Mortar Calculator
Convert a mortar volume into cement, sand and water quantities for a given mix ratio.
Formula
The wet volume is increased by the dry volume allowance, then split between cement and sand in the mix ratio.
- Wet volume (m³) = Required volume × (1 + wastage % ÷ 100)
- Dry volume (m³) = Wet volume × (1 + dry allowance % ÷ 100)
- Cement volume (m³) = Dry volume × cement parts ÷ total parts
- Sand volume (m³) = Dry volume × sand parts ÷ total parts
- Cement weight (kg) = Cement volume × Bulk density
- Bags = ceiling( Cement weight ÷ Bag weight )
Worked example
1 m³ of wet 1:4 mortar with a 33% dry allowance, 10% wastage and 50 kg bags.
- Wet volume = 1 × 1.10 = 1.10 m³
- Dry volume = 1.10 × 1.33 = 1.463 m³
- Cement = 1.463 × 1/5 = 0.293 m³
- Sand = 1.463 × 4/5 = 1.170 m³
- Cement weight = 0.293 × 1440 = 421 kg
- Bags = 421 ÷ 50 = 8.4, so 9 bags
9 bags of cement and about 1.17 m³ of sand.
Assumptions and limitations
- Mix ratios are by volume, which is standard site practice for mortar. Batching by weight gives more consistent results but is less common on small jobs.
- The dry volume allowance of around 33% is a long-standing site convention, not a laboratory value. It varies with sand grading and bulking.
- Cement bulk density of 1440 kg/m³ is the loose bulk density of bagged cement, not the specific gravity of the cement powder.
- Sand is assumed at normal site moisture. Very wet sand bulks significantly and will change the volume you actually need.
What this calculator does not do
- Does not select the mix. Which ratio is appropriate depends on the masonry unit, exposure and the specification for the job.
- Does not calculate how much mortar a given wall needs. Enter the wet mortar volume, or take it from the brick calculator, which works it out from the wall.
- Does not cover admixtures, plasticisers, pigments or pre-mixed silo mortar, all of which change the proportions.
Practical tips
- Sand bulking is real and often ignored. Damp sand can occupy 20–30% more volume than the same sand dry, which throws off volume batching.
- Use a gauge box rather than a shovel count. Shovel-batching drifts through the day and produces inconsistent mortar strength and colour.
- Do not mix more mortar than can be used within its working life. Retempering a stiffening mix with extra water weakens it.
Frequently asked questions
How much cement do I need for 1 m³ of mortar?
For a 1:4 mix with a 33% dry allowance, about 0.27 m³ of cement, which is roughly 380 kg or 8 bags of 50 kg — before wastage. The calculator adjusts for whatever ratio and bag size you use.
What does a 1:4 mix mean?
One part cement to four parts sand, measured by volume. It is a mid-strength general-purpose mortar. Richer mixes like 1:3 are stronger, leaner mixes like 1:6 are weaker and more workable.
Why multiply by 1.33 for dry volume?
Because dry sand and cement lose volume when mixed with water — the cement paste fills the voids between the sand grains. Adding roughly a third is the standard site allowance for that loss.
How much water should I add?
That is judged on site by workability, not calculated in advance. The sand already carries moisture that varies through the day, so a fixed water figure would be misleading.
Related calculators
- Brick & Block Calculator Calculate how many bricks or blocks a wall needs, including walls more than one unit thick, along with the mortar between them.
- Plaster Calculator Calculate plaster or render volume for a wall, and the cement and sand it needs.
- Concrete Bags Calculator Convert a concrete volume into the number of pre-mixed bags needed, for small pours where ready-mix is not practical.
- Concrete Volume Calculator Work out the volume of concrete needed for a rectangular pour, with an allowance for wastage.
Estimating tool, not a design. This calculator works out quantities from the dimensions you enter. It does not size any structural member, verify a design, or confirm compliance with any building code. Check every result against your project specification and applicable local standards, and involve a qualified engineer where the work requires one.