MistriCalc → RCC Slab Calculator
RCC Slab Calculator
Estimate the concrete volume, M20 cement, sand and aggregate, and the steel reinforcement for a reinforced cement-concrete slab from its dimensions.
Materials & quantities explainer
Run your numbers first, then read how it works. Start with the calculator below — the example values are pre-filled — then keep scrolling for the method, a worked example and the questions builders ask most. Everything runs in your browser; nothing you type is stored.
Calculator
For an RCC slab, first find the concrete volume = length × width × thickness. Then size the materials from the M20 (1:1.5:3) nominal mix using a dry-volume factor of 1.54, and estimate the reinforcement at about 80 kg of steel per cubic metre of concrete. A 10 × 10 m slab at 150 mm gives 15 m³ of concrete and roughly 1,200 kg of steel.
Key takeaways
- Concrete volume = L × W × thickness (same units).
- Dry volume = wet × 1.54 before splitting into cement/sand/aggregate.
- M20 = 1:1.5:3, so the parts add to 5.5.
- Cement bags = cement volume ÷ 0.0347 (50 kg bag).
- Steel ≈ 80 kg/m³ for residential slabs — confirm with the design.
How the slab quantities are worked out
The slab is a flat box, so its concrete volume is simply length × width × thickness. Because loose dry materials bulk up, we multiply the wet volume by 1.54 to get the dry volume, then divide it in the M20 ratio (1 part cement : 1.5 parts sand : 3 parts aggregate, totalling 5.5). Cement volume is converted to 50 kg bags at 0.0347 m³ each. Steel is estimated separately as a weight per cubic metre of concrete.
The concrete volume is also shown in cubic feet (1 m³ = 35.3147 CFT). The same logic powers the dedicated concrete calculator if you want a different grade.
Worked example: a 10 × 10 m roof slab, 150 mm thick
Concrete volume = 10 × 10 × 0.15 = 15 m³. Dry volume = 15 × 1.54 = 23.1 m³. Cement = 23.1 × (1 ÷ 5.5) = 4.2 m³ ÷ 0.0347 ≈ 121 bags. Sand = 23.1 × (1.5 ÷ 5.5) = 6.3 m³. Aggregate = 23.1 × (3 ÷ 5.5) = 12.6 m³. Steel at 80 kg/m³ = 15 × 80 = 1,200 kg (1.2 tonne). Budget the build with the construction cost calculator.
Nominal concrete mix ratios
| Grade | Ratio (C:S:A) | Sum of parts | Typical use |
|---|---|---|---|
| M15 | 1 : 2 : 4 | 7 | PCC, levelling |
| M20 | 1 : 1.5 : 3 | 5.5 | Residential slabs (default) |
| M25 | 1 : 1 : 2 | 4 | Slabs, columns (durable) |
Nominal mixes; design mixes are proportioned by weight following standard trade practice.
Frequently asked questions
How do I calculate concrete volume for a slab?
Length × width × thickness in the same units. A 10 × 10 m slab at 150 mm (0.15 m) gives 10 × 10 × 0.15 = 15 m³.
How much steel is needed in an RCC slab?
About 80 kg per m³ of concrete is a residential rule of thumb, so a 15 m³ slab is roughly 1,200 kg. The real figure comes from the structural design.
How many cement bags for an M20 slab?
Dry volume = wet × 1.54; cement = dry ÷ 5.5. For 15 m³, cement ≈ 4.2 m³ ÷ 0.0347 ≈ 121 bags.
What grade of concrete is used for slabs?
M20 (1:1.5:3) is a common minimum; many codes specify M25+. This tool defaults to M20 — check your drawings.
Why multiply wet volume by 1.54 for dry volume?
Loose dry materials contain voids and bulk up, needing about 54% more volume than the final wet concrete. The 1.54 factor accounts for this.
How thick should an RCC slab be?
Residential roof slabs are commonly 100–150 mm. Thickness is a structural decision based on span and load — use the value from your engineer's drawings.
Nominal mix ratios (M20 = 1:1.5:3) and the 1.54 dry-volume factor follow standard trade practice. The cement-bag figure uses 50 kg = 0.0347 m³. The ~80 kg/m³ steel figure is a widely used residential rule of thumb, not a code value — actual reinforcement follows the structural design.
Last reviewed 2026-06-14