Concrete Calculator
Work out concrete volume for slabs, beams, columns and footings, with cement bags, sand and aggregate quantities for any nominal mix.
Inputs
Formula
- Wet volume = L × W × D
- Dry volume = wet volume × 1.54
- Cement volume = dry volume × 1 / (a + b + c); bags = cement volume × 1440 / 50
- Sand = dry volume × b / Σ; Aggregate = dry volume × c / Σ
Fresh concrete occupies less space than its dry ingredients because sand fills the voids between aggregate and cement paste fills the voids in sand. The accepted bulking factor is 1.54 for nominal mixes. Ingredient volumes then follow the mix proportions, and cement volume converts to 50 kg bags using a bulk density of 1440 kg/m³.
Step-by-step calculation
Wet volume incl. wastage
L × W × D × (1 + waste)
3.15 m³
Dry volume
wet × 1.54
4.851 m³
Cement volume
dry × 1/5.5
0.882 m³
Cement bags
volume × 1440 / 50
25.4 bags
Sand
dry × 1.5/5.5
1.323 m³
Aggregate
dry × 3/5.5
2.646 m³
Worked example
- Slab 5 m × 4 m × 0.15 m → wet volume = 3.0 m³.
- Dry volume = 3.0 × 1.54 = 4.62 m³; for M20 (Σ = 5.5) cement = 4.62/5.5 = 0.84 m³.
- Cement = 0.84 × 1440 / 50 = 24.2 bags; sand = 1.26 m³; aggregate = 2.52 m³.
Assumptions
- Bulking factor 1.54 for dry volume, cement bulk density 1440 kg/m³, 50 kg bags.
- Nominal volumetric mixes to IS 456; design mixes must come from a mix design report.
Tips
- Order ready-mix in 0.5 m³ steps and always include the wastage allowance.
- One cubic metre of M20 concrete weighs about 2400 kg — check slab loading before pouring.
Warnings
- Nominal mixes are permitted only up to M20 under IS 456. Above that, use a designed mix.
- A water–cement ratio above 0.55 sharply reduces durability and strength.
Standards & references
- IS 456:2000
- IS 10262 (mix design)
- IS 383 (aggregates)
Frequently asked questions
How many cement bags are in 1 m³ of M20 concrete?
About 8.06 bags of 50 kg, i.e. 403 kg of cement, based on a dry volume factor of 1.54 and a 1 : 1.5 : 3 mix.
Why multiply the wet volume by 1.54?
Because dry ingredients bulk up: voids in aggregate and sand are filled during mixing, so 1.54 m³ of dry material yields roughly 1 m³ of compacted concrete.
What water quantity do I need?
Water = w/c ratio × cement mass. At w/c 0.5 with 403 kg of cement you need about 202 litres per cubic metre.
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