Concrete Calculator
Concrete quantities come from the dry volume, which is the wet volume multiplied by 1.54 (ACI 318). Split that dry volume by the mix ratio. A 20 x 15 x 0.5 ft slab in M20 (1:1.5:3) needs about 36 cement bags, 62 cu ft of sand and 125 cu ft of aggregate.
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Formula
- Wet volume = Length x Width x Depth x quantity
- Purchase volume = Wet volume x (1 + wastage%)
- Dry volume = Purchase volume x 1.54
- Cement = Dry volume x cement part / total parts, divided by 1.25 cu ft per bag
- Sand = Dry volume x sand part / total parts
- Aggregate = Dry volume x aggregate part / total parts
Worked Example
| Input | Value |
|---|---|
| Slab | 20 x 15 x 0.5 ft |
| Wet volume | 150 cu ft |
| Wastage | 5% |
| Purchase volume | 157.5 cu ft |
| Dry volume | 242.6 cu ft |
| Mix | M20 = 1:1.5:3 |
| Cement | 36 bags |
| Sand | 66.2 cu ft |
| Aggregate | 132.3 cu ft |
| Volume | 5.83 cubic yards |
Frequently Asked Questions
What is the mix ratio for M20 concrete?
M20 is 1 part cement, 1.5 parts sand and 3 parts coarse aggregate by volume, giving a characteristic strength of 20 MPa at 28 days.
Why multiply wet volume by 1.54?
Dry loose materials contain voids that fill with water and compact during mixing. The 1.54 factor converts finished wet volume back to the dry material required, per ACI 318.
How many cubic feet is one bag of cement?
A 50 kg or 94 lb bag of cement is about 1.25 cubic feet loose.
What is the difference between M15, M20 and M25?
M15 is 1:2:4 for non-structural work, M20 is 1:1.5:3 for slabs and footings, and M25 is 1:1:2 for structural columns and beams.
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Estimates only. Quantities follow recognised standards (ACI 318, CRSI, IRC 2021, IS-1077, PBQS); actual site consumption varies with workmanship, wastage and detailing. Confirm with a licensed engineer before ordering.