How to Order Ready-Mix Concrete

What you need

The slab's length, width and depth, the truck's access route, a wheelbarrow or pump if the truck cannot reach, a straight edge, a spirit level and gloves, eye protection and boots. Have the formwork built and the sub-base compacted before you order — the truck will not wait while you finish.

Step 1 — Measure the slab

Measure the length and width inside the formwork, and the depth from the sub-base to the top of the formwork. Measure the depth in several places: formwork is rarely perfectly level, and the shallowest point is not the average. Use the average depth, not the best one.

Step 2 — Work out the volume

Multiply length × width × depth for the volume, then add 10% wastage — a slab always takes more than the maths says, because the sub-base absorbs some and the formwork flexes. The concrete calculator gives you the volume in cubic feet, cubic metres and cubic yards, with and without wastage. Order by the cubic yard: that is how ready-mix is sold.

Step 3 — Decide: bags or a truck?

For a small pour, bagged concrete mixed on site can be cheaper. For anything over about a cubic yard, a truck is usually cheaper and far faster — and the mix is consistent. Compare the two with the calculator: it gives you the bag count alongside the volume.

Step 4 — Book the truck and pour

Book the delivery for the time the site is ready, with the truck's access confirmed — a mixer cannot reverse down a narrow lane it has never seen. Have help on site: concrete sets in a couple of hours, and a slab poured by one person is a slab finished badly. Pour, level with a straight edge, and keep it damp while it cures.

Worked example

A slab 12 ft × 10 ft and 4 in deep is 40 ft³ (1.13 m³, or 1.48 yd³). Add 10% wastage: 44 ft³ (1.25 m³, or 1.63 yd³) — that is the volume to order. The same slab in 80 lb bags would need 74 bags, which is why a truck makes sense here. Those are exactly the numbers the concrete calculator gives you.

Common mistakes

• Ordering the exact volume. A slab always takes more than the maths says. Order with 10% wastage or pay for a second, small delivery.
• Adding water on site. It makes the mix easier to pour and weaker when it sets. If the mix is too stiff, that is a problem for the supplier, not for your hose.
• No help on the day. Concrete waits for nobody. Have at least one other person on site for the pour.

Safety

Wet concrete burns skin and eyes — wear gloves, eye protection and boots, and wash splashes off straight away. A mixer truck is heavy: make sure the access route and the ground it parks on can take it. If the slab is structural or the ground is poor, get an engineer involved before you pour.

Try these calculators

• Concrete Calculator — volume, wastage and bags for a slab.
• Concrete Volume Calculator — volume for rectangular and round shapes.
• Cubic Yard Calculator — convert any volume to cubic yards.