Quick answer: Multiply length × width × depth (in feet) then divide by 27 to get cubic yards. A 10×10ft slab at 4" thick needs 1.23 cubic yards — about 74 bags of 60lb concrete or half a ready-mix truck.
Related Calculators
How thick should a concrete driveway be?
Residential driveways should be at least 4" (100mm) thick for passenger cars, 5–6" (125–150mm) for light trucks or SUVs. Use a 4000 PSI (C25/30) mix. A 10×20ft driveway at 4" needs about 2.5 cubic yards — enter your dimensions above for an exact figure.
How much concrete do I need for fence posts?
Each fence post hole typically needs 0.25–0.5 cubic feet of concrete. A standard 6" hole, 2ft deep needs about 0.37 cubic feet — roughly one 50lb bag. For a 100ft fence with posts every 8ft, you need about 13 bags of 80lb concrete.
What is the difference between concrete and cement?
Cement is an ingredient in concrete — it's the binding powder. Concrete is the finished product: cement + sand + gravel + water. When people say 'cement driveway' they mean concrete. You cannot use pure cement for structural work — it's too brittle without aggregate.
How long should I wait before driving on new concrete?
Wait at least 7 days before light foot traffic, 28 days before vehicle traffic. Concrete reaches 70% of its final strength in 7 days and full strength at 28 days. In cold weather (below 50°F/10°C) curing is slower — use blankets and allow extra time.
Can I pour concrete in cold weather?
Yes, but with precautions. Don't pour if ground is frozen or temperature will drop below 20°F (-7°C). Use heated water in the mix, cover with insulating blankets and allow 50% more curing time. Below 40°F (4°C), hydration slows dramatically — accelerating admixtures help.
How much does a cubic yard of concrete weigh?
A cubic yard of standard concrete weighs approximately 4,050 lbs (1,840 kg). A cubic metre weighs about 2,400 kg. This is useful for calculating structural load — a 4" slab weighs about 50 lbs per square foot (245 kg/m²).
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.