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 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 25 kg bag, 0.019 m³ for 40 kg)
- 4Ready-mix: order in m³ — 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 m × 3 m × 0.1 m depth = 1.2 m³. With 8% waste: 1.3 m³.
You need 109 bags of 25 kg premix, or approximately 5–6 bags of 40 kg premix per m² at 100 mm depth. For ready-mix, order 1.5 m³ (rounded up to nearest 0.25 m³ delivery increment).
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 40 bags of 40 kg premix.
What concrete mix should I use for a driveway?
C25/30 (equivalent to 4000 psi) is the minimum for a driveway. 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. Driveways: 100–125 mm. Garage floors: 100 mm. Structural floor slabs: 150–200 mm 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 and gives more consistent quality. Below 1 m³, bagged premix is more practical. Ready-mix requires access for a mixer truck — check site access before ordering.