How many concrete blocks do I need per square metre of wall?

This depends entirely on your block size and mortar joint thickness, so there is no single universal number. For a common 390mm x 190mm block with a 10mm joint, it works out to roughly 12-13 blocks per square metre; for a larger 440mm x 215mm block it is closer to 10-11 per square metre; and for a US 16in x 8in (nominal) block it is roughly 12.5 blocks per square metre. Use the calculator with your actual block size for an accurate figure rather than relying on a rule of thumb.

Do hollow concrete blocks need to be filled with concrete?

Not always — many non-structural, boundary, and partition walls use hollow block unfilled. Structural, load-bearing, or high-wind/seismic walls typically need selected cores filled with grout or concrete around vertical rebar for strength. This calculator estimates the block and mortar quantities; a dedicated concrete-block-fill calculator (linked below) covers the separate core-fill volume.

How much mortar do I need for a concrete block wall?

A commonly used rule of thumb is around 3 bags of mortar mix per 100 standard 16in x 8in blocks, which this calculator uses as its default — enter your own ratio if your mortar supplier or block size differs, since a larger block or thinner joint changes the mortar needed per block.

Can I use this calculator for a cinder block or breeze block wall?

Yes — "cinder block," "breeze block," and "concrete block" (CMU) all refer to essentially the same masonry unit family under different regional names, and the same area-divided-by-unit-footprint method applies as long as you enter that unit's actual face length and height.

Do I need Building Regulations approval for a block wall in the UK?

Freestanding garden walls under 1m high next to a footpath or 2m high elsewhere generally do not need Building Regulations approval, but any block wall forming part of a house structure, an extension, or a retaining wall over a certain height usually does — check with your local authority building control team before you order material.

What is the difference between dense block and aircrete (thermalite) block in the UK?

Dense concrete block is heavier and stronger, and is typically used below ground and for the outer leaf of some walls, while aircrete (aerated) block is lighter, has better thermal insulation, and is the standard choice for the inner leaf of most modern UK cavity walls. Both come in the same common face sizes, so this does not change your block count, only your cost per block and the wall's thermal performance.

Concrete Block Calculator — How Many Blocks Do I Need? | UK

How many concrete blocks do I need for a wall?

The number of concrete blocks you need depends on three things: the size of the wall, the size of the block (CMU) you are laying, and how thick the mortar joint between each block is. This calculator does the arithmetic for you: enter your wall length and height, pick or type in your block size, set the mortar joint thickness and a waste allowance, and it works out exactly how many blocks to order, plus roughly how many bags of mortar mix you will need to lay them.

Concrete block sizes are not standardised worldwide. A US 16in x 8in CMU, a UK/EU 440mm x 215mm dense block, and an Indian or Southeast Asian 400mm x 200mm block are all different face sizes, and most countries also stock two or three common formats side by side (hollow load-bearing block, solid block, and lightweight aerated-concrete block, for example). Rather than forcing you to guess, this tool ships with the real, sourced block dimensions used in your country, so the default answer already matches what your local builders' merchant sells.

Whether you are building a boundary wall, a foundation/footing course, a garden retaining wall, or the structural walls of a building, the same core method applies: work out the wall face area, work out how much wall each block-plus-joint actually covers, divide, and add a sensible waste margin for cuts, breakages, and mistakes. The sections below walk through exactly how that maths works, with a worked example, plus the mortar-bag estimate this calculator gives alongside the block count, so you can sanity-check the numbers the calculator gives you.

How to use the concrete block calculator

Enter the wall length and wall height — the total face area you actually need to cover with block (measure around any large openings, like a garage door, you plan to omit entirely).
Select your block size from the country presets, or enter a custom block length and height if you are using a non-standard block, a lightweight aerated-concrete block, or a reclaimed unit.
Set the mortar joint thickness — most blockwork uses a 10mm joint (⅜" in imperial-sized CMU), but confirm this against the block you are actually buying.
Add a waste allowance (5-15% is typical) to cover cuts, breakages, and delivery damage.
Enter the mortar mix ratio as bags per 100 blocks (the default of 3 bags per 100 blocks is a commonly used rule of thumb for a 16in x 8in block and a standard mortar mix — adjust it up for a larger block or a leaner joint).
Enter a price per block to get a total estimated material cost for the wall.

