This is the single most asked question on the day before a slab pour: "how many cement bags do I need?". The answer for a 1000 sqft slab at 125 mm thickness in M20 grade comes out to about 75 bags of cement + 95 cft of sand + 190 cft of 20mm aggregate — but the math is worth understanding so you can adjust for different sizes, thicknesses and grades.
🧮 Skip the math: Use the Cement Calculator → Roof Slab tab — enter L × W × thickness, pick grade, get bags + sand + aggregate with live prices.
The setup
We're estimating for a typical Chennai slab:
- Dimensions: 30 ft × 33.3 ft = 1000 sqft
- Thickness: 125 mm (standard residential)
- Grade: M20 concrete (1 : 1.5 : 3 mix ratio)
Convert to metric:
- Length 30 ft = 9.144 m
- Width 33.3 ft = 10.15 m
- Thickness 125 mm = 0.125 m
Step 1 — Wet concrete volume
Volume = 9.144 × 10.15 × 0.125 = 11.6 m³
Step 2 — Dry volume
Concrete shrinks when wet ingredients mix. We multiply by 1.54 to get the dry volume needed:
Dry volume = 11.6 × 1.54 = 17.9 m³
Step 3 — Split by M20 mix ratio (1 : 1.5 : 3)
Total parts = 1 + 1.5 + 3 = 5.5
- Cement = 17.9 × (1 / 5.5) = 3.25 m³
- Sand = 17.9 × (1.5 / 5.5) = 4.88 m³
- Aggregate = 17.9 × (3 / 5.5) = 9.76 m³
Step 4 — Convert to ordering units
Cement is sold in 50 kg bags. Cement density = 1440 kg/m³.
Cement kg = 3.25 × 1440 = 4,680 kg
Cement bags = 4,680 ÷ 50 = 93.6 → 95 bags (round up)
Wait — 95 bags feels high. Indian construction practice uses a slightly different convention where the dry volume factor for cement is computed independently, giving about 7-8 bags per cubic metre of concrete. Cross-checking with that rule of thumb:
Bags = 11.6 m³ × 7 bags/m³ ≈ 81 bags
The Suppliable Cement Calculator uses the conservative 7.5 bags/m³ for M20, returning ~87 bags. We recommend ordering 90 bags for a 1000 sqft × 125 mm M20 slab — gives a small safety buffer for spillage.
For sand and aggregate, convert m³ → cft (1 m³ = 35.31 cft):
Sand = 4.88 × 35.31 = 172 cft → order ~180 cft (~8 tonnes M-sand)
Aggregate = 9.76 × 35.31 = 345 cft → order ~360 cft (~16 tonnes 20mm jelly)
The order
| Item | Quantity | Live Suppliable price (Chennai, Jun 2026) | Cost |
|---|---|---|---|
| UltraTech / Ramco PPC 50 kg bag | 90 bags | ~₹410/bag | ₹36,900 |
| M-sand | 8 tonnes | ~₹1,400/tonne | ₹11,200 |
| 20 mm jelly (aggregate) | 16 tonnes | ~₹1,300/tonne | ₹20,800 |
| Materials subtotal | ~₹68,900 |
Plus steel (3.5 tonnes Fe 500D TMT for a 1000 sqft slab) at ~₹62/kg = **₹2,17,000**.
Grand total for the slab materials: ~₹2,85,000.
Adjustments for different grades
The same slab in higher concrete grade:
| Grade | Mix ratio | Cement bags (for 11.6 m³) | Why use it |
|---|---|---|---|
| M20 | 1 : 1.5 : 3 | ~90 bags | Standard residential slabs |
| M25 | 1 : 1 : 2 | ~107 bags | Multi-storey, heavier load |
| M30 | 1 : 0.75 : 1.5 | ~130 bags | Industrial, very heavy load |
For most G+0 / G+1 Chennai homes, M20 is the right call. M25 only if your structural engineer specifies it.
Pre-pour checklist
The day before a slab pour:
- ✅ Steel cage tied + measured (3.5 tonnes for 1000 sqft is the usual number)
- ✅ All 90 cement bags on site by 6 AM pour day (don't take chances with delivery on pour day)
- ✅ Sand and aggregate piles within 50 ft of the mixer
- ✅ Water source confirmed
- ✅ Pump truck booked (if pouring upper floors)
- ✅ Mason crew of 8-10 confirmed
- ✅ Dr. Fixit Pidiproof LW+ in stock for integral waterproofing — 200 ml per bag × 90 bags = 18 L
That last point is critical for Chennai. The monsoon hits hard; integral waterproofing in the slab itself is the single best protection.
Common mistakes that cost half-days
- Under-ordering cement by 5%. Pour starts, masons hit 70 bags, send someone for 5 more. 90 minutes lost.
- Wrong aggregate size. Use 20 mm jelly for slabs — not 10 mm (fines, too dense) or 40 mm (too coarse for proper bedding).
- Skipping LW+. The slab will leak in monsoon 18 months later. Topcoat (Newcoat / Pidifin 2K) alone won't fix it.
Use the calculator instead
This was a worked example. For your actual slab dimensions, open the Cement Calculator → Roof Slab tab, punch in your L × W × thickness, pick the grade, and the same calculation runs in 2 seconds — with live cement bag prices from our catalogue.
Pouring a slab in Chennai soon? WhatsApp +91 87786 27926 with the slab size and we'll quote the full materials BOM (cement + sand + aggregate + LW+) within an hour.
🧮 Free tool: Try our Cement Calculator — three tabs for plastering, columns, and roof slabs.