Running short on a pour is one of the most expensive mistakes on a small job. The truck has gone, one edge is starting to stiffen, and you are mixing bags in a hurry or paying for a second delivery with a short-load fee on top. Over-ordering wastes money too, but far less of it. A good concrete estimate comes down to four habits: measure what is really there, keep your units straight, add a waste allowance that suits the site, and order in the steps your supplier actually sells.
1. Measure the real dimensions
Measure inside the formwork, not off the drawing. Forms are rarely set exactly to plan, and an extra 25 mm on each side of a large slab adds up. For irregular shapes, split the plan into rectangles, triangles and circles. Work out each area separately and add them together, and keep the sketch with your notes so you can check it later.
Thickness is where most errors creep in. A sub-base that dips by 10–15 mm across a large area can add several percent to the volume. Run a string line or laser across the forms and measure down to the base at regular points: at each corner, along the edges and through the middle. Use the average, not the design figure. If one area is much deeper, for example where a soft spot was dug out and backfilled, measure it as its own piece.
Look for features that need extra concrete:
- Thickened edges under walls or along a garage door opening, often 200–300 mm deep for a strip 300 mm wide.
- Internal beams or pads under point loads such as posts or a wood heater.
- Steps and ramps formed in the same pour.
- Drainage falls. A slab that falls 25 mm over 3 m is thicker at one side, so measure both.
2. Convert everything to one unit
The classic trap is mixing metres and millimetres. A 6 × 4 m slab that is 100 mm thick is 6 × 4 × 0.1 = 2.4 m³. Always turn millimetres into metres (divide by 1,000) before you multiply.
Formula: volume = length × width × thickness, with every dimension in the same unit. For a circle, volume = π × radius² × thickness.
Some useful figures: a 100 mm slab takes 1 m³ for every 10 m². A 150 mm slab takes 1 m³ for every 6.7 m².
3. Add a waste allowance
Concrete is lost to uneven sub-bases, slight form bulges, spillage, what's left in the pump line or barrow, and the slurry you can't get out of the drum. Typical allowances:
| Situation | Allowance |
|---|---|
| Formed slab on a well-compacted, level base | 5% |
| Typical residential slab, patio or shed base | 8–10% |
| Hand-dug trenches, uneven ground, pumped pours | 10–15% |
| Very small pours (under 1 m³) | An extra bag or two rather than a percentage |
Waste is cheaper than a second delivery. If you are unsure, round up.
4. Decide how to supply it
Below about 1 m³, pre-mixed bags are usually simplest. Above that, premix from a concrete truck, or a mini-mix or mix-on-site truck is normally cheaper and far less work, and the concrete is more consistent. Our bags vs ready-mix guide covers the break-even point in detail.
Premix is usually ordered in 0.2 m³ steps (some plants go to 0.1 m³), so round up. Expect a short-load fee below about 3–4 m³, and ask about waiting-time charges. When you phone the order through, have ready:
- The volume, rounded up to the supplier's step.
- The mix: the strength grade, such as N20 for paths and N25 or N32 for driveways and house slabs, plus the slump (often 80–100 mm) and the maximum aggregate size.
- Access: how close the truck can get, and whether you need a pump or a longer chute.
- Time on site. Plan to discharge within the free unloading time, typically 5–10 minutes per cubic metre.
5. Don't forget reinforcement and extras
The concrete line is only part of the takeoff. A complete slab estimate usually includes:
- Compacted road base or crushed rock, compacted in layers no thicker than about 75 mm.
- A 0.2 mm polyethylene vapour barrier and termite protection to AS 3660.1 where the slab is inside a building.
- Reinforcing mesh (SL72 or SL82) or reo bars, plus chairs or spacers so it sits in the slab, not on the ground.
- Edge forms, stakes, expansion joint strip, and fixings.
- Curing compound or sheeting, especially in hot or windy weather.
- Pump hire if the truck can't reach.
- Saw-cutting control joints, typically at spacings of about 24–30 times the slab thickness.
A worked example
A 7 × 5 m garage slab, 125 mm thick, with a 300 × 300 mm thickened edge along the 7 m door side. The slab is 7 × 5 × 0.125 = 4.375 m³. The thickened edge adds 7 × 0.3 × 0.175 = 0.37 m³ (only the extra 175 mm below the slab). Total 4.74 m³. Add 8% waste for 5.12 m³, so order 5.2 m³.
The concrete slab calculator does the main slab for you, with the ready-mix order, every bag size and a site-mix recipe side by side. Add the edge with the footing calculator, add both to the same job, and the quote carries the totals.
Common mistakes
- Using the drawing thickness instead of the measured one.
- Forgetting thickened edges, pads and steps.
- Rounding the order down to save money, then paying for a second truck.
- Ordering the wrong mix for the job, for example a general-purpose mix for a driveway.
- No plan for access. If the truck can't reach, the price of the pour changes completely.
This guide is general information for estimating. It is not engineering, legal or tax advice. Check the NCC, Australian Standards and your building surveyor or certifier for your project.