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 inch 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 base that dips by half an inch 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 8–12" deep for a strip 12" wide.
- Internal beams or pads under point loads such as posts or a wood stove.
- Steps and ramps formed in the same pour.
- Drainage falls. A slab that falls 1" over 10 ft is thicker at one side, so measure both.
2. Convert everything to one unit
In Canada the trap is mixing systems: the plans are in feet and inches, but ready-mix is sold by the cubic metre. A 20 × 12 ft slab that is 4 inches thick is 20 × 12 × (4 ÷ 12) = 80 cubic feet, not 960. Divide cubic feet by 35.3 to get cubic metres: 80 ÷ 35.3 = 2.27 m³.
Formula: volume = length × width × thickness, with every dimension in the same unit. For a circle, volume = π × radius² × thickness.
Some useful conversions: 1 m³ = 35.3 cu ft = 1.31 cu yd. A 4" slab takes 1 m³ for every 106 sq ft. A 6" slab takes 1 m³ for every 71 sq ft.
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, ready-mix 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.
Ready-mix is ordered by the cubic metre, usually in 0.25 or 0.5 m³ steps, so round up. Most plants charge a short-load fee below their minimum load, and a winter heat charge in cold months, so compare those against a bag pour. When you phone the order through, have ready:
- The volume, rounded up to the supplier's step.
- The mix: strength in MPa and the exposure class: 32 MPa air-entrained concrete (CSA class C-2) for driveways, garage floors and outdoor flatwork that sees de-icing salt, and 20–25 MPa for footings and foundation walls, plus the slump.
- 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:
- Granular A or ¾" crusher run base, compacted in layers no thicker than about 3".
- Vapour barrier or polyethylene where the slab is inside a building.
- 10M or 15M rebar or welded wire mesh, plus chairs or spacers so it sits in the slab, not on the ground.
- Edge forms, stakes, expansion joint strip, and fasteners.
- Curing compound or plastic 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 24 × 16 ft garage slab, 5 inches thick, with a 12 × 12 inch thickened edge along the 24 ft door side. The slab is 24 × 16 × (5 ÷ 12) = 160 cu ft. The thickened edge adds 24 × 1 × (7 ÷ 12) = 14 cu ft (only the extra 7 inches below the slab). Total 174 cu ft = 4.93 m³. Add 8% waste for 5.32 m³, so order 5.5 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 local codes and your inspector for your project.