Rectangular & Square Slab Guide
Estimate concrete volume for flat roofs, ground-floor slabs, suspended slabs, and any rectangular or square concrete slab.
Circular Slab Guide
For round slabs, circular tanks, manhole covers, circular column pads, and ring slabs.
L-Shaped Slab Guide
For L-shaped floor plans, corner slabs, and any slab with an internal corner cutout. Enter the overall rectangle and the size of the rectangular cutout.
Triangular Slab Guide
For triangular slabs, wedge-shaped sections, corner fills, and any slab where the plan shape is a triangle.
Slab Volume Formulas — How Each Estimate Works
The Four Slab Volume Formulas
Rectangular: V = L × W × (T ÷ 1000) Circular: V = π × (D ÷ 2)² × (T ÷ 1000) L-Shaped: V = [(L1 × W1) − (L2 × W2)] × (T ÷ 1000) Triangular: V = 0.5 × B × H × (T ÷ 1000)
L, W, B, H, Dare entered in metres.T(thickness) is entered in millimetres and divided by 1,000 to convert to metres before multiplying. All output volumes are in cubic metres (m³).
Dry volume factor:Material quantities use the standard 1.54 dry-to-wet conversion factor — 1 m³ of wet concrete requires 1.54 m³ of dry loose materials. See ourHow We Estimatepage for full methodology.
Cement bagsare estimated as: CEILING[ (cement fraction × dry volume) ÷ 0.0347 ] where 0.0347 m³ is the volume of one 50 kg cement bag. Bags are always rounded up — never down.
Wastageis applied to the net wet volume before any material estimate: Final Volume = Net Volume × (1 + Wastage% ÷ 100). All material quantities scale proportionally with the wastage adjustment.
Slab Thickness Reference Table — Pakistan & International Practice 2026
Typical concrete slab thicknesses for common slab types in Pakistan residential and commercial construction — for reference when selecting thickness in the guide. For related background, see Slab Construction Guide.
- Slab Type: Ground-Bearing Slab on Grade; Typical Thickness: 100–150 mm; Concrete Grade: M15–M20; Span Range: N/A; Notes: Non-structural. Sub-base compaction critical. No moment demand from supports.
- Slab Type: Suspended RC Flat Slab (Residential); Typical Thickness: 125–175 mm; Concrete Grade: M20; Span Range: 3.0–5.0 m; Notes: Span ÷ 28 rule (ACI 318) gives starting point. Structural engineer to confirm.
- Slab Type: Suspended RC Flat Slab (Commercial); Typical Thickness: 175–250 mm; Concrete Grade: M20–M25; Span Range: 5.0–8.0 m; Notes: Heavier imposed loads. Check deflection and punching shear at columns.
- Slab Type: One-Way RC Slab (with beams); Typical Thickness: 100–150 mm; Concrete Grade: M20; Span Range: 3.0–5.0 m; Notes: Span ÷ 35 for simply supported; ÷ 40 for continuous (ACI 318 Table 7.3.1.1).
- Slab Type: Roof Slab (Pakistan Standard); Typical Thickness: 125–150 mm; Concrete Grade: M20; Span Range: Up to 4.5 m; Notes: Most common in Pakistan residential construction. Add waterproofing screed above.
- Slab Type: Flat Plate (No Drop Panels); Typical Thickness: 200–280 mm; Concrete Grade: M25; Span Range: 5.0–9.0 m; Notes: Punching shear governs. Often requires column heads or drop panels for economy.
- Slab Type: Post-Tensioned Flat Slab; Typical Thickness: 200–300 mm; Concrete Grade: M30–M35; Span Range: 8.0–12.0 m; Notes: PT tendons significantly reduce thickness vs conventional RC for same span.
- Slab Type: Transfer Slab (above podium); Typical Thickness: 500–1,200 mm; Concrete Grade: M30–M40; Span Range: Varies; Notes: Heavy-loaded. Extensive structural engineering design required. Not a standard slab.
- Slab Type: Raft Foundation Slab; Typical Thickness: 300–600 mm; Concrete Grade: M20–M30; Span Range: Building footprint; Notes: Thickness governed by punching shear and bending at column positions.
- Slab Type: Precast Hollow-Core Slab; Typical Thickness: 150–400 mm; Concrete Grade: Factory-set C35+; Span Range: 6.0–16.0 m; Notes: Not site-cast — ordered from precast manufacturer. No concrete volume estimate needed.
Quick Material Reference — Per 1 m³ of Finished Slab Concrete
Materials required per 1 m³ of finished wet concrete (before wastage) for each mix grade. Use this table to cross-check worked estimates or for manual estimates.
