How many bags of concrete do I need for 1 cubic yard?

1 cubic yard is 27 cubic feet. At Quikrete's yields that is 45 bags of 80 lb (27 ÷ 0.60), 60 bags of 60 lb (27 ÷ 0.45) or 90 bags of 40 lb (27 ÷ 0.30).

How many 40 kg bags of concrete make a cubic metre?

About 54. A 40 kg bag makes roughly 18.8 litres of concrete, and 1,000 ÷ 18.8 = 53.2, so 54 bags with nothing to spare. 20 kg bags make half as much each, so about 107.

How do I work out cubic yards of concrete?

Multiply length × width × thickness in feet, then divide by 27. A 10 × 10 ft slab 4 inches thick is 10 × 10 × 0.333 = 33.3 cu ft, which is 1.23 cubic yards.

How thick should a concrete slab be?

That depends on the load and the local code, not on this calculation. The page takes whatever thickness you enter; anything carrying vehicles needs more than a garden path, so take the figure from the plans or the building department.

Is it cheaper to use bags or ready-mix?

For a few bags' worth — post holes, a step, a small pad — bags are simplest. Once the count runs into dozens, get a ready-mix quote for the cubic yards shown here and compare; a truck has a minimum load and a delivery charge.

Concrete Calculator: Volume, Cubic Yards and Bags Needed

Concrete for a slab in m³ or cubic yards, and how many 20–40 kg or 40–90 lb bags it takes.

Inputs

Length(m)

0.01 m100 m

Width(m)

0.01 m100 m

Depth (thickness)(m)

0.01 m5 m

Units

Bag size

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Concrete volume

1.2 m³

Working: 4 m × 3 m × 0.1 m = 1.2 m³
40 kg bags: 64 — 1.2 m³ ÷ 18.8 L (0.0188 m³) a bag = 63.9, rounded up
Ready-mix order: 1.2 m³
Footprint area: 12 m²

A bag makes about 0.47 L of concrete per kg of mix (Quikrete's data sheet: 17 L from an 80 lb / 36.2 kg bag). Other brands differ a little — the bag's own yield wins.

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What each Concrete input means

Length: The slab's length in metres, or feet in US units. For an L-shape, work out each rectangle separately and add the volumes.
Width: The slab's width, in the same unit.
Depth (thickness): The finished thickness — in metres or feet, not centimetres or inches: 10 cm is 0.1 m and 4 inches is 0.333 ft.
Units: Metric gives cubic metres; US units give cubic feet and the cubic yards ready-mix is ordered in.
Bag size: The bag you will buy. Yields come from Quikrete's data sheet — an 80 lb bag makes 0.60 cu ft — and metric bags are scaled from the same 17 litres per 36.2 kg.

Concrete formula

volume = length × width × depth · bags = volume ÷ yield of one bag, rounded up · 1 cu yd = 27 cu ft

V = l \times w \times d, \qquad n = \left\lceil \frac{V}{V_{\text{bag}}} \right\rceil

A slab is a box, so its volume is length × width × thickness — in metres that gives cubic metres, in feet cubic feet, and dividing cubic feet by 27 gives the cubic yards a ready-mix plant sells by. Bagged concrete is sold by weight but used by volume, so the bag count depends on what one bag makes when mixed. Quikrete's data sheet gives the figures: an 80 lb bag makes 0.60 cubic feet (17 litres), a 60 lb bag 0.45 and a 40 lb bag 0.30 — the same 0.47 litres of concrete per kilogram of mix at every size, which is how the metric bags here are worked out: a 40 kg bag makes about 18.8 litres. Divide the slab's volume by one bag's yield and round up; a part bag still has to be bought. The working is shown with the result, so you can swap in the yield printed on your own bag.

Concrete worked example

A 4 × 3 m patio slab 10 cm thick: 4 × 3 × 0.1 = 1.2 m³. A 40 kg bag makes about 18.8 litres (0.0188 m³), so 1.2 ÷ 0.0188 = 63.9 — 64 bags. In US units the same slab is about 13 × 10 ft at 4 inches: 42.4 cu ft, or 1.57 cu yd, which is 71 bags of 80 lb. At that size a ready-mix truck is usually the better buy; bags suit post holes, steps and small pads.

Where the Concrete calculator can give the wrong answer

Yields differ between brands and mixes. Quikrete's are used here because they are published; a high-strength or fast-setting mix can make a little more or less per bag, and the figure on your bag wins.
Depth is the finished thickness. A 4-inch slab is 0.333 ft, not 4 — and in metric, 10 cm is 0.1 m.
Uneven ground, forms that bow and spillage all eat concrete, so the exact bag count is a minimum. Rebar, mesh and anything else in the slab displaces almost nothing and is ignored.
Round columns and post holes are cylinders: π × radius² × depth. Work the volume out separately and add it.
Until 2 Oct 2026 this page assumed 0.023 m³ per 40 kg bag, a figure with no source that was 23% too high; metric counts from before then are about a fifth too low.

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Concrete reference table

Bags of concrete mix per cubic yard and per cubic metre
Bag	Yield of one bag	Bags per cubic yard	Bags per cubic metre
40 lb	0.30 cu ft	90	118
50 lb	0.375 cu ft	72	95
60 lb	0.45 cu ft	60	79
80 lb	0.60 cu ft (17 L)	45	59
90 lb	0.675 cu ft	40	53
20 kg	about 9.4 L	82	107
25 kg	about 11.7 L	66	86
40 kg	about 18.8 L	41	54

Source: Quikrete Concrete Mix (No. 1101) data sheet, revised October 2022, for the US bags; metric bags at the same 17 L per 36.2 kg; counts rounded up

Concrete FAQ

: 1 cubic yard is 27 cubic feet. At Quikrete's yields that is 45 bags of 80 lb (27 ÷ 0.60), 60 bags of 60 lb (27 ÷ 0.45) or 90 bags of 40 lb (27 ÷ 0.30).
: About 54. A 40 kg bag makes roughly 18.8 litres of concrete, and 1,000 ÷ 18.8 = 53.2, so 54 bags with nothing to spare. 20 kg bags make half as much each, so about 107.
: Multiply length × width × thickness in feet, then divide by 27. A 10 × 10 ft slab 4 inches thick is 10 × 10 × 0.333 = 33.3 cu ft, which is 1.23 cubic yards.
: That depends on the load and the local code, not on this calculation. The page takes whatever thickness you enter; anything carrying vehicles needs more than a garden path, so take the figure from the plans or the building department.
: For a few bags' worth — post holes, a step, a small pad — bags are simplest. Once the count runs into dozens, get a ready-mix quote for the cubic yards shown here and compare; a truck has a minimum load and a delivery charge.