What the Cement and Sand Ratio Calculator does
This calculator converts a mortar or plaster mix such as 1:4 or 1:6 into the number of cement bags and the volume of sand to buy. Give it the area and thickness of plaster, the size of a brick wall, or a wet mortar volume; it applies the dry-volume factor - the extra dry material needed because sand and cement shrink when water fills the gaps - and splits the result by the ratio.
It is for mortar and plaster, not structural concrete. For slabs, footings and columns, use the Concrete Calculator.
How to use it
- Choose the work type. For plastering, enter the area and thickness. For brickwork, enter the wall size, brick size and thickness; the mortar volume comes from the gap between bricks. Or enter a wet mortar volume directly.
- Choose the mix ratio of cement to sand by volume. Common choices are 1:4 for internal plaster and ceilings, 1:5 for render, and 1:6 for general brickwork; use custom parts for anything else.
- Keep the dry-volume factor at 1.33 unless your specification says otherwise (1.27 and 1.35 are also used).
- Choose the bag size, add a wastage allowance if you want one, and optionally a water-cement ratio for a water estimate.
Reading the results
Bags to buy is the exact cement weight divided by the bag size, rounded up. The exact figure is shown too, so you can pool several jobs before rounding.
Sand is given in cubic metres and cubic feet because it is sold by volume in some places and by weight in others; damp sand bulks up, so buy a little more than the figure if yours is wet.
The working table shows each step, so you can check it against your specification or a contractor's estimate.
Worked example: plastering 100 m² at 12 mm with 1:4
100 m² of wall plastered 12 mm thick needs 100 x 0.012 = 1.2 m³ of wet plaster. With a dry-volume factor of 1.33, the dry materials measure 1.2 x 1.33 = 1.596 m³.
A 1:4 mix has 5 parts, so cement is 1.596 / 5 = 0.3192 m³ and sand is 1.596 x 4 / 5 = 1.2768 m³. At 1,440 kg/m³, the cement weighs 0.3192 x 1,440 = 459.6 kg, which is 9.19 bags of 50 kg - buy 10.
The sand, 1.277 m³, is about 45.1 cubic feet. For a 1:6 brickwork mortar the cement share would drop from one fifth to one seventh of the dry volume.
Formulas and scoring rules
- Wet volume
plaster: area x thickness; brickwork: wall volume - brick volume (from the brick size and joint); or entered directly- Dry volume
dry = wet x dry-volume factor (default 1.33)- Split by ratio
cement = dry x c / (c + s); sand = dry x s / (c + s)- Cement bags
bags = cement m³ x 1,440 kg/m³ / bag kg1,440 kg/m³ is the loose bulk density used for volume-batched mixes; a 50 kg bag is about 0.0347 m³.- Water (optional)
litres = cement kg x water-cement ratio
How this differs from the Concrete Calculator
The Concrete Calculator works out the volume of concrete for slabs, footings and columns. This calculator is for mortar and plaster: it starts from a mix ratio such as 1:4 or 1:6 for brickwork or plastering, applies the dry-volume factor to the wet volume, and converts the result into cement bags and sand. There is no coarse aggregate here, and the inputs - plaster thickness, brick and joint size - are ones the concrete tool does not ask for.
Limitations: what the result does not prove
- Mixes here are by volume, as batched on most small sites. Specifications that give mortar by designation, strength class or weight need converting first.
- The dry-volume factor is a convention covering voids and some bulking; very wet sand, uneven walls and thick dubbing coats use more.
- Lime and plasticisers change proportions; this calculator handles cement and sand only.
- The right mix depends on the bricks, exposure and structural role. Follow the specification, your engineer or local code rather than choosing a ratio from this page.
Privacy: where your data goes
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Standards and sources
- Portland Cement Association - Masonry mortar
- US Army Corps of Engineers / UFGS 04 20 00 - Unit masonry (mortar proportions)
Frequently asked questions
How many bags of cement do I need for 100 m² of plaster?
At 12 mm thick and a 1:4 mix, about 9.2 bags of 50 kg - so 10 bags - plus about 1.28 m³ of sand. At 1:6 it drops to about 6.6 bags. Thicker coats scale in proportion: 20 mm needs two thirds more.
What is the dry-volume factor and why 1.33?
When water is added, it fills the air gaps between sand grains and the mix shrinks, so you need more dry material than the finished volume. Practice uses factors from about 1.27 to 1.35; 1.33 (a third extra) is the most common, and the calculator lets you change it.
What is the best cement to sand ratio for brickwork?
General brickwork often uses 1:5 or 1:6, with richer 1:3 or 1:4 mixes for load-bearing walls, below damp-proof level or exposed positions. The right mix depends on the bricks and the specification, so check it before you buy.
How much sand do I need for a bag of cement?
In a 1:6 mix, one 50 kg bag (about 0.0347 m³) goes with six times its volume of sand, about 0.21 m³. In a 1:4 mix it is about 0.14 m³. The ratio is by volume, not weight.
Why use 1,440 kg per cubic metre for cement?
It is the conventional loose bulk density of cement used when mixes are measured by volume. It gives about 0.0347 m³ per 50 kg bag, the figure most site estimates use.
Can I use this for concrete?
No. Concrete has coarse aggregate and different dry-volume factors, and it is sized by the volume of the slab, footing or column. Use the Concrete Calculator for that.
Last reviewed by the A2Z.Tools team against the sources listed above.
