A 5 m × 4 m patio slab, 15 cm thick
Wet volume with 5% waste is about 3.15 m³, needing roughly 24 bags of 25 kg cement with a 1:2:4 mix.
Concrete volume & bag dosage calculator
Result
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How it works
The wet (poured) volume is simply length × width × thickness, plus a waste allowance for spillage and an uneven subgrade. To find the dry material quantities, that volume is multiplied by 1.54 — the standard "dry volume factor" that accounts for the air gaps between dry cement, sand and gravel particles before they are mixed with water and compact together.
The dry volume is then split between cement, sand and gravel according to the chosen mix ratio (by volume), and each share is converted to a weight using its typical density, so cement bags can be counted.
Formulas used
Concrete volume
V = L × l × e × (1 + waste%)
- L
- Length
- l
- Width
- e
- Thickness
Dry material volume
V_dry = V × 1.54
The constant 1.54 is an average value used across the construction industry; it can vary slightly with material moisture and grading.
Cement quantity
V_cement = V_dry × (cement parts / total parts)
Worked examples
A structural footing, 1:1.5:3 mix
A richer mix increases the cement share relative to sand and gravel, raising strength at a higher material cost.
Assumptions and limits
- Materials are ordinary Portland cement, natural sand and coarse gravel at the densities stated above.
- The 1.54 dry volume factor is an industry average, not a lab-measured value for a specific material batch.
- This does not verify structural adequacy — it estimates material quantities only.
Frequently asked questions
Why is the dry volume larger than the poured volume?
Dry cement, sand and gravel are made of separate particles with air gaps between them. When mixed with water and compacted, those gaps close and the material settles into a smaller volume — the fresh concrete you actually pour. Buying dry materials on the wet volume alone would fall short.
Which mix ratio should I use?
A richer mix (1:1.5:3) suits structural elements like beams, columns and footings. General slabs and paths commonly use 1:2:4. A leaner 1:3:6 mix suits mass, non-structural fill such as blinding concrete. For any load-bearing structure, follow the ratio specified by a structural engineer or the applicable building code rather than a rule of thumb.
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