How Much Concrete, Cement and Steel for a Roof Slab? A Worked Example

Short answer: a 12 m × 10 m roof slab, 125 mm thick, holds 15.0 m³ of concrete. With 3% wastage and M20 nominal mix (1 : 1.5 : 3), it needs about 125 bags of cement, 6.5 m³ (229 cft) of sand and 13.0 m³ (458 cft) of 20 mm aggregate. Steel for the slab alone is typically about 0.8–1.2 tonnes by thumb rule; the actual figure comes from the bar bending schedule.

Below is the full calculation, step by step, so you can repeat it for your own slab size.

Data and assumptions

The slab is taken as one plain rectangle with no openings, sunken portions or drops. Beams below the slab are dealt with separately (see further down).

Step 1: Net concrete volume

Volume = length × breadth × thickness = 12.0 × 10.0 × 0.125 = 15.0 m³ (about 530 cft).

Deduct openings larger than the limit in your measurement specification (staircase cut-outs, shafts). Under IS 1200 practice, small openings are usually not deducted; follow your tender's mode of measurement.

Step 2: Add wastage

15.0 × 1.03 = 15.45 m³ of wet concrete to be produced. Wastage covers spillage, concrete left in the mixer and pans, slight extra thickness and uneven shuttering.

Step 3: Dry volume of materials

15.45 × 1.54 = 23.79 m³ of dry materials. Sum of ratio = 1 + 1.5 + 3 = 5.5.

Step 4: Cement

• Cement volume = 23.79 × 1/5.5 = 4.326 m³
• Cement mass = 4.326 × 1440 = 6229 kg
• Bags = 6229 / 50 = 124.6 → 125 bags

Step 5: Sand and aggregate

• Sand = 23.79 × 1.5/5.5 = 6.49 m³ = 6.49 × 35.31 = 229 cft
• 20 mm aggregate = 23.79 × 3/5.5 = 12.98 m³ = 12.98 × 35.31 = 458 cft

Stock a little more sand than calculated, because damp sand bulks and some is lost in handling. If you batch by volume, correct the sand box for bulking.

Step 6: Mixing water

Water = 0.50 × 6229 = about 3115 litres of mixing water. This excludes water for wetting shuttering, washing the mixer and curing. A 125 mm slab cured by ponding for at least 7 days (longer for blended cements) uses far more water than the mix itself; see curing of concrete.

Summary of materials

Step 7: Steel by thumb rule, and a quick cross-check

For estimating before the bar bending schedule is ready, slab steel is often taken as 0.7–1.0% of the concrete volume. This is a rough rule that varies with span, loading and design; it is not a code value.

• At 1.0%: 15.0 × 0.010 × 7850 = 1178 kg (about 78.5 kg/m³)
• At 0.7%: 15.0 × 0.007 × 7850 = 824 kg

Cross-check with an assumed layout of 10 mm bars at 150 mm c/c both ways at the bottom (unit weight d²/162 = 100/162 = 0.617 kg/m):

• Bars running along 12 m: 10/0.15 = 66.7 → 67 spaces + 1 = 68 bars × 12 m × 0.617 = 503 kg
• Bars running along 10 m: 12/0.15 = 80 spaces + 1 = 81 bars × 10 m × 0.617 = 500 kg
• Bottom mesh ≈ 1003 kg, before laps, top bars over supports, distribution bars and chairs

So for this assumed layout the slab would land near the upper end of the thumb-rule range or a little above it, once top extras are added. With 8 mm bars (0.395 kg/m) at the same spacing it would be much lower. Always take final steel from the designed drawing and a proper BBS: see how to calculate steel quantity and the bar bending schedule guide.

Don't forget the beams

When the slab is cast monolithically with beams, the beam portion below the slab is extra concrete. Example: 60 running metres of 230 mm × 450 mm beams under a 125 mm slab. Depth below slab = 450 − 125 = 325 mm, so extra concrete = 60 × 0.230 × 0.325 = 4.49 m³. That is almost 30% on top of the slab volume, and it is the single most common reason slab-day concrete runs short.

If you order ready-mixed concrete, 15.45 m³ is about 2.6 loads of a 6 m³ transit mixer; with the beams added, plan for the total and confirm the last load only after checking the remaining volume on site.

Shuttering area (for completeness)

Soffit = 12 × 10 = 120 m²; edge shuttering = perimeter 44 m × 0.125 m = 5.5 m². Beam sides and bottoms are additional.

The Nirmaan house estimator does slab, beam, steel and cost together if you want the whole building at once.

Common mistakes

• Skipping the dry volume factor, which makes the cement and aggregate short by about one third.
• Measuring thickness at the edge only. Sagging shuttering can add 10–15 mm in the middle of a bay, which is 8–12% extra thickness over that area of a 125 mm slab.
• Forgetting beams, sunken toilet portions, chajjas and parapet bands that are cast with the slab.
• Adding water at the end of the pour to finish faster; it weakens the top surface, which is exactly where the slab is most exposed.
• Using exposure-blind grades. A roof slab exposed to rain is moderate exposure under IS 456, for which the minimum RCC grade is M25 (design mix). Use the grade on your structural drawing.