Concrete Calculator
Calculate how much concrete you need for slabs, footings, columns, and steps. Get the volume in cubic metres, pre-mix bags (20kg and 25kg), ready-mix truck loads, and raw material quantities if mixing yourself. Includes 10% wastage allowance.
Frequently asked questions
How do I calculate concrete volume?▾
Multiply length x width x depth in metres to get cubic metres (m3). For circular columns, use pi x radius squared x height. Always add 10% for wastage and slight over-filling.
How many bags of concrete do I need?▾
A standard 25kg bag of pre-mixed concrete makes approximately 0.012m3 (12 litres). A 20kg bag makes about 0.009m3 (9 litres). Divide your total volume by the bag yield to get the number needed.
When should I use ready-mix vs bags?▾
For volumes under 0.5m3, pre-mix bags are practical. For 0.5-2m3, consider a mini-mix delivery. For over 2m3, ready-mix trucks (6m3 capacity) are more cost-effective and produce more consistent concrete.
What concrete mix ratio should I use?▾
For general purpose work: 1:2:3 (cement:sand:gravel) or C20 strength. For foundations: 1:2.5:3.5 or C25. For paths and driveways: 1:1.5:2.5 or C30. Higher cement ratio = stronger concrete.
How deep should a concrete slab be?▾
Garden paths: 75mm. Patios: 100mm. Driveways: 100-150mm. Garage floors: 100-150mm. Building foundations: as specified by structural engineer (typically 150-300mm for strip foundations).
Do I need to add a sub-base?▾
Yes, for most applications. A compacted sub-base (Type 1 MOT) of 100-150mm provides drainage and prevents the concrete from cracking. Only very small non-structural slabs can go directly on firm ground.
How long does concrete take to cure?▾
Concrete is workable for 1-2 hours after mixing. It reaches initial set in 24-48 hours and gains most of its strength within 7 days. Full cure strength (28 days) is used for structural calculations.
Can I pour concrete in cold weather?▾
Avoid pouring when temperatures are below 3C or if frost is expected within 48 hours. Cold concrete sets slower and can crack. Use frost blankets to protect freshly poured concrete in borderline conditions.
How much does concrete cost in the UK?▾
Pre-mix bags cost approximately 5-7 pounds per 25kg bag (330-470 pounds per m3). Ready-mix delivery costs approximately 65-120 pounds per m3 depending on strength and volume, plus a delivery charge of 15-50 pounds.
Do I need reinforcement in my concrete?▾
For driveways and structural slabs, use steel mesh (A142 or A193) or fibre reinforcement. For paths and small patios, reinforcement is usually not needed unless on soft ground or spanning voids.