How to Calculate Cement Bags for Any Construction Job
Whether you're pouring a slab, a footing, or a small concrete pad, the underlying calculation for cement, sand, and aggregate is always the same. Here's exactly how it works — the same formula behind our free concrete calculator.
Step 1: Work out the wet volume
Wet volume is simply the volume of concrete you need, in cubic feet or cubic metres — length × width × thickness. If you're pouring a 10 ft × 10 ft slab at 5 inches (0.42 ft) thick, that's 10 × 10 × 0.42 = 42 cft of wet concrete.
Step 2: Convert to dry volume
Concrete ingredients (cement, sand, aggregate) take up more space loose than they do once mixed and set, because mixing removes air gaps. The standard multiplier is 1.54 — so dry volume = wet volume × 1.54.
Step 3: Apply your mix ratio
The mix ratio tells you how the dry volume splits between cement, sand, and aggregate. Common nominal mixes:
- M15 — 1 : 2 : 4 (cement : sand : aggregate)
- M20 — 1 : 1.5 : 3 (the most common for residential slabs and footings)
- M25 — 1 : 1 : 2
Add the parts together (for M20: 1 + 1.5 + 3 = 5.5). Each material's volume is its own part divided by that total, multiplied by the dry volume.
Step 4: Convert cement volume to bags
Cement weighs approximately 1,440 kg per cubic metre. Once you have the cement volume in cubic metres, multiply by 1,440 to get kilograms, then divide by 50 (the standard bag size in India) to get the number of bags.
Worked example
For 100 cft of M20 concrete (1 : 1.5 : 3):
- Wet volume: 100 cft = 2.83 cum
- Dry volume: 2.83 × 1.54 = 4.36 cum
- Cement: 4.36 × (1/5.5) = 0.79 cum → 1,138 kg → ~22.8 bags
- Sand: 4.36 × (1.5/5.5) = 1.19 cum (~42 cft)
- Aggregate: 4.36 × (3/5.5) = 2.38 cum (~84 cft)
Skip the manual maths
Our free calculator does all of this instantly — just enter your volume and mix ratio and it works out cement bags, sand, aggregate, and water quantity for you. It also covers brickwork, plaster, tiling, paint, steel weight, and water tank sizing.