How Much Concrete Do I Need? A Step-by-Step Guide
Ordering concrete comes down to one calculation — length × width × thickness — but the units, the waste allowance and the way you buy it all change the number you end up ordering. This guide walks through the maths with real examples in both imperial and metric.
The basic formula
Concrete is ordered by volume. For a rectangular slab, volume is length × width × thickness. The catch is that thickness is almost always measured in inches or centimetres while length and width are in feet or metres, so you need to convert first.
In imperial units: cubic feet = length (ft) × width (ft) × thickness (in) ÷ 12. Cubic yards = cubic feet ÷ 27. In metric: cubic metres = length (m) × width (m) × thickness (cm) ÷ 100. Those two divisions — by 12 and by 27 — are where most hand calculations go wrong.
Worked example: a 12 × 16 ft shed base
- Area: 12 × 16 = 192 sq ft.
- Volume: 192 × 4 in ÷ 12 = 64 cu ft.
- Cubic yards: 64 ÷ 27 = 2.37 cu yd.
- With 10% waste: 2.37 × 1.10 = 2.61 cu yd — order 2¾ yd of ready-mix.
In bags, 64 cu ft × 1.10 ÷ 0.60 cu ft per 80 lb bag is about 118 bags, or more than four tons of premix to lift. That is the point where a ready-mix truck becomes the sensible option.
Worked example: a 3 × 3 m patio
- Area: 3 × 3 = 9 m².
- Volume at 10 cm: 9 × 0.10 = 0.90 m³.
- With 10% waste: 0.99 m³ — about 1 m³ of ready-mix, or roughly 85 bags of 25 kg premix at ~11.7 L per bag.
Choosing thickness
Thickness is a design decision, and it changes the volume in direct proportion. These are common starting points for non-structural work; check local codes for anything that carries a building or vehicles.
| Use | Imperial | Metric |
|---|---|---|
| Garden path, patio, shed base | 4 in | 100 mm |
| Hot tub pad, heavier shed | 4–6 in | 100–150 mm |
| Driveway, garage floor | 5–6 in | 125–150 mm |
| Post holes | to depth of hole | to depth of hole |
Waste: how much extra to order
Concrete fills every dip in the sub-base and every bow in the forms. Even careful DIY pours rarely finish with exactly the calculated volume. A 5% allowance suits a well-compacted, screeded base with rigid forms; 10% is a good default for most patios and shed bases; 10–15% is wiser for hand-dug edges or uneven ground. Post holes vary even more because their sides are rarely straight — 10–20% is common.
Running short is worse than having a little left over. A second delivery or a dash to the store leaves a cold joint in the slab where the first pour has started to set.
Round slabs, footings and post holes
For a circle, area is π × radius². A round 8 ft hot-tub pad has a radius of 4 ft, so area = 3.1416 × 16 = 50.3 sq ft; at 4 in thick that is 16.8 cu ft, or 0.62 cu yd. For a cylindrical footing or post hole, multiply the circular area by the depth — and subtract the post if it stays in the hole.
Continuous strip footings are just long, narrow slabs: length × width × depth. Remember that footings are structural; their size is set by code or an engineer, not by a calculator.
Turning volume into an order
- Under about 1 cu yd (0.76 m³): bags are usually simplest. Use the bag calculator to compare sizes.
- 1–3 cu yd: get quotes for both a mini-mix delivery and ready-mix; short-load fees vary.
- Above 3 cu yd: ready-mix, with a pump if the truck cannot reach the forms.
- Tell the plant the thickness and the use so they can suggest an appropriate mix.
Slopes, steps and irregular pours
A slab that falls for drainage is thicker at one edge than the other. Use the average thickness: a patio that is 4 in at the house and 5 in at the far edge averages 4.5 in. Steps are a stack of rectangular blocks — calculate each tread block from the ground up and add them, or treat the whole flight as a series of slabs of decreasing length. For a free-form patio, divide it into rectangles and triangles, or lay a grid of 1 ft or 0.5 m squares on your sketch and count them.
Thickened edges are common on shed bases and garage slabs: the outer 8–12 in of the slab is deeper to act as a simple footing. Calculate the main slab first, then add a strip along the perimeter for the extra depth. A 12 × 16 ft slab with an edge thickened from 4 in to 8 in over a 12 in width adds about 56 ft × 1 ft × 4/12 ft ≈ 18.7 cu ft — almost 0.7 cu yd, which is easy to overlook.
Base, reinforcement and timing
The gravel or hardcore base is ordered separately. A typical patio uses 4 in (100 mm) of compacted crushed stone, which is the same area times that depth — use the gravel calculator for tonnes. Reinforcement mesh is sold in sheets or rolls; rebar by the length. Neither changes the concrete volume meaningfully.
Book a ready-mix delivery for a morning in mild weather when possible. Very hot, windy or freezing conditions shorten or lengthen the working time and can harm the finished surface. Have tools, helpers and access ready before the truck arrives, because waiting time is usually charged.
Common mistakes
- Multiplying feet by inches without dividing by 12.
- Forgetting to divide cubic feet by 27 to get cubic yards.
- Measuring the outside of the form boards instead of the inside.
- Ignoring the gravel base, which is a separate order.
- Ordering exactly the calculated volume with no allowance for waste.
Quick questions
How many cubic yards of concrete for a 10 × 10 slab?
At 4 in thick, 1.23 cu yd before waste, or about 1.36 cu yd with 10% extra.
How many 80 lb bags make a yard?
About 45 bags, at a typical yield of 0.6 cu ft per bag.
Calculators for this guide
- Concrete Slab Calculator — Estimate cubic yards, cubic metres and bags of concrete for a slab, patio or pad — with waste factor, the formula and a shopping list.
- Concrete Bag Calculator — How many bags of concrete do you need? Compare 40, 60 and 80 lb or 20, 25 and 30 kg bags, total weight and cost — with the math shown.
- Gravel Calculator — Estimate gravel, crushed stone or pea gravel in cubic yards, cubic metres, short tons and tonnes for paths, driveways and bases.
- Cubic Yard Calculator — Convert length, width and depth into cubic yards, cubic feet, cubic metres and litres for soil, mulch, gravel or concrete orders.
More guides
Estimates only. This guide explains general methods. Verify quantities with your supplier and follow local building codes; for structural or safety-critical work consult a licensed professional. Editorial policy · Report an error.