RCC Slab Calculator
Enter the slab size, thickness, mix and the bar size and spacing from your drawings. You get the concrete to order, cement bags, sand and aggregate for site mixing, the number of rebar lengths to buy and the total steel weight.
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Concrete = length × width × thickness + waste. Steel = bars in each direction × bar length × weight per foot. A 20 × 15 ft slab, 5 in thick, with #4 bars at 6 in and 8 in is 4.86 yd³ with 5% waste. That is 5 yd³ of ready-mix or 30 bags of 50 kg cement for an M20 site mix, plus 58 bars of 20 ft #4 (764 lb of steel).
How to use this RCC slab calculator
- Enter the slab length, width and thickness from your drawings.
- Choose the concrete mix. Nominal mixes are shown. If you buy ready-mix, order by the strength specified instead.
- Enter the bar size and spacing in both directions, the cover and whether there is one layer or two.
- Set the extra steel for laps, bends and offcuts. 10% suits most simple slabs.
- Read the order: concrete, cement bags, sand, aggregate, bar count and steel weight.
How to calculate concrete and steel for a slab
bars one way = floor((W − 2 × cover) ÷ spacing) + 1, each L − 2 × cover long
steel weight = total bar length × (1 + laps) × weight per foot
For site mixing, multiply the wet volume by 1.54 for dry materials and split it by the mix ratio. Cement is taken at 1,440 kg/m³.
| Bar | Diameter | Weight | Stock length |
|---|---|---|---|
| #3 | ⅜ in (9.5 mm) | 0.376 lb/ft | 20 ft |
| #4 | ½ in (12.7 mm) | 0.668 lb/ft | 20 ft |
| #5 | ⅝ in (15.9 mm) | 1.043 lb/ft | 20 ft |
| 8 mm | 8 mm | 0.395 kg/m | 12 m |
| 10 mm | 10 mm | 0.617 kg/m | 12 m |
| 12 mm | 12 mm | 0.888 kg/m | 12 m |
| 16 mm | 16 mm | 1.580 kg/m | 12 m |
Metric bar weight is d² ÷ 162 kg/m. Stock lengths are typical; longer bars are available from many suppliers. A quick sense check: 1% steel by volume is 78.5 kg per m³ of concrete.
Cement, sand and aggregate per cubic meter
| Nominal mix | Cement | 50 kg bags | Sand | Aggregate |
|---|---|---|---|---|
| M25 (1 : 1 : 2) | 554 kg | 11.1 | 0.39 m³ | 0.77 m³ |
| M20 (1 : 1.5 : 3) | 403 kg | 8.1 | 0.42 m³ | 0.84 m³ |
| M15 (1 : 2 : 4) | 317 kg | 6.3 | 0.44 m³ | 0.88 m³ |
Per cubic meter of finished concrete, before waste. Design mixes from a lab or ready-mix plant usually need less cement than nominal mixes for the same grade.
Worked example
- Slab 20 × 15 ft, 5 in thick
- 125 ft³ = 3.54 m³
- Add 5% waste
- 3.72 m³ = 4.86 yd³ (order 5 yd³)
- M20 cement (× 1.54 ÷ 5.5 × 1,440)
- 1,499 kg = 30 bags of 50 kg
- #4 bars at 6 in, running lengthwise
- 30 bars × 19.8 ft
- #4 bars at 8 in, running widthwise
- 30 bars × 14.8 ft
- Total + 10% for laps
- 1,144 ft = 58 bars of 20 ft, 764 lb
Frequently asked questions
How do I calculate materials for an RCC slab?
L × W × thickness plus waste for concrete; × 1.54 and split by the ratio for cement, sand and aggregate. For steel, count bars each way from the spacing, multiply by their lengths and by the bar weight.
How do I calculate the number of bars in a slab?
Take the cover off both edges, divide by the spacing, round down and add one. A 15 ft slab with 1 in cover and bars at 6 in needs 30 bars running lengthwise.
How much does rebar weigh per foot?
#3 is 0.376 lb/ft, #4 is 0.668 lb/ft and #5 is 1.043 lb/ft. Metric bars weigh d² ÷ 162 kg/m: 0.395 for 8 mm, 0.617 for 10 mm, 0.888 for 12 mm.
How many bags of cement for an RCC slab?
For M20 nominal (1:1.5:3), about 8.1 bags of 50 kg per m³. A 20 × 15 ft slab 5 in thick with 5% waste needs about 30 bags.
How thick should an RCC slab be?
It depends on span, loads and supports, so thickness and reinforcement must come from a structural engineer or your local code. This calculator only turns the design into quantities.
How much extra steel should I add for laps?
10% is a common allowance for laps, bends, chairs and offcuts on simple slabs. Use the lap lengths on your drawings for an exact figure.
What this calculator assumes
- A rectangular slab with straight bars in two directions at the spacing you enter, held back from the edges by the cover.
- Laps, bends, chairs and offcuts are covered by the extra steel percentage. Beams, drop panels and extra bars at openings are not included.
- Site-mix quantities use a 1.54 dry volume factor, cement at 1,440 kg/m³ and typical sand and aggregate densities of 1,600 and 1,550 kg/m³.
- This is a quantity estimate, not a design. Slab thickness, concrete grade, bar size and spacing must come from a structural engineer and meet your local code.