What this calculator works out
This calculator splits a dry material volume into cement, sand and aggregate according to the ratio you enter.
It starts from a dry volume rather than a finished concrete volume. If you know the size of the slab you are pouring, use the Concrete Mix Calculator instead — it applies the dry volume factor first.
Enter your details
Before you calculate
- The default of 1.54 m³ is the dry volume needed for 1 m³ of finished concrete.
- To work from a finished volume, multiply it by 1.54 before entering it here.
- Ratios are by volume. 1:2:4 means one part cement to two of sharp sand to four of aggregate.
- Use sharp sand for concrete. Building sand is finer and intended for mortar.
A 1:2:4 mix is seven parts in total, so cement is one seventh of the dry volume rather than a quarter or a third. Reading ratios as proportions of the whole rather than as parts is a common source of a mix that comes out far too rich or far too weak.
How this calculator works
A proportional split:
Total parts = cement + sand + aggregate
Each material = dry volume × its parts ÷ total partsThe calculation is purely proportional, so it works for mortar ratios and screed mixes as well as concrete — enter the appropriate ratio and the dry volume you need.
Worked example: 1.54 m³ at 1:2:4
Using the default figures — 1.54 m³ of dry material at a 1:2:4 ratio:
- Total parts: 1 + 2 + 4 = 7
- Cement: 1.54 ÷ 7 = 0.22 m³
- Sand: 0.44 m³
- Aggregate: 0.88 m³
Cement is 14% of the mix by volume, not 25%. Anyone reading the 1 in 1:2:4 as a quarter would order nearly twice the cement needed — and a cement-rich mix is not a stronger one. Excess cement increases shrinkage and cracking rather than strength.
Common mistakes
- Entering a finished concrete volume. Multiply by 1.54 first, or use the Concrete Mix Calculator.
- Reading ratio parts as fractions of the whole. 1:2:4 is one seventh cement, not a quarter.
- Assuming more cement means stronger concrete. Beyond the design ratio it increases cracking.
- Using building sand in concrete. Sharp sand is the correct material.
- Mixing ratios by weight when they are specified by volume.
Frequently asked questions
How do I convert volume to bags?
A 25 kg bag of cement is roughly 0.017 m³ loose, so 0.22 m³ is about 13 bags. Sand and aggregate are usually bought loose by the tonne or in bulk bags; a bulk bag holds roughly 0.5 to 0.6 m³ depending on supplier and material, so 0.88 m³ of aggregate is around one and a half bags.
Does this work for mortar?
Yes. Enter your mortar ratio — commonly 1:4 or 1:5 cement to building sand, sometimes with lime as a third component — and the dry volume required. The proportional arithmetic is identical; only the materials and ratios change.
What is the difference between sharp sand and building sand?
Sharp sand has larger, more angular grains and is used in concrete and screeds where strength matters. Building or soft sand is finer and more workable, which suits mortar and rendering. Substituting one for the other produces either unworkable mortar or weak concrete.
How accurate does the ratio need to be?
Reasonably, but consistency between batches matters more than precision within one. Using the same container for every part and keeping the water content steady produces more uniform concrete than agonising over exact volumes. Variation between batches is what shows up as colour banding and differential cracking.
Is what I enter stored?
No. Volumes and ratios are processed in your browser and never transmitted or retained.
Related tools
References
- Health and Safety Executive — safe handling of cement and control of silica dust
- GOV.UK — building regulations guidance on structural materials
Sources are checked at publication and can change — how I choose and check references.