- Grade: M10; Mix Ratio: 1 : 3 : 6; Cement (kg): 222; Sand (kg): 666; Aggregate (kg): 1,332; Water (L): 133; 50 kg Bags / m³: 5; Typical Use: Blinding, non-structural fill, lean concrete
- Grade: M15; Mix Ratio: 1 : 2 : 4; Cement (kg): 317; Sand (kg): 634; Aggregate (kg): 1,268; Water (L): 174; 50 kg Bags / m³: 7; Typical Use: Ground-bearing floor slabs, pathways, mass concrete
- Grade: M20; Mix Ratio: 1 : 1.5 : 3; Cement (kg): 403; Sand (kg): 605; Aggregate (kg): 1,210; Water (L): 202; 50 kg Bags / m³: 9; Typical Use: Structural suspended slabs — standard residential & commercial
- Grade: M25; Mix Ratio: 1 : 1 : 2; Cement (kg): 554; Sand (kg): 554; Aggregate (kg): 1,108; Water (L): 249; 50 kg Bags / m³: 12; Typical Use: Heavy-loaded slabs, transfer slabs, slabs in seismic zones
Worked Examples — Slab Volume Estimates 2026
1 Kanal House Roof Slab — Lahore
Slab dimensions 18 m × 15 m × 150 mm
Gross volume 18×15×0.15 = 40.5 m³
Wastage (5%) 40.5 × 1.05 = 42.53 m³
Dry volume (×1.54) 42.53 × 1.54 = 65.5 m³
Cement (M20: 1÷5.5) 65.5÷5.5÷0.0347 = 343 bags
42.5 m³ · 343 bags · 25.7 t sand · 51.4 t agg
Circular Slab — Water Tank Base
Diameter 5.0 m (radius = 2.5 m)
Thickness 200 mm
Volume = π × 2.5² × 0.2 3.927 m³
Wastage (10%) 3.927 × 1.10 = 4.32 m³
M25 cement 4.32 × 1.54 × (1÷4) = 1.663 m³
Bags = CEIL(1.663÷0.0347) = 48 bags
4.32 m³ · 48 bags cement
L-Shaped Slab — Corner Apartment
Overall L1 × W1 12 m × 10 m = 120 m²
Cutout L2 × W2 5 m × 4 m = 20 m²
Net plan area 120 − 20 = 100 m²
Volume (150 mm thick) 100 × 0.15 = 15.0 m³
With 5% wastage 15.0 × 1.05 = 15.75 m³
15.75 m³ · 127 bags cement
Triangular Slab — Corner Fill
Base length 6.0 m
Perpendicular height 4.5 m
Thickness 125 mm
Area = 0.5 × 6.0 × 4.5 13.5 m²
Volume = 13.5 × 0.125 1.688 m³
With 5% wastage 1.688 × 1.05 = 1.772 m³
1.772 m³ · 15 bags cement
Tips for Accurate Slab Concrete Estimation
Always Add 5% Wastage Minimum
A flat roof slab with clean formwork and ready-mix supply still loses 3–5% to over-pour at upstands, beam edges, and column head thickenings. Never order the exact estimated volume — you risk stopping the pour mid-slab, which creates a cold joint that must be chased out and repaired.
Measure in the Right Units
Length and width go in as metres; thickness goes in as millimetres. The most common data-entry error is entering thickness in metres (e.g., typing 0.15 instead of 150 mm) or entering length in feet when the field expects metres. The guide converts thickness to metres automatically — just type the mm value. For related background, see Concrete Wall Guide.
Use M20 for Structural Slabs
M15 is adequate for ground-bearing slabs on grade (with no structural loading from above) but M20 is the minimum for any suspended reinforced concrete slab per ACI 318-19 and BCP 2021. Using M15 for a structural roof slab to save cost is a false economy — the strength difference is significant and the additional cement cost is modest.
Deduct Openings Before Ordering
For slabs with openings (staircases, lift shafts, service penetrations), estimate the gross slab area first, then deduct the total opening area before applying the thickness. A 1.2 m × 2.4 m staircase opening in a 10 × 12 m slab reduces the concrete volume by about 3.5% — worth deducting for large slabs.
Cure for 7 Days — Do Not Rush
A concrete slab in Pakistan’s summer heat loses moisture rapidly if not cured. Cover immediately after floating with wet hessian and polythene sheet, and keep continuously wet for 7 days minimum. Cutting curing short at 3–4 days reduces final strength by 10–20% and dramatically increases long-term cracking and permeability.
Check Supplier Minimum Batch Size
Ready-mix plants in Pakistan typically have a minimum batch size of 3–5 m³. For small slabs below this threshold, you will be charged for the minimum regardless. For very small pours (below 2 m³), site-batching with a mixer may be more economical — adjust to 10% wastage to account for batch losses.
Frequently Asked Questions — Slab Concrete Guide
How many bags of cement do I need for a 10 × 10 m slab, 150 mm thick? +
What thickness should a roof slab be in Pakistan? +
How do I estimate the volume of an L-shaped slab? +
What is the dry volume factor (1.54) and why does the guide use it? +
Can I use this guide for a concrete driveway or pathway? +
Slab Guide 2026 — Results are estimates for planning and material ordering. Always verify quantities with your site engineer before procurement. Estimates use IS 456 / ACI 211.1 nominal mix methodology with a 1.54 dry volume factor. SeeHow We EstimateandDisclaimerfor full details.