Concrete Calculator — Volume, Bags & Mix Strength
Calculate concrete volume in cubic metres (or cubic yards), number of bags required and the right mix strength for slabs, footings, posts and walls. Supports 25 kg and 40 kg bags in metric, and 60 lb and 80 lb bags in imperial.
How much concrete does a bag make?
A 25 kg bag of ready-mix yields roughly 0.012 m³ and a 40 kg bag roughly 0.019 m³ — about 80 or 53 bags to the cubic metre. In US sizes an 80 lb bag makes about 0.6 cu ft and a 60 lb bag about 0.45, so a cubic yard takes roughly 45 or 60 bags. Past half a cubic metre (or two thirds of a yard), ready-mix by the load is usually cheaper and always less work.
How do I calculate concrete volume?
Length × width × depth in the same units throughout. A 4 × 3 m slab at 100 mm deep is 1.2 m³; a 12 × 10 ft slab at 4 in is about 1.5 cubic yards. Add 5–10% for spillage, over-dig and formwork that is never quite true, and round a ready-mix order up to the nearest 0.25 m³ or quarter yard.
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How the concrete calculator works
- 1Volume = length × width × depth (m³ or ft³)
- 2Add 5–10% for waste, uneven formwork and spillage
- 3Bags needed: volume ÷ yield per bag (0.012 m³ for a 25 kg bag, 0.019 m³ for 40 kg; 0.45 cu ft for a 60 lb bag, 0.6 cu ft for an 80 lb bag)
- 4Ready-mix: order in m³ or cubic yards — the calculator rounds up to the nearest 0.25 m³
- 5Mix strength: C20/25 for most domestic slabs, C25/30 for driveways, C30/37 for structural footings
- 6Water-cement ratio: typically 0.45–0.55 for structural concrete
Worked example
Concrete slab: 4 × 3 m at 0.1 m deep — 13 × 10 ft at 4 in — = 1.2 m³. With 8% waste: 1.3 m³, or 1.7 cubic yards.
You need 109 bags of 25 kg premix — or 77 bags of 80 lb — and approximately 5–6 bags of 40 kg premix per m² at 100 mm depth. For ready-mix, order 1.5 m³ (rounded up to the nearest 0.25 m³ delivery increment), which is 2 cubic yards.
Frequently asked questions
How many bags of concrete do I need for a 10×10 slab?
A 10×10 ft (3.05×3.05 m) slab at 4 inches (100 mm) deep = 0.93 m³. With 8% waste: ~1 m³. You need approximately 83 bags of 25 kg premix or 53 bags of 40 kg premix (each 40 kg bag yields approx. 0.019 m³).
What concrete mix should I use for a driveway?
C25/30 (roughly comparable to 3,600–4,350 psi depending on cylinder vs cube basis) is a common choice for a driveway, but the right mix depends on the loads, exposure and local practice — confirm the specification locally. For freeze-thaw climates, use air-entrained concrete with 4–7% air content and a water-cement ratio below 0.45.
How thick should a concrete slab be?
Pedestrian patios: 75–100 mm (3–4 in). Driveways: 100–125 mm (4–5 in). Garage floors: 100 mm (4 in). Structural floor slabs: 150–200 mm (6–8 in) with rebar. Always check local building codes.
How long does concrete take to cure?
Concrete reaches approximately 70% of its design strength after 7 days and 99% after 28 days. Keep it moist for at least 7 days. Do not load a slab within 7 days of pouring.
When should I use ready-mix instead of bags?
Ready-mix is more economical above approximately 1 m³ — around 1.3 cubic yards — and gives more consistent quality. Below that, bagged premix is more practical. Ready-mix requires access for a mixer truck — check site access before ordering.