TryBuildCalcRoof Slab Concrete Calculator (RCC Volume, Cement, Sand & Aggregate)
Calculate slab concrete volume and materials.
Calculate roof slab concrete volume and material quantities. Adjust slab dimensions, thickness, mix ratio, and wastage for your RCC estimate.
🕒 Last updated: August 6, 2026
Inputs
ℹ️Typical: 100–150 mm
Optional Deductions
Deduct staircase openings, ducts, lift shafts, or other voids from the slab area before calculating concrete volume.
Cost
Concrete Volume: 7.079 m³ / 250.0 cft
Wet Volume: 7.079 m³ / 250.0 cft
Dry Volume: 10.902 m³ / 385.0 cft
Sand: 2.973 m³ / 105.0 cft
Aggregate: 5.947 m³ / 210.0 cft
Cement Required (Including 5% Wastage): 60 bags
Extra (Wastage): 2.86 bags
Planning the shuttering and prop layout for this slab? Slab Shuttering Process Guide →
Approximate results for planning only. Verify with a professional.
Looking for the verification checklist, reference tables, tips, or common mistakes?See the complete Concrete Slab Calculator.
Roof slab concrete estimate
This page starts with a typical roof slab dimension in feet for quick concrete planning.
The calculator is pre-filled for this slab use case. Edit any input and the worked example updates from the active values.
- Default roof slab: 30 ft x 20 ft.
- Thickness: 5 in.
- Includes material split.
How does the concrete slab calculator work?
The calculator follows standard civil engineering formulas to estimate concrete volume and material quantities step-by-step, from the net slab area down to cement bags.
Step 1 — Calculate Net Slab Area
For a single rectangular slab this is just Length × Width. Multiple Sections sums each rectangle (for L-shaped/irregular footprints); Optional Deductions subtracts any staircase, duct, or void areas.
Step 2 — Calculate Wet Volume
Step 3 — Convert to Dry Volume
The factor 1.54 accounts for voids, bulking of sand, and material losses during mixing.
Step 4 — Calculate Material Proportions
Step 5 — Convert Cement Volume to Bags
Step 6 — Add Wastage
Step 7 — Cost (Optional)
All three quantities are priced at their wastage-adjusted amount. Sand and aggregate prices can be entered per m³ or per cft — whichever unit you enter is converted to match automatically.
Worked Example: Concrete Slab Calculation
This example uses the active inputs above and follows the same steps as the Formula section.
Input Values Used
| Input | Value |
|---|---|
| Length × Width | 30 ft × 20 ft = 55.742 m² |
| Thickness | 5 in (0.127 m) |
| Concrete Mix | M20 (1:1.5:3) |
| Wastage | 5% |
Step 1 — Net Area & Wet Volume
| Calculation | Substitution | Result |
|---|---|---|
| Net Area = Gross − Openings | 55.742 − 0 | 55.742 m² |
| Wet Volume = Net Area × Thickness | 55.742 × 0.127 | 7.079 m³ / 250.0 cft |
Step 2 — Dry Volume
| Calculation | Substitution | Result |
|---|---|---|
| Dry Volume = Wet Volume × 1.54 | 7.079 × 1.54 | 10.902 m³ / 385.0 cft |
Step 3 — Materials & Cement Bags
| Material | Share of Mix (1:1.5:3) | Result |
|---|---|---|
| Sand | 1.5 / 5.5 | 2.973 m³ / 105.0 cft |
| Aggregate | 3 / 5.5 | 5.947 m³ / 210.0 cft |
| Cement Bags (with 5% wastage) | 1 / 5.5 | 60 bags |
Therefore, this slab needs approximately 7.079 m³ / 250.0 cft of concrete, requiring 60 cement bags, 2.973 m³ / 105.0 cft sand, and 5.947 m³ / 210.0 cft aggregate (including 5% wastage on cement).
Cross-check against the Standard Concrete Mix Ratios table in the complete Concrete Slab Calculator.