.com

HOW TO USE

1
Enter the wall length and height, then deduct any doors and windows. Openings are taken off the face area, since a hole in a wall needs no blocks.
2
Choose your block. The US sizes are listed by their nominal names because that is how they are sold, but the calculator uses the actual dimensions underneath and adds your joint to get the module.
3
Check the joint thickness. Ten millimetres is standard in metric work and three eighths of an inch in US work, and the coverage figure shown underneath updates as you change it.
4
Read the block count, then the courses and blocks per course beneath it. Those two are what you set out from — and if the height does not land on a whole course, the calculator says so and tells you the next height that would.

THE FORMULAS

Module(block L + joint) × (block H + joint)

Blocks per m²1 / module area

Net wall arealength × height − openings

Blocksnet area × per m² × (1 + waste)

Coursesceil(height / module height)

Per courseceil(length / module length)

Mortar per block(module − block face) × thickness

Cementmortar × 1.30 × c/(c+s) × 1440

BLOCK COVERAGE REFERENCE

Block	Actual face	Module	Per m²	Per 100 ft²
440 × 215 × 100 mm	440 × 215 mm	450 × 225 mm	9.88	91.8
440 × 215 × 140 mm	440 × 215 mm	450 × 225 mm	9.88	91.8
440 × 215 × 215 mm	440 × 215 mm	450 × 225 mm	9.88	91.8
390 × 190 × 190 mm	390 × 190 mm	400 × 200 mm	12.50	116.1
8 × 8 × 16 in (nominal)	15.625 × 7.625 in	16 × 8 in	12.11	112.5
6 × 8 × 16 in (nominal)	15.625 × 7.625 in	16 × 8 in	12.11	112.5
4 × 8 × 16 in (nominal)	15.625 × 7.625 in	16 × 8 in	12.11	112.5
12 × 8 × 16 in (nominal)	15.625 × 7.625 in	16 × 8 in	12.11	112.5

Metric figures assume a 10 mm joint, US a 3/8 inch one. Notice that the US module comes out at exactly 16 × 8 inches — the block is deliberately made undersize so that block plus joint lands on a round number. That is why a "8 × 8 × 16" block measures 15⅝ × 7⅝, and why 1.125 blocks per square foot is the figure the trade quotes.

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Last updated: July 29, 2026 · Formula verified · Eagle-eyed accuracy for every calculation.