The concrete block calculation formula

The core formula is the same logic used for brick, scaled up to block size: the number of blocks equals the wall face area divided by the area that one block, plus its mortar joint, actually occupies on the face of the wall.

blocks = wall area ÷ [(block length + joint) x (block height + joint)]

The joint thickness is added to both the block length and the block height because every block in a course is separated from its neighbours by mortar on all sides, so the "footprint" each block claims on the wall is always slightly bigger than the block itself. Skip this step and your block count will be too low, because you have effectively assumed the blocks are laid with no mortar at all.

Once the raw count is worked out, you apply your waste percentage on top and round up to a whole number of blocks, since merchants sell block by the unit (or by the pack/cube, which this calculator's "blocks needed" figure lets you convert against your supplier's pack size).

Worked example: a wall 6m long and 2m high, using a 400mm x 200mm block face with a 10mm mortar joint and 10% waste. Wall area = 6 x 2 = 12 m². Each block, with its joint, occupies (0.4 + 0.01) x (0.2 + 0.01) = 0.41 x 0.21 = 0.0861 m². Raw block count = 12 ÷ 0.0861 ≈ 139.4. Add 10% waste: 139.4 x 1.1 ≈ 153.3, rounded up to 154 blocks.

For the mortar estimate, the calculator multiplies the block count by your chosen "bags per 100 blocks" ratio and rounds up. Using the default rule of thumb of 3 bags per 100 blocks: 154 x 3 ÷ 100 = 4.62, rounded up to 5 bags. This ratio is only ever a starting estimate — it was originally sourced for a 16in x 8in block with a standard mortar mix, so a much larger or much smaller block, or a thicker or thinner joint, will use more or less mortar per block than that.

Typical concrete block sizes (reference only)

Concrete block (CMU) face sizes vary by country and by block type, which is why this calculator loads your country's real, sourced standard sizes as presets rather than a single global default. The figures below are illustrative examples only — always check the reference table for your own country (shown further down this page) or the size printed on your supplier's spec sheet.

North American CMU is typically quoted as a nominal 16in x 8in or 12in x 8in block, which manufactures about ⅜in smaller in each dimension to allow for the mortar joint (the calculator already uses the actual, not nominal, size).
UK, EU, Australian and New Zealand dense/aircrete block is commonly 390-440mm long by 190-215mm high, in a range of thicknesses (blockwork thickness does not affect the block count, only the wall's structural design).
Lightweight autoclaved aerated concrete (AAC) block — sold under brand names like Ytong, Siporex, or Hebel in different markets — is usually a larger single unit than a standard hollow block and is counted and priced the same way, per block, once you enter its actual face size.

UK concrete block standards

UK dense and aircrete (aerated) concrete blocks are specified under BS EN 771-3 (Specification for masonry units — Aggregate concrete masonry units), and the dominant face size sold by builders' merchants is 440mm x 215mm, with a secondary 390mm x 190mm face size also common. Block thickness (typically 100mm, 140mm, or 215mm) is chosen for structural and thermal performance and does not change the block count for a given wall face area — only the face length and height matter for that.

Pro tips and common mistakes

Hollow vs. solid matters for both structure and cost. Hollow-core block is lighter, cheaper, and much more common for standard load-bearing and boundary walls; solid block is heavier, stronger in some applications, and usually priced higher per unit. Make sure the price per block you enter matches the type you are actually buying, not a generic average.

Nominal size and actual size are not the same thing. A "16 x 8 in" US block is manufactured about ⅜in smaller in both directions than its nominal name, specifically so that a 3/8in mortar joint brings the finished module back up to a clean 16in/8in grid. If you measure an actual block on a pallet and it looks smaller than its stated name, that is expected, not a mistake — enter the true face size, or use this calculator's presets, which already account for it.

Do not forget openings. This calculator gives you the block count for the full wall area you enter; if your wall has door, window, or vent openings you are not blocking over, subtract their area from your wall length x height before entering it.

Structural block walls are often reinforced and core-filled. Load-bearing block walls, retaining walls, and anything built to resist lateral load typically need vertical rebar through the hollow cores and a concrete or grout fill — this calculator gives you the block and mortar count, not the fill volume, so budget separately for grout/fill and rebar on structural runs.

A common and expensive mistake is ordering block by wall area alone and forgetting the joint thickness entirely, which under-counts by roughly 5-10% depending on block size — always use a joint-aware calculation like this one rather than a flat "area ÷ block face area" shortcut.

At a typical UK builders' merchant price of around £2.10 per standard dense block, the worked example on this page — a 6m x 2m wall needing 154 blocks including 10% waste — comes out at roughly £323 in block cost alone, before mortar, sand, cement, and labour.