How to Estimate Concrete for a Slab

What you need

A tape measure, string line and pegs, a spirit level, a wheelbarrow, a shovel, a float or trowel, and formwork timber to hold the edges. Wear gloves and eye protection — wet concrete burns skin. Order the concrete only after the formwork is built and the depth is checked.

Step 1 — Measure the slab

Measure the length and width of the slab, and the depth (thickness). A typical house slab is 100 mm (4 in) thick; a driveway is usually 125–150 mm (5–6 in). Measure the depth in several places — the ground is rarely level.

Step 2 — Work out the volume

Multiply length × width × depth, all in the same unit. A 10 ft × 10 ft slab, 4 in deep, is 10 × 10 × ⅓ = 33.33 ft³, which is 1.23 yd³ or 0.94 m³.

Step 3 — Add wastage

Add 10% for spillage, uneven ground and formwork gaps. The slab above becomes 1.23 × 1.10 = 1.35 yd³. Running short mid-pour is far worse than having a little extra.

Step 4 — Bags or ready-mix?

Small jobs (under about 1 yd³) usually use bags: an 80 lb bag yields about 0.60 ft³. Bigger jobs use ready-mix, ordered by the cubic yard or metre. The concrete calculator gives you both — bags and ready-mix volume.

Worked example

Slab 3 m × 3 m × 100 mm: volume = 3 × 3 × 0.1 = 0.9 m³. With 10% wastage: 0.99 m³ — order 1 m³ of ready-mix, or 72 bags of 25 kg without wastage (each yields 0.0125 m³, so 0.9 ÷ 0.0125 = 72). With 10% wastage that is 0.99 ÷ 0.0125 = 79.2, so about 80 bags. Check the bag yield on the packaging, because it varies.

Common mistakes

• Mixing inches and feet. Convert the depth to feet first (4 in = ⅓ ft) or the whole calculation is wrong.
• Forgetting wastage. Uneven ground eats concrete. 10% is the minimum.
• Trusting one depth measurement. Measure in several places and use the deepest — the slab is only as good as its thinnest point.

Safety

Wet concrete is caustic: it burns skin and eyes. Wear gloves, long sleeves, boots and eye protection, and wash any concrete off your skin with clean water straight away. A 25 kg bag is heavy — lift with your legs, not your back, and get help with a full wheelbarrow. Never work alone on a large pour, and keep children and pets away from wet concrete.

When to hire a professional

Hire a contractor for any structural slab, foundation or driveway that will carry vehicles, for slabs that need steel reinforcement, and whenever a permit is required. A professional is also the right call for large pours: ready-mix concrete sets on a clock, and a crew that has done it before will finish the slab before it starts to set.

Frequently asked questions

• How long before I can use the slab? You can usually walk on it after a day or two. Wait about a week before driving on it, and about four weeks before it reaches full strength. Keep it damp in hot weather so it cures instead of drying out.
• Bags or ready-mix — which is cheaper? For small jobs, bags are practical and easy to handle. For larger volumes, ready-mix is usually cheaper per cubic metre. Compare both with the concrete calculator, and see our guide to ordering ready-mix concrete before you book a truck.
• Can I pour concrete in the rain? Light rain is manageable if the surface is covered, but heavy rain weakens the top of the slab. Do not pour in heavy rain — wait for a dry window.
• Do I need reinforcement? Small garden slabs often need none. Driveways and structural slabs usually need steel mesh or bars — and a footing needs its own design. Our guide to building a concrete footing explains when reinforcement matters, and a professional can confirm it for your slab.

Try these calculators

• Concrete Calculator — volume, bags and ready-mix.
• Concrete Slab Calculator — slab-specific quantities.
• Cubic Yard Calculator — pure volume in yd³, ft³ and m³.