Estimating concrete for a slab has two parts: the volume of concrete the slab needs, and the materials to make that volume on site. Ready-mix concrete is ordered by volume; site-mixed concrete needs the materials.
Step 1: wet volume
Convert everything to metres first. Slab thickness is often given in inches or millimetres: 5 in = 0.127 m, 125 mm = 0.125 m.
WORKED EXAMPLE
A roof slab 12 × 10 ft, 5 inches thick.12 ft = 3.658 m · 10 ft = 3.048 m · 5 in = 0.127 m
Volume = 3.658 × 3.048 × 0.127 = 1.416 m³ (≈ 50 cft)
For beams and footings the formula is the same. For a round column, volume = π × radius² × height.
Step 2: dry volume
Cement, sand and aggregate occupy more volume dry than they do once mixed with water and compacted, because water fills the voids between particles. Estimators therefore scale the wet volume up:
1.54 is the most commonly used factor; you will also see figures between about 1.52 and 1.57. It is an estimating convention, not a property of any particular mix.
Step 3: split by the mix ratio
For a nominal mix of cement : sand : aggregate = 1 : a : b, the sum of parts is 1 + a + b.
| Grade (nominal) | Ratio | Sum of parts |
|---|---|---|
| M10 | 1 : 3 : 6 | 10 |
| M15 | 1 : 2 : 4 | 7 |
| M20 | 1 : 1.5 : 3 | 5.5 |
Cement (bags) = cement m³ × 1440 ÷ 50
Sand (m³) = dry volume × a ÷ sum
Aggregate (m³) = dry volume × b ÷ sum
1,440 kg/m³ is the bulk density used for cement in estimating, and a bag holds 50 kg.
WORKED EXAMPLE, CONTINUED · M20
Dry volume = 1.416 × 1.54 = 2.181 m³Cement = 2.181 ÷ 5.5 = 0.397 m³ → 0.397 × 1440 ÷ 50 = 11.4 bags (buy 12)
Sand = 2.181 × 1.5 ÷ 5.5 = 0.595 m³ (≈ 21 cft)
Aggregate = 2.181 × 3 ÷ 5.5 = 1.190 m³ (≈ 42 cft)
Nominal mix or design mix?
IS 456 allows nominal mix concrete for grades up to M20. For M25 and above, concrete should be designed (proportioned) for the materials actually used, following IS 10262, and the design will specify its own quantities. The nominal ratios above are for estimating and for smaller works where nominal mixes are acceptable.
Allowances
Add a small allowance for spillage, uneven formwork and over-excavation, often 2–5% depending on the site. When ordering ready-mix, round up to the supplier's minimum increment.
Do it on your phone
The NANDIX concrete calculator does these steps for slabs, beams, footings and round columns, in feet or metres, and shows every assumption so you can change it.
