Running short on a pour is one of the most expensive mistakes on a small job. The truck has gone, the edge is starting to set, and you are mixing bags in a panic or booking a second delivery with a short-load charge on top. Over-ordering wastes money too, but much less of it. Estimating concrete well means measuring carefully, converting units without slips, and adding a waste allowance that fits the site.
1. Measure the real dimensions
Measure the inside of the formwork, not the drawing. Formwork is rarely set exactly to plan, and an extra inch on each side of a large slab adds up. For irregular shapes, break the plan into rectangles, triangles and circles, work out each area, and add them together.
Thickness is where most errors creep in. A sub-base that dips by half an inch across a large area can add several percent to the volume. Use a string line or laser and check the depth from the top of the forms down to the sub-base at several points. Use the average, not the design figure.
2. Convert everything to one unit
In the US, the trap is mixing feet and inches. A 20 × 12 ft slab that is 4 inches thick is 20 × 12 × (4 ÷ 12) = 80 cubic feet, not 960. Then divide by 27 to get cubic yards: 80 ÷ 27 = 2.96 cu yd.
Formula: volume = length × width × thickness, with every dimension in the same unit.
3. Add a waste allowance
Concrete gets lost to uneven sub-bases, slight form bulges, spillage, what's left in the pump line or barrow, and the bit you can't get out of the drum. Typical allowances:
| Situation | Allowance |
|---|---|
| Formed slab on a well-compacted, level base | 5% |
| Typical residential slab or patio | 8–10% |
| Hand-dug trenches, uneven ground, pumped pours | 10–15% |
4. Decide how to supply it
Below about a cubic yard, pre-mixed bags are usually simplest. Above that, ready-mix or a mix-on-site truck is normally cheaper and far less work. Our bags vs ready-mix guide covers the break-even point in detail.
Ready-mix is usually ordered in quarter-yard steps, so round up. Many plants charge a short-load fee below 3–4 yards, so compare the fee against a bag pour.
5. Don't forget reinforcement and extras
The concrete line is only part of the takeoff. A complete slab estimate usually includes:
- Gravel base, compacted in layers
- Vapour barrier or DPM where the slab is inside a building
- Rebar grid or welded wire mesh, plus chairs or spacers
- Edge forms, stakes, expansion joint strip and curing compound
- Pump hire if the truck can't reach
A worked example
A 24 × 16 ft garage slab at 5 inches: 24 × 16 × (5 ÷ 12) = 160 cu ft = 5.93 cu yd. Add 8% waste for 6.40 cu yd, then order 6.5 yards.
The concrete slab calculator does all of this for you. It shows the ready-mix order, every bag size and a site-mix recipe side by side, and drops the materials straight into a job.
This guide is general information for estimating. It is not engineering, legal or tax advice. Check local codes and your inspector for your project.