Concrete Slab Calculator

Concrete for a patio, shed base or driveway slab: enter length, width and thickness to get cubic yards, m³, weight and the number of premixed bags.

A slab's volume is length × width × thickness, with the thickness converted to the same unit as the sides. In feet: 12 ft × 10 ft × 4 in is 12 × 10 × 0.333 = 40 cubic feet, divided by 27 is 1.48 cubic yards. In metres: 3.6 m × 3 m × 100 mm is 3.6 × 3 × 0.1 = 1.08 m³. Enter your slab's sizes and this calculator gives the volume in cubic yards, cubic feet and cubic metres, the premixed bags in US, UK or Australian sizes, the weight, and an optional cost. Slabs that aren't rectangles can be built from several areas: add a rectangle, a triangle and a half-circle and they're totalled for you.

Units
Bag sizes
Pour
Area
ft
ft
in
Feet and inches work too: type 12'6 or 12.5.
%
Concrete
1.36 yd³

36.67 ft³ · 1.038 m³

Bags
62

80 lb bag (0.60 ft³)

Weight when mixed
5,494 lb

2.75 tons

Premixed bags needed by size
80 lb bag (0.60 ft³)62
60 lb bag (0.45 ft³)82
50 lb bag (0.375 ft³)98
40 lb bag (0.30 ft³)123
20 kg bag113
25 kg bag89
30 kg bag74
That's 62 × 80 lb bags. At this size many people order ready-mix instead: ask suppliers for 1.36 cubic yards, and check their minimum load and how they round.
How it was worked out
  1. Area: 10 ft × 10 ft = 100 ft²; × 4 in (0.333 ft) deep = 33.33 ft³
  2. 33.33 ft³ ÷ 27 = 1.23 yd³
  3. Plus 10% extra = 36.67 ft³ (1.36 yd³)
  4. 80 lb bag (0.60 ft³): 36.67 ft³ ÷ 0.6 ft³ made per bag = 61.11, rounded up to 62 bags

Bag yields are typical figures: 0.30, 0.375, 0.45 and 0.60 ft³ for 40, 50, 60 and 80 lb bags, and about 108 × 20 kg bags per m³. Check the yield printed on the bag you buy. Mixed concrete is taken as 2,400 kg/m³ (about 4,050 lb per cubic yard).

Common use cases

  • Size a patio, garden path or hot tub pad
  • Work out concrete for a shed or garage base
  • Plan a driveway pour and compare bags with ready-mix
  • Total an L-shaped or curved slab from simple shapes

How to use

  1. Choose your units and bag sizes.
  2. Enter the slab's length, width and thickness. For odd shapes, add extra areas and switch their shape.
  3. Keep 10% extra, or more if the base is uneven.
  4. Read the cubic yards or m³, the bags of each size and the weight.

FAQ

How thick should a patio, shed base or driveway be?

Common figures are 4 inches (100 mm) for patios, paths and shed bases, and 5 to 6 inches (125 to 150 mm) for driveways. Heavier loads, soft ground and local codes can call for more, and slabs often thicken at the edges.

Do I need gravel under a concrete slab?

Usually, yes. A compacted base of crushed stone (often 4 inches in the US) or MOT Type 1 (often 100 mm in the UK) gives the slab an even, well-drained bed. Use the crushed stone or Type 1 calculators to work out the base.

Does rebar or mesh change how much concrete I need?

Not in any way that matters. Steel takes up a tiny share of a slab's volume, much less than the 5 to 10% extra most people add anyway.

How do I measure a slab that isn't a rectangle?

Split it into rectangles, triangles and circles, enter each as its own area, and the calculator adds them. For a half-circle, enter the full circle in a separate calculation and halve it, or use Known area with the area you've measured.

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