Concrete Calculator
Concrete is ordered in cubic metres and sold in 20 kg bags, and the conversion between them surprises people every time.
Working out the volume
The depth unit is where mistakes happen. A 4 × 3 m slab at 100 mm thick is 4 × 3 × 0.1 = 1.2 m³. Treating the depth as 100 rather than 0.1 gives an answer a thousand times too large, which is at least an obvious error. Treating it as 10 cm without converting gives one ten-times too large, which is not.
For a cylindrical post hole, the volume is πr²h. A 300 mm diameter hole 600 mm deep is π × 0.15² × 0.6 = 0.042 m³ — about two 20 kg bags per hole.
Why bags are so disappointing
Concrete weighs roughly 2,400 kg per cubic metre. A 20 kg bag of dry pre-mix therefore yields only about 0.0083 m³ once mixed — a little over eight litres.
That 1.2 m³ slab needs around 160 bags of 20 kg mix — more than three tonnes of material to carry, and a great deal of mixing. This is the calculation that persuades most people to order ready-mixed concrete instead.
The crossover point. Below about 0.3 m³, bags are usually more practical. Above roughly 1 m³, ready-mix delivery is cheaper per cubic metre as well as vastly less work. Most suppliers have a minimum order, often 1 m³, and charge a part-load fee below it.
Mix ratios and strength
If mixing your own, the ratio determines the strength:
| Mix (cement : sand : aggregate) | Strength | Use |
|---|---|---|
| 1 : 3 : 6 | C10–C15 | Kerb bedding, backfill, blinding |
| 1 : 2 : 4 | C20 | General purpose, paths, shed bases |
| 1 : 1.5 : 3 | C25–C30 | Driveways, foundations, floor slabs |
| 1 : 1 : 2 | C35+ | Structural work, heavy loading |
The strength grade C25 means a characteristic compressive strength of 25 N/mm² at 28 days. Concrete reaches roughly 70 % of its final strength in seven days and continues gaining for years.
Water is the variable that ruins more concrete than any other. Excess water makes the mix easier to work and substantially weaker — a water-to-cement ratio above about 0.6 can cut strength by half. Aim for a mix that holds its shape when heaped rather than one that pours easily. In cold weather, do not pour below 5 °C without precautions; in hot weather, keep the surface damp while it cures.
Frequently asked questions
How thick should a concrete slab be?
Typically 100 mm for a shed base or footpath, 150 mm for a driveway taking cars, and more with reinforcement for heavier loads. Building regulations and ground conditions govern anything structural — check locally before pouring foundations.
Is ready-mix cheaper than bags?
Above about one cubic metre, almost always — and it saves enormous labour. Below 0.3 m³ bags usually win, because ready-mix suppliers apply minimum charges and part-load fees. Between those figures, get a quote.
How long before I can walk on it?
Roughly 24 to 48 hours for light foot traffic, seven days before heavier use, and 28 days to reach design strength. Vehicles should stay off a new driveway for at least a week, longer in cold weather.