Concrete Block Calculator
Find how many concrete blocks (CMU, cement or hollow blocks) you need for a wall, plus the mortar. Advanced adds more walls, doors and windows, your own block size, core fill, rebar and cost.
Typical: 11.61 per m² with 10 mm joints.
10 mm or ⅜ in is the usual joint. US blocks are sold by nominal size (actual + ⅜ in). Changing a size switches to "My own size".
Typical door about 0.9 × 2.1 m (3 × 7 ft), window about 1.2 × 1.2 m (4 × 4 ft).
5–10% extra blocks is typical: more for many corners and openings.
Works out the bed joint under each block and the joint at its end.
1:4 for strong or below-ground work, 1:5 or 1:6 for general block walls (typical). Follow your local rules.
Typical: an 80 lb (36 kg) bag of mortar mix makes about 0.67 cu ft (19 L). Check your bag.
About 45% hollow is a rough typical figure for two-core blocks. Use the maker's figure if you have it. Bag yields are typical (80 lb makes about 0.6 cu ft).
One bar at each end of every wall, then one every spacing. Extra length covers laps into the footing or bond beam. Bar size and spacing are set by your engineer or local code.
How your blocks are worked out
Each block plus its joint takes a little area. Divide the wall by it, add extra, then add the mortar each block needs.
- 1
Blocks per m²
(400+10) × (200+10)= 0.0861 m² each1 ÷ that = 11.61
- 2
Wall area
5 × 3= 15 m²
- 3
Count the blocks
15 × 11.614 = 174.2× 1.05 = 182.9= 183 blocks
- 4
Mortar per block
174.2 × 1.22 L+ 10% = 0.234 m³2 × 50 kg cement
Read more: the formula, worked examples in metres and feet, typical sizes and tips
Each block takes up its own face plus one mortar joint along the side and one along the top. So the number of blocks in a square metre (or square foot) is 1 divided by that bigger area. Multiply by the wall area, less doors and windows, then add a little extra for cuts and breakage. Mortar is worked out per block from a stated assumption, which you can change. Core fill is the hollow part of each block you choose to fill.
The formula
blocks per m² = 1 ÷ ((block length + joint) × (block height + joint))
wall area = length × height × how many − doors and windows
blocks = wall area × blocks per m² × (1 + extra % ÷ 100), rounded up
mortar per block = joint × (length + joint + height) × bedded width (full depth, or two face shells)
mortar = blocks before extra × mortar per block × (1 + extra mortar % ÷ 100)
dry mix = mortar × 1.33 · cement = dry mix ÷ (1 + sand parts) × 1440 kg/m³ · sand = dry mix × sand parts ÷ (1 + sand parts) × 1600 kg/m³
core fill = blocks before extra × length × height × depth × hollow % (or bars × courses × half that, for cells with bars only), + extra %
bars per wall = (length ÷ spacing, rounded up) + 1
bags = amount ÷ what one bag holds or makes, rounded up
Worked example
In metres. A wall 5 m long and 3 m high is 15 m². A 400 × 200 × 200 mm block with 10 mm joints takes 0.41 × 0.21 = 0.0861 m², so 1 ÷ 0.0861 = 11.614 blocks per m². The wall needs 15 × 11.614 = 174.22 blocks, and with 5% extra 174.22 × 1.05 = 182.93, so buy 183 blocks. It is 3 ÷ 0.21 = 14.3 courses high.
Bedded across the full 200 mm, each block takes 0.01 × (0.4 + 0.01 + 0.2) × 0.2 = 0.00122 m³ (1.22 L) of mortar. 174.22 × 0.00122 = 0.21254 m³, and with 10% extra 0.2338 m³ of wet mortar. The dry mix is 0.2338 × 1.33 = 0.31095 m³. In a 1:6 mix, cement is 0.31095 ÷ 7 × 1440 = 63.97 kg (2 bags of 50 kg) and sand is 0.31095 × 6 ÷ 7 × 1600 = 426.4 kg. If you fill every core at 45% hollow, each block holds 0.45 × 0.4 × 0.2 × 0.2 = 0.0072 m³, so 174.22 × 0.0072 = 1.2544 m³, or 1.38 m³ with 10% extra.
