Mortar Calculator & Mix Ratios

How to Use the Mortar Calculator

Start with the volume of finished mortar you need — the wet material that actually ends up in the joints. The easiest source is the brick & block calculator: enter your wall, take its mortar row, and type that figure here. Working by hand, the joint volume approach gives the same answer: joint cross-section (roughly unit depth × joint thickness, allowing for perpends) times total joint length. Typical trade figures land near 0.026 m³ of mortar per m² of metric brickwork and 0.012 m³ per m² of blockwork at a 10 mm joint.

The ratio is cement to sand by volume. Typical practice — and it does vary by country, tradition and the units being laid — runs roughly like this: 1:3 for very strong, exposed work and below damp-proof course in some traditions; 1:4 for general-purpose strong work; 1:5 for everyday bricklaying; 1:6 for internal lightweight blockwork, where a strong mix is harder than the block itself and cracks across the joints. Modern practice replaces the lime that older mixes used with a liquid plasticiser at around a capful per batch, which makes 1:5 and 1:6 mixes workable without lime. Block manufacturers publish recommended mortars for their products — follow those where they exist.

Why the arithmetic is not just "volume divided by parts": dry ingredients and finished mortar occupy different volumes. When you mix cement and sand with water, the cement paste fills the voids between sand grains, and the finished mortar comes out at roughly three-quarters of the dry ingredients' bulk. So one cubic metre of finished mortar needs about 1.33 m³ of dry ingredients — a standard trade factor, and the source of the constant in the formula below. The waste factor then covers what stays in the mixer, the bucket and the wheelbarrow; 10% is a typical default.

Mass follows from two standard constants: cement at 1440 kg/m³ and sand at 1600 kg/m³, both loose bulk densities the trade uses for ordering. One cubic metre of mortar at 1:4 therefore works out to about 383 kg of cement — call it 16 bags of 25 kg — and about 0.44 m³ of sand. A richer 1:3 mix takes about 479 kg (roughly 20 bags) and a leaner 1:6 about 274 kg (11 bags); the sand share moves the other way. Your actual cement may come in 94 lb (42.6 kg) sacks if you are in the US — divide the kilogram figure accordingly.

Two honest limits. First, this is a cement-and-sand tool: lime mortars for older buildings are a different material, gauged on putty or hydraulic lime, and the supplier's guidance beats any generic calculator. Second, mortar is not concrete — it has no coarse aggregate, and using it as one is a false saving. For slabs and footings, see the mortar mix ratios guide for the full background, or the concrete mix ratios guide for the structural stuff.

The Math Behind the Calculator

Three steps: inflate the wet volume to dry, split it by ratio parts, then convert volumes to masses with standard bulk densities.

The constants are the standard ones the trade uses: 1.33 for the dry-to-wet bulk factor, 1440 kg/m³ for loose bulk cement, 1600 kg/m³ for loose dry sand. Actual densities vary a little by grading and moisture, and bag weights by market — the constants get you within a bag on realistic volumes, which is closer than most batches get mixed.

Worked Example: 0.25 m³ at 1:4

A garden wall from the brick calculator needs 0.52 m³ of mortar; halve that for a shorter wing wall and you have 0.25 m³ to mix at 1:4, allowing 10% waste. Enter volume 0.25, ratio 1:4, waste 10.

1. Dry volume: 0.25 × 1.33 × 1.10 = 0.366 m³.
2. Parts: 1 + 4 = 5.
3. Cement volume: 0.366 ÷ 5 = 0.0732 m³ → × 1440 ≈ 105.4 kg → 105.4 ÷ 25 = 4.2 → 5 bags.
4. Sand volume: 0.366 × 4 ÷ 5 = 0.293 m³.
5. Sand mass: 0.293 × 1600 ≈ 468 kg — about nineteen 25 kg bags of dry sand, or roughly half a typical 850 kg bulk bag.

Field check: one cubic metre of 1:4 mortar by the same math is 383 kg of cement — 16 bags — and 0.44 m³ of sand, which matches the trade shorthand that a bulk bag of sand and a dozen-plus bags of cement keep a bricklayer busy for a good stretch. Scale that to 0.25 m³ and you are back to 5 bags and half a bulk bag of sand: consistent.

Assumptions & Limitations

Every number above rests on these constants. If your project differs, change the matching input — or read the linked guide for the full reasoning.

• Dry factor 1.33 — dry ingredients bulk about a third more than finished wet mortar (standard trade constant)
• Cement 1440 kg/m³ and sand 1600 kg/m³ loose bulk densities — standard constants; actual loads vary with grading and moisture
• Cement bags are 25 kg — divide the kg row for other bag sizes (a US 94 lb sack is about 42.6 kg)
• Waste is applied to the dry volume before splitting by ratio
• Sand figures are dry loose volume — damp sand bulks and occupies more space per kg
• Quantities only — no plasticiser, lime, pigments or mixing water scheduled