What this calculator works out
This calculator takes a finished concrete volume and a mix ratio, applies the standard dry volume factor, and splits the result into cement, sand and aggregate.
The factor is the part most people miss. Dry materials do not add up to the volume of concrete they produce — water fills the voids between particles, so you need roughly 54% more dry volume than the finished result.
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Before you mix
- Choose a ratio suited to the job. 1:2:4 is the general-purpose default; 1:1.5:3 is stronger for structural work; 1:3:6 is a weak mix for filling and bedding.
- Ratios are by volume, measured with a consistent container such as a bucket, not by weight.
- Aggregate size matters. 20 mm is standard for slabs; 10 mm suits thin sections and areas with reinforcement.
- Cement is sold by weight. A 25 kg bag is roughly 0.017 m³ loose.
Dry sand and aggregate contain air between the particles. When mixed with water and cement, that space fills with paste and the total volume shrinks. The 1.54 factor used here is the standard construction allowance for that consolidation, and it is why measuring your dry ingredients to the finished volume leaves you short.
How this calculator works
A dry volume, then a proportional split:
Dry volume = finished volume × 1.54
Total parts = cement + sand + aggregate
Cement = dry volume × cement parts ÷ total parts
Sand and aggregate follow the same proportionThe 1.54 factor is an accepted rule of thumb rather than a measured constant. It varies a little with aggregate grading and how tightly the materials are compacted.
Water is deliberately not calculated. The right amount depends on the aggregate's existing moisture and the workability you need, and too much water is the single most common cause of weak concrete.
Worked example: 1 m³ at 1:2:4
Using the default figures — 1 m³ of finished concrete at a 1:2:4 ratio:
- Dry volume: 1 × 1.54 = 1.54 m³
- Total parts: 7
- Cement: 1.54 × 1 ÷ 7 = 0.22 m³, roughly 13 bags of 25 kg
- Sand: 0.44 m³
- Aggregate: 0.88 m³
In bucket terms, that is one bucket of cement to two of sand to four of aggregate, repeated until you have mixed enough. Working in whole buckets is far more reliable than trying to measure cubic metres on site, and consistency between batches matters more than precision within one.
Common mistakes
- Measuring dry materials to the finished volume. You need about 54% more.
- Adding water to make mixing easier. Excess water is the main cause of weak concrete.
- Mixing ratios by weight. These are volume ratios; use a consistent container.
- Using building sand instead of sharp sand. Building sand is for mortar; concrete needs sharp sand.
- Handling cement without protection. Wet cement is strongly alkaline and causes serious burns.
Frequently asked questions
Which mix ratio should I use?
1:2:4 cement to sharp sand to aggregate is the standard general-purpose mix, suitable for paths, shed bases and non-structural slabs. 1:1.5:3 is stronger and used where more load is expected. 1:3:6 is a weak fill mix. For anything structural, the specification should come from a designer rather than a rule of thumb.
How much water do I add?
As little as will produce a workable mix — typically around half the weight of the cement, but the aggregate's own moisture content changes it. Add water gradually and stop when the mix holds its shape on a shovel without being sloppy. Every extra litre reduces the final strength, and the loss is permanent.
Can I mix concrete by hand?
For small quantities, yes, on a board or in a barrow. Beyond about a quarter of a cubic metre it becomes genuinely hard work and consistency suffers between batches. A mixer can be hired inexpensively and pays for itself in both effort and quality on anything larger.
What safety precautions do I need?
Wet cement is caustic and causes chemical burns that can be severe, often without immediate pain — waterproof gloves, long sleeves and eye protection are essential, and any contact should be washed off immediately. Cutting or grinding cured concrete releases silica dust, which requires water suppression or extraction and a suitable mask.
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 — cement burns, silica dust control and personal protective equipment
- GOV.UK — building regulations guidance on concrete and foundations
Sources are checked at publication and can change — how I choose and check references.