In feet. A wall 20 ft long and 8 ft high is 160 sq ft. An 8 × 8 × 16 in CMU with ⅜ in joints takes 16 × 8 = 128 sq in, so 144 ÷ 128 = 1.125 blocks per sq ft. 160 × 1.125 = 180 blocks, and with 5% extra 189 blocks. Laid on two 1¼ in face shells, each block takes 0.375 × (15.625 + 0.375 + 7.625) × 2.5 = 22.15 cu in of mortar. 180 × 22.15 = 3,987 cu in = 2.307 cu ft, and with 10% extra 2.538 cu ft, which is 2.538 ÷ 0.67 = 3.79, so 4 bags of 80 lb mortar mix. Filling all cores at 45% hollow: 0.45 × 15.625 × 7.625 × 7.625 = 408.8 cu in per block, 180 × 408.8 ÷ 1728 = 42.58 cu ft, and with 10% extra 46.84 cu ft (1.73 cu yd), or 79 bags of 80 lb that make 0.6 cu ft each. With a bar every 48 in, the wall needs 240 ÷ 48 = 5, + 1 = 6 bars.
Typical block sizes
| Block (typical) | Per area |
|---|---|
| 400 × 200 mm face (200, 150 or 100 mm deep), 10 mm joint | 11.61 per m² (12.5 if joints are ignored) |
| UK block 440 × 215 mm (100 or 140 mm deep), 10 mm joint | 9.88 per m² (about 10) |
| US CMU 16 × 8 in nominal (15⅝ × 7⅝ in actual), ⅜ in joint | 1.125 per sq ft (112.5 per 100 sq ft) |
Sizes, hollow shares and bag yields are typical and vary by maker and country, so check your blocks and bags. The mortar per block is an estimate from the joint size, not a standard: real use depends on the bedding, the joint and the mason, so enter your own figure if you know it. The 1.33 dry-mix factor and the 1440 kg/m³ cement and 1600 kg/m³ sand densities are common rules of thumb, checked 13 October 2026. Reinforcement, grout strength and mortar type are set by your engineer and local building rules.
Tips
- Count corner, half and lintel (bond beam) blocks separately if your supplier sells them as different items.
- For large fills, ready-mixed grout or concrete from a truck is often cheaper than bags.
- Keep blocks dry and off the ground. Mix only as much mortar as you can use in about two hours.
Common questions
How many concrete blocks are in a square metre?
A 400 × 200 mm block with a 10 mm joint takes up 410 × 210 mm, so 1 ÷ (0.41 × 0.21) = 11.61 blocks per m². The figure of 12.5 often quoted ignores the joints (1 ÷ (0.4 × 0.2)), so it gives about 7% spare. A UK 440 × 215 mm block with 10 mm joints is 9.88 per m², usually called 10.
How many CMU blocks per square foot?
A standard 8 × 8 × 16 in CMU is 15⅝ × 7⅝ in, and with a ⅜ in joint each block takes 16 × 8 = 128 sq in. 144 ÷ 128 = 1.125 blocks per sq ft, or 112.5 per 100 sq ft. A 20 × 8 ft wall (160 sq ft) needs 180 blocks, or 189 with 5% extra.
How much mortar does one block need?
It depends on the joint and how the block is bedded, so the calculator shows its assumption and lets you change it. With a 10 mm joint under and at the end of a 400 × 200 × 200 mm block bedded across its full width, the joints hold 0.01 × (0.4 + 0.01 + 0.2) × 0.2 = 1.22 L. Hollow CMU laid on its two face shells only use much less: about 22 cu in (0.36 L) for an 8 in block. Spills and droppings add more, so allow extra.
How much grout fills the cores of a hollow block?
Multiply the block's size by its hollow share. An 8 in CMU is 15⅝ × 7⅝ × 7⅝ in (908 cu in); if 45% is hollow, each block holds about 409 cu in (0.24 cu ft) of grout. Makers publish the real hollow share for their blocks, so use theirs if you have it. Many walls only fill the cells that hold a bar.
What is the difference between nominal and actual block size?
US blocks are sold by nominal size, which includes a ⅜ in joint: an 8 × 8 × 16 in CMU really measures 7⅝ × 7⅝ × 15⅝ in. Metric hollow blocks such as 400 × 200 × 200 mm are usually named by their real size, so the joint is added on top. Check the label or the maker's sheet.
How many extra blocks should I order?
About 5% is typical for straight walls, and up to 10% for walls with many corners, openings or cut blocks. Corner, half and lintel blocks are often sold separately, so count those on their own if your wall needs them.
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