Concrete Calculator — Slabs, Footings & Bags Needed
Free tool · Works offline · Metric & imperial · Bags or ready-mix
How to calculate how much concrete you need
Concrete volume is simply length × width × depth. The tricky part is converting between units and accounting for waste. A 10×10 ft slab at 4" depth is 10 × 10 × (4/12) = 33.3 cubic feet = 1.23 cubic yards. Always add 5–10% for waste — formwork is never perfectly square, and you always lose some on the ground. This calculator handles the conversion automatically and tells you exactly how many bags to buy or whether ready-mix makes more sense.
How many bags of concrete for a 10×10 slab?
A 10×10 ft (3×3 m) slab at 4" (100mm) depth needs about 1.3 cubic yards (1 m³) of concrete. That's approximately:
- 45 × 60 lb bags (Quikrete or similar)
- 30 × 80 lb bags
- Or about 1.3 yd³ of ready-mix
For anything over 1 cubic yard (27 cubic feet), ready-mix is usually more economical — mixing that many bags by hand is exhausting and the result is less consistent. Use bags for small repairs, fence posts, and pours under 0.5 m³.
How thick should a concrete slab be?
Slab thickness depends on what it carries. Patios and paths: 75–100mm (3–4") is standard for foot traffic. Garage floors: 100–150mm (4–6") to handle vehicle weight — go to 150mm if you park heavy vehicles or store machinery. Driveways: 100mm (4") minimum, 150mm for heavy vehicles. Structural slabs with rebar: consult an engineer — typically 150–250mm depending on loads and span.
What concrete mix should I use?
For most residential work, a C20 mix (1:2:4 cement:sand:aggregate) is standard — suitable for slabs, paths, steps and non-structural footings. Use C25 (1:1.5:3) for garage floors, driveways and lightly reinforced elements. C30 (1:1:2) for structural beams, columns and anything an engineer has specified. Bagged products like Quikrete General Purpose are typically equivalent to C20–C25 — check the bag for compressive strength (usually 4000 psi / 27 MPa at 28 days).
Ready-mix vs bagged concrete — which is cheaper?
Bagged concrete costs roughly £5–8 per 25kg bag (covering about 0.015 m³). Ready-mix is typically £90–130 per m³ delivered. The crossover point is around 0.5–1 m³ — below that, bags are cheaper and more convenient. Above 1 m³, ready-mix saves time, money and back pain. Note that ready-mix has a minimum order (usually 1–2 m³) and needs to be poured quickly — have your formwork ready before the truck arrives.
Frequently asked questions
How many bags of concrete for fence posts?
A standard 100mm diameter fence post hole at 600mm deep needs about 0.005 m³ — roughly one 25kg bag per post. For larger posts or deeper holes use the column tab in the calculator.
How much does a yard of concrete cover?
One cubic yard (0.76 m³) covers 81 sq ft at 4" depth, or 54 sq ft at 6" depth. In metric: 1 m³ covers 10 m² at 100mm or 6.7 m² at 150mm.
How long does concrete take to cure?
Initial set: 24–48 hours (keep off it). Walking strength: 3–7 days. Full strength: 28 days. Keep concrete moist for the first 7 days — cover with plastic sheeting or damp burlap to prevent cracking.
Do I need rebar in a concrete slab?
For slabs over 3m span, vehicle traffic, or anywhere there's risk of settlement, yes. Use 10mm rebar at 300mm centres for a standard garage floor. Always check local building codes.