How Much Concrete Do I Need? Slabs, Pads, Bags and Ready-Mix

By the SAA Tool editor, an IT support engineer · Last reviewed 9 October 2026 · 7 min read

Concrete is bought by volume, so every estimate comes down to length × width × thickness. People go wrong in the details: multiplying feet by inches, leaving out the extra for waste, or dividing by the wrong bag yield. The examples cover a slab, a round pad and a set of small pads, in feet and in metres.

It's general information for planning. A driveway, a garage floor, a house slab or anything structural needs proper plans and must meet local building rules, so ask a qualified professional or your building authority.

Step 1: get the volume in one unit

Put the length, width and thickness in the same unit before you multiply. Thickness is usually given in inches or centimetres and the rest in feet or metres, and that mismatch causes most bad estimates.

• Imperial: a 10 × 10 ft slab, 4 in thick. 4 in ÷ 12 = 0.333 ft, so 10 × 10 × 0.333 = 33.3 cubic feet. Multiply 10 × 10 × 4 and you get 400, twelve times too much.
• Metric: a 3 × 3 m slab, 10 cm thick. 10 cm = 0.1 m, so 3 × 3 × 0.1 = 0.9 m³. Multiply 3 × 3 × 10 and you get 90, a hundred times too much.

Measure the depth in several places and use the average. The US National Ready Mixed Concrete Association (NRMCA) notes that an extra 1/8 in on a 4 in slab means a 3% shortage. By the same sum, a slab 11 cm thick instead of 10 cm needs 10% more.

Cubic feet, cubic yards and cubic metres

A foot is exactly 0.3048 m and a yard exactly 0.9144 m (NIST), so these conversions are fixed:

So 33.3 cubic feet ÷ 27 = 1.23 cubic yards (0.94 m³), and 0.9 m³ is 1.18 cubic yards. Ready-mix is generally priced per cubic yard in the US and per cubic metre in metric countries. More on the factors in our metric and imperial guide.

How much extra to order

Real pours use more than the drawing. The NRMCA's sheet on discrepancies in yield blames spillage, over-digging, forms that bow under wet concrete, an uneven base and some loss of volume as it hardens. It recommends ordering 4 to 10% more than your estimate, and more for stop-start work than for repetitive pours.

Running short is worse than having some left. Quikrete's Concrete Mix data sheet says to fill forms to full depth and avoid partial-depth fills, which can leave cold joints: weak lines where fresh concrete meets concrete that has started to set. Stopping halfway to fetch more bags is how that happens.

With 10% extra, the imperial slab becomes 36.7 cubic feet (1.36 cubic yards) and the metric slab 0.99 m³.

How many bags of premix

Divide the volume with extra by what one bag makes, and round up. These are Quikrete's yields from its Concrete Mix data sheet:

So the 10 × 10 ft slab takes 62 bags of 80 lb (36.7 ÷ 0.6 = 61.1, rounded up): nearly 5,000 lb of mix, and each 80 lb bag needs roughly 6 to 9 pints (2.8 to 4.3 litres) of water. The mix is designed for pours 2 in (50 mm) thick or more.

Metric bags are less standard. Premix comes in sizes such as 20, 25 and 40 kg, and the yield depends on the brand and mix, so read the bag. Quikrete's yield per kilogram gives about 9.4 litres for 20 kg, 11.7 litres for 25 kg and 18.7 litres for 40 kg, which is what our calculator uses; Quikrete's own 25 kg bag of Fast-Setting Concrete Mix, sold in Canada, is rated at about 11.6 litres. The metric slab needs 0.99 ÷ 0.0117 = 84.6, so 85 bags of 25 kg (106 of 20 kg, or 53 of 40 kg).

Bags or ready-mixed concrete?

There's no universal cut-off; it depends on the volume, access, helpers and local prices.

• Bags let you work at your own pace and reach places a truck can't. The cost is labour: every bag is lifted, mixed and barrowed, and you have to keep fresh concrete coming so the slab is placed to full depth.
• Ready-mixed concrete arrives mixed, in one go. You need the base and forms ready, a way from the truck to the forms (chute, barrows or pump) and enough people to place and finish it before it stiffens. Ask about minimum loads and small-load charges. The NRMCA advises re-checking the volume near the end and telling the supplier in good time, rather than finishing with a small top-up load.

Compare a delivered quote with bag count × bag price, plus your time. Our calculator suggests ready-mix once the answer passes 60 bags, but that's a nudge, not a rule.

Round pads and several pads

For a circle, the area is π × diameter² ÷ 4.

• Imperial: a round pad 8 ft across and 4 in thick is 3.1416 × 64 ÷ 4 = 50.3 sq ft, × 0.333 ft = 16.8 cu ft (0.62 cubic yards). With 10% extra, 31 bags of 80 lb.
• Metric: a pad 2.4 m across and 10 cm thick is 3.1416 × 5.76 ÷ 4 = 4.52 m², × 0.1 m = 0.45 m³. With 10% extra, 43 bags of 25 kg.

For identical pads, work out one and multiply. Eight pads 2 × 2 ft and 6 in deep are 16 cu ft; with 10% extra, 17.6 cu ft or 30 bags of 80 lb. Eight pads 60 × 60 cm and 15 cm deep are 0.432 m³; with extra, 41 bags of 25 kg. Round once for the whole job: rounding each pad up separately would mean 32 bags instead of 30, or 48 instead of 41 in metric. Split odd shapes into rectangles and circles and add them up.

How thick should it be?

It depends on the load, the ground and local rules. Quikrete's guide to sidewalks and small slabs says sidewalks (paths) should be at least 4 in (100 mm) thick and simple slabs are normally 4 to 6 in (100 to 150 mm), depending on the load. For larger jobs or poorly drained ground, it suggests adding several inches of gravel or crushed stone underneath; the Gravel Calculator and our guide to how much gravel you need cover that layer. For driveways, garages and house floors, the plans and the building rules where you live set the thickness, reinforcement and base.

Working it out with the Concrete Slab Calculator

1. Open the Concrete Slab Calculator and choose Units: Imperial (feet, inches) or Metric (metres, centimetres).
2. Pick the Shape: Rectangle or square, or Circle (round pad or column).
3. Enter the Length and Width, or the Diameter, and the Thickness. Each box shows its unit, so there's nothing to convert.
4. For identical pads, fill in How many (same size), and choose Extra for waste: None, 5%, 10% (the default) or 15%.
5. Choose the Premix bag size: 40, 50, 60 or 80 lb, or 20, 25 or 40 kg. Results update as you type, or press Calculate; Reset restores the defaults.
6. Read the cubic yards, cubic feet, cubic metres and bags. The Details table shows the volume with and without the extra and the yield per bag.

The defaults match the examples above: 1.23 cubic yards and 62 bags of 80 lb, or 0.9 m³ and 85 bags of 25 kg (after switching units, press Reset to load that system's defaults). It works in your browser, assumes an even thickness and leaves out the base, reinforcement and formwork. The Volume Converter turns cubic feet or cubic metres into litres (it doesn't list cubic yards).

Common mistakes checklist

• Multiplying feet by inches, or metres by centimetres.
• Measuring the thickness in one spot instead of averaging.
• No extra. Allow 4 to 10%, more for rough ground or many small pours.
• Using another product's yield instead of the one printed on your bag.
• Rounding bags down, or rounding each pad separately.
• Forgetting thickened edges, steps or footings, or counting the gravel base as concrete.
• Booking a truck without checking access, helpers and the units on the quote.

Setting fence posts instead? See how many fence posts you need for the concrete per post.

Spotted a mistake or something out of date? Tell us and we'll fix it.