Block Wall Mortar Calculator
Find the mortar for a concrete block wall: in litres, bags of mortar mix, or sand and cement.
Measure your job
Your answer
For about 119 blocks (100 mm blocks), mixed 1 : 5.
Bulk bags are 800 kg. Bags are rounded up.
This answer is a guide. For footings, walls or anything that holds weight, check your local building rules or ask a builder or engineer.
What it costs
UK middle prices from October 2026, with VAT. Cement: £8.39 a 25 kg bag (7 prices from 3 sellers). Bulk bags: £75, building sand £67.35 (3 sellers each). Few prices, so treat them as a rough guide.
Quick answers
- Mortar per block
- About 1 litre for a 100 mm block, says Cemex. Other makers range from 0.67 to 1.25 litres.
- 100 blocks
- About 100 litres of mortar, or 9 bags of 25 kg mortar mix.
- Blocks per m²
- Kilsaran uses 9.88 blocks of 440 × 215 mm per m². That matches 10 mm joints (our sum).
- One bag of mortar mix
- Cemex's 25 kg bag lays about 12 blocks of 100 mm.
How to work it out
- Count the blocks, or measure the wall's length and height in metres and multiply them.
- Each m² takes 9.88 blocks of 440 × 215 mm. Round up.
- Allow about 1 litre of mortar for each 100 mm block.
- For 25 kg bags of mortar mix, divide the blocks by 12. Round up.
- To mix your own, allow about 1.54 kg of building sand per block. The calculator adds the cement.
6 × 2 m = 12 m² × 9.88 = 118.6 → 119 blocks
119 × 1 = 119 litres of mortar
119 ÷ 12 = 9.9 → 10 bags of 25 kg
Mortar for 100 blocks, and other counts
Pick the row closest to your number of blocks. Both columns use Cemex's figure of about 1 litre of mortar per block, as the calculator does.
| Blocks | Mortar (litres) | 25 kg bags of mortar mix |
|---|---|---|
| 50 | 50 | 5 |
| 100 | 100 | 9 |
| 200 | 200 | 17 |
| 300 | 300 | 25 |
| 500 | 500 | 42 |
Bags: 12 blocks each (Cemex). 10 mm joints.
Mixing your own? The calculator above gives the building sand and cement. Our sand and cement calculator has the full buying list. To count the blocks first, use our concrete block calculator.
Mortar per block: why makers differ
| Maker | What they say | Litres per block |
|---|---|---|
| Blue Circle | 1 m³ lays 1,500 blocks | 0.67 |
| MPA | 1 tonne lays 600 blocks | 0.83–0.98 (our sum) |
| Cemex | 25 kg bag (12 litres) lays about 12 | 1 |
| Kilsaran calculator (Ireland) | – | 1.11 |
| Kilsaran flyer (Ireland) | 1 m³ lays about 800 blocks | 1.25 |
A 10 mm joint under and at one end of a 100 mm block holds 0.67 litres (our sum). That matches Blue Circle's figure, so it allows nothing spare. Kilsaran's flyer allows almost twice as much.
3 figures out of 5 land close to 1 litre, from 0.83 to 1.11. That is why the calculator works with Cemex's 1 litre a block. It adds no extra for spills unless you pick 5% or 10%.
Thicker blocks
Blue Circle's figures give about 0.5 litre for a 75 mm block and 1 litre for 150 mm (our sums). We found no UK maker figure for 140 or 215 mm blocks. Kilsaran gives 1.54–2.22 litres for a 215 mm hollow block. For a 100 mm block laid flat, it gives 2.22 litres.
How many blocks per bag of mortar?
- Cemex, 25 kg dry mortar: about 12 blocks of 100 mm. So 100 blocks take 8.3, or 9 bags. Mix each bag with 3.5–4 litres of water. Use it up within 2 hours.
- Heidelberg, 20 kg ready to use: it makes 0.011 m³. Spread over the makers' range, that is about 8–16 blocks a bag. That is our sum: Heidelberg only gives a brick figure.
We found no checked UK price for mortar mix bags. So the costs in the calculator cover sand and cement only.
Which mortar for a block wall?
- M4: Cemex sells it for laying bricks and blocks, repointing and patio slabs. A 1 : 5 mix of cement to building sand is M4.
- M12: the stronger mix, for below ground and at DPC level, says Cemex. That is about 1 : 3.
- Cold days: don't use mortar when it is 2 °C and falling, says the BDA.
If your block wall holds weight, check your local building rules or ask a builder or engineer.
Questions people ask
How do you estimate mortar for a block wall?
Find the m² (length × height), then multiply by 9.88 for the blocks. Allow about 1 litre of mortar per block. A 6 × 2 m wall takes 119 blocks and about 119 litres.
How much mortar per block?
About 1 litre for a 100 mm block, says Cemex. Blue Circle's figure works out at 0.67 litres and Kilsaran's at 1.25.
How much mortar to lay 100 blocks?
About 100 litres. That is 9 bags of 25 kg mortar mix, at Cemex's 12 blocks a bag.
How much mortar for a concrete block wall?
A 10 m² wall has 99 blocks. At about 1 litre a block, that is about 99 litres, or 9 bags of mortar mix.
How much mortar goes between concrete blocks?
Cemex's figures are for 10 mm joints. Kilsaran's 9.88 blocks per m² matches 10 mm joints too (our sum).
What mortar should I use for a block wall?
Cemex sells M4 mortar for laying bricks and blocks, and M12 for below ground and at DPC level. A 1 : 5 mix of cement and building sand is class M4.
Do thicker blocks need more mortar?
Yes. Blue Circle's figures give about 1 litre for a 150 mm block, against 0.67 for 100 mm. Kilsaran, an Irish maker, gives 1.54–2.22 litres for a 215 mm hollow block. The calculator covers 100 mm blocks only.
Where our numbers come from
- Blocks per m² and mortar per block: Kilsaran calculator
- Mortar per bag: Cemex (25 kg lays 12 blocks or 20 bricks)
- Mortar per brick: Blue Circle Quality Assured Mortar data sheet
- Mortar per block: Kilsaran K-Spread mortar flyer (Ireland)
- Ready-mixed mortar bags: Heidelberg Sand & Cement Mortar data sheet (20 kg lays about 19 bricks)
- Aircrete block weight: H+H Celcon Standard (100 mm = 7 kg)
- Mortar and render mixes, cement and mortar per tonne of sand: MPA sheet 04
- Mortar mixes: Heidelberg Multicem data sheet
- Bricks per tonne: Mineral Products Association Mortar Data Sheet 01
- Sand per 1,000 bricks: Brick Development Association, Mortar for Brickwork
- Mix ratios and 25 kg bags: Heidelberg General Purpose cement data sheet
- Ballast weight: Wickes ballast bulk bag (800 kg covers 9.7 m² at 50 mm)
- Cement price: the middle of 7 prices from 3 UK sellers (25 kg, Oct 2026)
- Bulk bag prices: the middle price from 3 UK sellers each (Wickes, Tippers, Lawsons; Oct 2026)
Made by BuildersCount. Last checked 4 Oct 2026.
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