Concrete Calculator
Calculate concrete volume for slabs, footings, and columns. Get cubic yards, cubic metres, and the exact number of bags — with a 10 % wastage allowance.
How It Works
Concrete volume is always Length × Width × Depth (or Height), but the unit conversions differ by shape:
Slab: Volume (ft³) = Length (ft) × Width (ft) × Thickness (ft). Convert inches to feet by dividing by 12. Then divide by 27 to get cubic yards (1 yd³ = 27 ft³).
Footing: Same formula as a slab but typically narrower and deeper. Add multiple footings and their volumes are summed.
Column (round): Volume (ft³) = π × radius² × Height. Radius = diameter ÷ 2.
Bag sizes and approximate yields:
- 40 lb bag ≈ 0.011 yd³ (0.30 ft³)
- 60 lb bag ≈ 0.017 yd³ (0.45 ft³)
- 80 lb bag ≈ 0.022 yd³ (0.60 ft³)
- 25 kg bag ≈ 0.010 m³
- 50 kg bag ≈ 0.020 m³
Always add 10 % waste to account for spillage, uneven sub-base, and minor over-pours. The calculator applies this automatically.
Example: A patio slab 10 ft × 12 ft × 4 in thick = 10 × 12 × (4/12) = 40 ft³ = 1.48 yd³. With 10 % wastage = 1.63 yd³. Using 80 lb bags: 1.63 ÷ 0.022 ≈ 74 bags.
Frequently Asked Questions
What concrete mix should I use?
For standard slabs and footings, use a 3000 psi (20 MPa) mix, which is the most widely available pre-mix concrete. For structural columns or high-load applications, use 4000 psi (25 MPa). For fence posts or small repairs, an 80 lb bag of standard mix is fine.
How do bag sizes compare in strength?
The weight of the bag indicates quantity, not strength. All standard pre-mix bags produce approximately 3000 psi once cured. Larger bags simply mean fewer trips, but the concrete is the same. Choose bag size based on what you can safely lift and mix.
How long does concrete take to cure?
Concrete is safe to walk on after 24–48 hours, but it only reaches its design strength after 28 days. Keep it moist for the first 7 days (cover with plastic sheeting or wet burlap) to prevent cracking. Avoid heavy loads for at least a week.
Do I need reinforcement (rebar or mesh) in my slab?
For driveways, garage floors, and structural slabs, use #4 rebar on 18-inch centres or welded wire mesh. For foot paths and decorative patios, wire mesh is usually sufficient. Footings for structural walls always require rebar. Check your local building code for specific requirements.
Why add 10 % wastage?
Sub-base surfaces are rarely perfectly flat, so concrete fills low spots you cannot easily measure. There is also spillage during mixing and pouring, and the last portion of a bag rarely yields full volume. 10 % is the standard industry allowance; add 15 % for complex shapes or rough ground.