Solve backwards
How big a slab will my bags of concrete make?
- 20 bagsThe example worked below
- Concrete CalculatorEvery figure is its own
You have
How much bagged concrete?
Worked by the Concrete Calculator at the settings below.
The settings it uses
- Slab Thickness
- 4 in
- Waste Factor (%)
- 10
What it will do
32.7 sq ft of slab
3.04 m²
One slab. Enough for a slab 5.7 × 5.7 ft at 4 in deep.
Or several. Less than one slab of the size the Concrete Calculator opens on (a slab 10 × 10 ft at 4 in deep).
Check it forwards
- Slab Length
- 5.72 ft
- Slab Width
- 5.72 ft
- It asks for
- 20 bags
- You have
- 20 bags
The Concrete Calculator asks for 20 bags at that size, and 21 bags one step larger.
If you had more, or less
Around your amount
Each worked by the Concrete Calculator at the same settings.
Half
10 bags
16.4 sq ft
a slab 4 × 4 ft at 4 in deep
Yours
20 bags
32.7 sq ft
a slab 5.7 × 5.7 ft at 4 in deep
Half as much again
30 bags
49.1 sq ft
a slab 7 × 7 ft at 4 in deep
Double
40 bags
65.5 sq ft
a slab 8.1 × 8.1 ft at 4 in deep
Common amounts
What each amount of bagged concrete covers
The sizes it is usually sold in, each worked by the Concrete Calculator at the settings above.
| You have | It covers | As one slab | Average slabs |
|---|---|---|---|
| 10 bags | 16.4 sq ft(1.52 m²) | a slab 4 × 4 ft at 4 in deep | under 1 |
| 20 bags | 32.7 sq ft(3.04 m²) | a slab 5.7 × 5.7 ft at 4 in deep | under 1 |
| 40 bags | 65.5 sq ft(6.08 m²) | a slab 8.1 × 8.1 ft at 4 in deep | under 1 |
| 60 bags | 98.2 sq ft(9.12 m²) | a slab 9.9 × 9.9 ft at 4 in deep | under 1 |
| 100 bags | 164 sq ft(15.2 m²) | a slab 12.8 × 12.8 ft at 4 in deep | 1 |
Six tenths of a cubic foot a bag
An 80 lb bag yields about 0.6 cubic feet of mixed concrete, 17 litres. With the calculator's ten per cent over the geometry, twenty bags pour about 32.7 square feet at 4 inches — a slab about 5.7 feet square — or 3.1 m² at 100 mm. At 6 inches (150 mm) the same twenty bags pour about 21.8 square feet, and a cubic yard takes 45 bags before any waste at all.
Bags are for small pours
A 10 by 10 foot patio at 4 inches takes 62 bags by the calculator's count, nearly 5,000 lb (2.2 t) mixed by hand, and a 3 by 3 m slab at 100 mm takes 59. Past about a cubic yard, the labour of mixing and the risk of a cold joint between batches start to outweigh the convenience, and a ready-mix load — which has a page of its own here — is worth pricing.
Yield is printed on the bag
The 0.6 cubic foot figure is the published nominal yield at the maker's water content; real yield shifts with the product and the water added. Other sizes scale by weight — the site's bag figures put a 60 lb bag at about 0.45 cubic feet and a 40 lb bag at 0.30, and the 25 kg bag sold in metric markets at about 11.7 litres. Fast-setting post mixes yield differently again.
How the answer is worked
Nothing here is a second formula. The Concrete Calculator runs forwards — a size in, a quantity out — and this page grows its own starting slab in proportion until the quantity it asks for is what you have. Where it rounds up to whole containers, the answer is the largest size before the next one would be needed.
Where the calculator adds an allowance for cuts or breakage when you order, the same allowance comes off here, because it is the same calculator: a stack you already own finishes less than its face value. Where it adds none, neither does this page — the settings beside the answer say which.
Concrete Calculator
runs it forwardsEstimate how much concrete a slab needs — in cubic yards or cubic metres, and as a count of premix bags.
Work out what to buy insteadSolve backwards
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Solve backwards
Start from what is already in the yard
Paint, sheets, rolls, bags and pieces, each read the other way by the calculator that orders it.
The calculator's own figures
Nothing is estimated twice: the page runs the same calculator you would order with.
The largest size that fits
Where a calculator rounds up to whole containers, the answer stops before the next one.
Both unit systems
Gallons and litres, square feet and square metres, side by side.
Frequently asked questions
- How much slab does 20 bags of bagged concrete cover?
- At the Concrete Calculator's own settings, 32.7 sq ft (3.04 m²) — a slab 5.7 × 5.7 ft at 4 in deep. Every figure follows the settings, which you can change in the calculator.
- How many 80 lb bags of concrete make a yard?
- 45 bags at the published yield of about 0.6 cubic feet (17 L) a bag, since a cubic yard is 27 cubic feet. With the calculator's ten per cent for spillage and an uneven base, plan on about 50. A cubic metre is about 59 bags before any waste.
- How many bags of concrete for a 10 by 10 slab?
- At 4 inches thick a 10 by 10 foot slab is 33.3 cubic feet; with ten per cent it is 36.7, or 62 bags of 80 lb. At 6 inches it is 92 bags. A metric 3 by 3 m slab at 100 mm needs 0.99 m³ with waste — 59 bags of 80 lb.
- How many square feet does an 80 lb bag of concrete cover?
- About 1.8 square feet at 4 inches thick before waste — 0.6 cubic feet divided by a third of a foot — or 1.6 with the calculator's ten per cent. At 2 inches (50 mm) it is about 3.6 square feet before waste. In metric, a bag covers about 0.17 m² at 100 mm.