Hong Life CalculatorsUnderstanding the mortar and plaster mix calculator — concept and principle
This calculator converts a volume of wet mortar or plaster into the amount of cement (in kg and 25 kg bags) and sand (in m³) for a cement : sand mix ratio by volume, such as 1 : 4. Use it to order materials for bricklaying, rendering, plastering or screeding.
How it is calculated
- Dry volume = wet volume × 1.33.
- One part = dry volume ÷ (cement parts + sand parts).
- Cement (kg) = one part × cement parts × 1,440 kg/m³.
- Cement bags = cement kg ÷ 25, rounded up.
- Sand (m³) = one part × sand parts.
Volumes are in cubic metres; the ratio is by volume, as it is usually specified on site.
The principle
Dry cement and sand contain a lot of air between their grains. When water is added and the mix is worked, the fine cement particles fill the gaps between sand grains and the air is driven out, so the wet mortar takes up noticeably less space than the loose dry ingredients. Multiplying by about 1.33 (a third more) is a common rule of thumb to restore the dry volume needed. That dry volume is then shared out by the parts of the ratio. Cement is converted to mass with its loose bulk density of about 1,440 kg/m³, the figure traditionally used in quantity estimating.
Worked example
- Inputs: wet volume 0.5 m³, mix 1 : 4.
- Dry volume = 0.5 × 1.33 = 0.665 m³.
- One part = 0.665 ÷ 5 = 0.133 m³.
- Cement = 0.133 × 1 × 1,440 = 191.52 kg; bags = 191.52 ÷ 25 = 7.66, rounded up to 8 bags.
- Sand = 0.133 × 4 = 0.532 m³.
| Sand part | Cement (kg) |
|---|---|
| 1 | 478.8 |
| 2 | 319.2 |
| 3 | 239.4 |
| 4 | 191.52 |
| 5 | 159.6 |
| 6 | 136.8 |
| 8 | 106.4 |
How to read the result
Richer mixes such as 1 : 3 are stronger and less permeable and are often used for renders or below-ground work; leaner mixes such as 1 : 5 or 1 : 6 are more workable and more forgiving of movement, often with lime or plasticiser added. Sand is sold by the tonne in many places; damp sand bulks up, so a given volume weighs less than dry sharp sand would suggest. Order a little extra sand, as some is lost on site.
Common mistakes
- Entering the dry volume as the wet volume, which counts the 1.33 factor twice.
- Using shovels as "parts" for cement and sand when shovels of damp sand hold a different amount.
- Choosing a mix stronger than the bricks, which can push cracking into the masonry units.
Frequently asked questions
What does a 1 : 4 mix mean?
One volume of cement to four volumes of sand, measured loose and dry, before water is added.
Why is the factor 1.33 and not something else?
It is an estimating rule of thumb. Other references use slightly different factors depending on the sand and on whether site wastage is included.
How much water do I add?
Enough to reach a workable consistency; too much water weakens mortar and increases shrinkage.
Sources: BS EN 998-1 and BS EN 998-2: Specification for mortar for masonry (rendering/plastering and masonry mortar), BSI/CEN · PD 6678: Guide to the specification of masonry mortar, BSI