How the concrete block calculator works
Concrete blocks are sold under a nominal size that already includes the mortar joint. An 8 × 8 × 16 in block really measures 7⅝ × 7⅝ × 15⅝ in, so that with a 3/8 in joint every block occupies exactly 16 × 8 in of wall. A European 20 × 20 × 50 cm block is made to its full size and laid with a joint of about 1 cm, so each one covers 51 × 21 cm. The calculator divides the net wall area by that face module and adds a breakage allowance.
Blocks = (length × height − openings) ÷ ((L + j) × (H + j)) × (1 + waste) · Mortar = area × T × (1 − L × H ÷ ((L + j) × (H + j))) × 1.2Mortar is estimated as if the bed and head joints ran across the full block thickness T, then increased by 20% for droppings. Hollow blocks laid on face shells only use less, but the extra covers mortar pushed into cores at ends and corners, which is how real jobs tend to go. Dry mix is taken as 1,900 kg per cubic metre of mortar, so an 80 lb bag of mortar mix makes roughly two-thirds of a cubic foot.
Worked example
A boundary wall 10 m long and 1.2 m high in 20 × 20 × 50 cm blocks covers 12 m². Each block plus joint takes 0.51 × 0.21 = 0.1071 m², which is 9.34 blocks per square metre, so the wall needs 112 blocks and 118 with 5% extra. It is 5.7 courses high, so in practice you would build six courses and adjust the footing or the top. The joints make up 6.6% of the wall face; across a 20 cm thickness that is 0.159 m³ of mortar, or 0.191 m³ with the allowance. At 1,900 kg per m³ that is about 363 kg of dry mix, or 15 bags of 25 kg.
Standard block sizes
| Block | Actual size | Joint | Blocks per m² | Blocks per 100 ft² |
|---|---|---|---|---|
| 8 × 8 × 16 in CMU | 7⅝ × 7⅝ × 15⅝ in | 3/8 in | 12.1 | 112.5 |
| 6 × 8 × 16 in | 5⅝ × 7⅝ × 15⅝ in | 3/8 in | 12.1 | 112.5 |
| 12 × 8 × 16 in | 11⅝ × 7⅝ × 15⅝ in | 3/8 in | 12.1 | 112.5 |
| 20 × 20 × 50 cm | 200 × 200 × 500 mm | 10 mm | 9.3 | 87 |
| 15 × 20 × 50 cm | 150 × 200 × 500 mm | 10 mm | 9.3 | 87 |
| UK dense block | 440 × 215 × 100 mm | 10 mm | 9.9 | 92 |
Thickness does not change the block count, only the mortar and the weight: a hollow 8 in block weighs roughly 35 to 40 lb, and a solid 440 × 215 × 100 mm dense block around 20 kg.
Practical tips
- Design to the module. Wall lengths in multiples of 8 in (or 25.5 cm for European blocks) and heights in whole courses avoid cutting, which saves both blocks and time.
- Count corner and end blocks separately. Exposed ends usually take corner or bullnose units and bond beams take special U-shaped blocks; order those in their own count.
- Grout and rebar are extra. Filling cores with grout for reinforced walls needs its own volume, worked out from the core size in the block maker's data sheet and the number of cores the design fills.
- Retaining and structural walls. Anything that holds back soil, supports a roof or exceeds about 1 m of freestanding height needs footings, reinforcement and often a permit; follow local code and an engineer's details.
Frequently asked questions
How many concrete blocks are in 100 square feet?
Standard 8 × 8 × 16 in blocks with 3/8 in joints come to 1.125 per ft², so 112.5 blocks per 100 ft². Add 5% for breakage and order about 118.
How many parpaings or 20x20x50 blocks per square metre?
With 1 cm joints each 20 × 20 × 50 cm block covers 0.1071 m², so a square metre takes 9.3 blocks. The rounded figure of 10 per m² often quoted includes waste.
How much mortar do I need for 100 concrete blocks?
For 8 × 8 × 16 in blocks the calculator's method gives about 4.7 ft³ of mortar per 100 blocks, roughly seven 80 lb bags of mortar mix.
How many blocks do I need for a 4 ft high, 32 ft long wall?
That wall is 128 ft², so 144 standard 8-inch blocks plus 5%, which is 152 blocks, laid in six courses.
Do I need to subtract windows and doors?
Yes. Enter their total area as openings; a 3 × 7 ft door removes about 24 blocks from the count. Keep a few extra for cutting around lintels.
Last reviewed: 2026-09-27 · How we calculate