Mortar Calculator (UK)
Last updated: 12 August 2026
Work out how much mortar you need to lay bricks or blocks, and what to buy: soft sand and cement for site mixing, or bags of ready-mixed mortar. Shows the assumptions so you can check the figures.
How the calculation works
Here is the derivation, because published figures vary by a factor of two. A brick bed joint is 215 × 102.5 × 10mm = 0.00022 m³, and the perpend is 102.5 × 65 × 10mm = 0.00007 m³ — about 0.0003 m³ per brick. A 100mm block works out at roughly 0.00065 m³. We multiply by your unit count and add 25% for waste, which is genuinely high between the spot board, the mixer and what goes off before it is used.
The volume is then split by your mix ratio, with sand converted to tonnes at about 1.5 t/m³ (soft sand) and cement to 25kg bags. Published figures across the web disagree wildly on this, so the assumptions are stated openly here rather than hidden.
Mortar mix ratios (BS EN 998-2 designations)
| Mix | Designation | Where to use |
|---|---|---|
| 1 : 3 | M6 / (ii) | Below DPC, exposed parapets, chimneys |
| 1 : 4 | M4 / (iii) | General external brickwork — the usual choice |
| 1 : 5–6 | M2 / (iv) | Internal walls, soft or reclaimed bricks |
Stronger is not better. Mortar should always be weaker than the brick, so that movement cracks the joint (repointable) rather than the brick face (not).
Sand and cement per 1,000 bricks
At a 4:1 mix with 10mm joints, 1,000 bricks need roughly 0.375 m³ of mortar including waste, which is about 0.45 tonnes of soft sand and 5 bags of cement — or 30 bags of ready-mixed mortar. If you see figures quoting double this elsewhere, check what joint thickness and waste allowance they assumed; most sites do not say, and several appear to double-count the perpend joint.
Frequently asked questions
How much sand and cement do I need for 1000 bricks?
At a 4:1 mix with 10mm joints, about 0.45 tonnes of soft sand and five 25kg bags of cement, which is roughly 0.375 cubic metres of mortar including 25% waste. Figures published elsewhere vary widely because they assume different joint thicknesses and waste allowances.
What is the correct mortar mix for brickwork?
4 parts soft sand to 1 part cement (an M4 mix) is standard for general external brickwork. Use 3:1 below damp-proof course and in exposed positions, and 5:1 or 6:1 for internal walls and soft or reclaimed bricks.
How many bricks does a bag of mortar lay?
A 25kg bag of ready-mixed mortar yields about 0.0125 cubic metres, which lays roughly 33 bricks with 10mm joints once waste is allowed for. Put another way, allow about three bags per 100 bricks.
What sand is used for mortar?
Soft sand, also called building sand, is used for bricklaying mortar. Sharp sand is too coarse and makes the mortar difficult to work. Rendering and floor screeds use different sands again.
Should mortar be stronger than the bricks?
No — mortar should always be weaker than the masonry units. Then any movement cracks the mortar joint, which can be raked out and repointed, rather than cracking the brick faces, which cannot be repaired.
Related calculators
Brick Calculator
How many bricks for your wall — UK sizes, mortar bags and wastage.
Concrete Block Calculator
Blocks, mortar and wall ties for blockwork.
Render Calculator
Sand, cement and coverage for scratch and top coats.
Estimates are for guidance only. Material dimensions, densities and coverage vary by supplier — always confirm quantities before ordering, and consult a qualified professional for structural work.