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How Much Concrete Do I Need? A Complete Ordering Guide
Concrete is the one material you cannot “pick up more of later” mid-pour. This guide walks through measuring, calculating and ordering the right amount — whether you are pouring a garden path or a garage slab.
Start with the volume formula
Every concrete order starts with the same formula: length × width × thickness. The trap is units — thickness is usually specified in centimetres or inches while length is in metres or feet. Convert everything to one unit before multiplying: 10 cm is 0.1 m, 4 inches is 0.333 ft.
For a quick answer, use the Concrete Calculator — it handles the unit conversion and adds a waste allowance automatically. For L-shaped slabs, split the plan into rectangles and add the volumes.
How thick should the slab be?
Thickness drives the volume more than any other dimension. Walkways and garden paths need 10 cm (4 in). Patios and shed bases: 10–15 cm. Driveways for cars: 15–20 cm (6–8 in) on a compacted gravel base. Strip footings for walls: 20–40 cm depending on load and soil.
Doubling the thickness doubles the concrete — and the cost. If you are unsure, check local building guidance for your soil type rather than guessing thicker.
Bags or ready-mix truck?
A 25 kg bag of dry mix yields about 12 litres (0.012 m³) of concrete — roughly 85 bags per cubic metre. An 80 lb bag yields 0.6 ft³, about 45 bags per cubic yard. Mixing more than 30–40 bags by hand is a full day of hard work.
The crossover point is around 1.5–2 m³ (2 yd³): above that, a ready-mix truck is cheaper per cubic metre and finishes the pour in an hour. Below 0.5 m³, bags win on flexibility. In between, compare local prices — and remember the Cement Calculator can split a volume into cement, sand and gravel if you want to mix from raw ingredients.
Always order extra — but not too much
Excavations are never perfectly square and sub-bases never perfectly level. Add 5–10% for slabs on prepared ground and up to 15% for footings dug into rough soil. Running short mid-pour means a cold joint — a visible, weak seam where the two pours meet.
Before the truck arrives, prepare the base: compacted gravel (see the Gravel Calculator for tonnage), formwork checked for level, and reinforcement in place. Concrete waits for no one.