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Create Your Free ProposalQuick reference: cubic metres of concrete needed per square metre of area, by slab thickness.
| Depth (mm) | m³ per m² | 25kg Bags per m² | Common Use |
|---|---|---|---|
| 50mm | 0.05 | ~3 bags | Paths, thin screeds |
| 75mm | 0.075 | ~4 bags | Shed bases, light-use areas |
| 100mm | 0.1 | ~6 bags | Patios, standard slabs |
| 150mm | 0.15 | ~8 bags | Driveways, garage floors |
| 200mm | 0.2 | ~11 bags | Heavy-duty bases, foundations |
| 300mm | 0.3 | ~17 bags | Strip foundations, footings |
Getting the right amount of concrete is essential for any building project. Order too little and you face delays and cold joints. Order too much and you waste money and have to dispose of excess. This guide helps you understand the calculation and make the right decisions.
Concrete volume is calculated as: Length (m) x Width (m) x Depth (m) = Volume (m³). Note that depth is usually specified in millimetres on plans, so you need to convert to metres first (divide by 1,000). For example, a 100mm slab is 0.1 metres deep.
For small jobs under 1m³, using 25kg bags of premix concrete is the most practical option. Each bag yields approximately 0.018m³ when mixed, so you need around 56 bags per cubic metre. For jobs over 1m³, ready-mix concrete delivered by lorry is significantly cheaper (typically £65-90 per m³) and saves a huge amount of mixing time.
Bulk bags (also called jumbo bags or ton bags) typically weigh around 850kg and yield approximately 0.4m³ of concrete each. They are a good middle ground between individual 25kg bags and full ready-mix delivery, ideal for jobs in the 0.5-2m³ range where lorry access might be difficult.
Do not forget the sub-base. Most concrete slabs should be laid on:
When quoting concrete jobs, always include materials, labour, sub-base preparation, formwork and finishing in your price. Use tools like QuoteSmith to build professional proposals that clearly break down every element for your customer, helping you win more work and avoid disputes.
Common questions about calculating and ordering concrete.
It depends on the depth of your slab. For a 100mm thick slab, you need 0.1m³ per square metre. For 75mm depth, it is 0.075m³ per m². For 150mm depth, it is 0.15m³ per m². Use the calculator above to work out the exact volume for your project dimensions.
A typical fence post hole is approximately 300mm x 300mm x 600mm deep. You will need 1-2 bags of 25kg postcrete or premix concrete per post hole. For a standard 6ft fence panel run, budget 2 bags per post to be safe. Postcrete (rapid-set) is the easiest option as it sets in about 10 minutes.
For small jobs under 1m³, 25kg bags are convenient and practical (£5-7 per bag). For anything over 1m³, ready-mix delivery is more cost-effective, typically costing £65-90 per cubic metre delivered. The breakeven point is usually around 1-1.5m³.
Shed base: 75-100mm. Patio: 100mm. Driveway: 100-150mm. Garage floor: 150mm. All slabs should be laid on a minimum of 100mm compacted hardcore (MOT Type 1) with a damp-proof membrane underneath.
Concrete becomes surface-hard in 24-48 hours, allowing light foot traffic. It reaches approximately 70% strength after 7 days and achieves full design strength at 28 days. Do not place heavy loads on new concrete for at least 7 days. In cold weather, protect fresh concrete from frost for the first 48 hours.
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