Concrete and hardscape

Concrete Calculator

Work out how much concrete you need for a slab, patio, path or round footing, in cubic yards, cubic feet and cubic meters, plus how many 40, 60 or 80 lb bags to buy.

Units
ft
ft
in
%
Our default of 10% is a rule of thumb, not a standard. Set 0 for the exact volume.

Enter your measurements and press Calculate.

How we calculate

We work out the volume in cubic feet, convert it to cubic yards (the unit ready-mix concrete is ordered in), then divide by how much one bag makes.

Slab: volume (cu ft) = length (ft) x width (ft) x thickness (in) / 12 Round footing: volume (cu ft) = 3.14159 x (diameter / 2)^2 x depth, all in feet Cubic yards = cubic feet / 27 (a cubic yard is 3 x 3 x 3 ft) Bags = volume x (1 + extra %) / bag yield, rounded up

Bag yields come from the manufacturer data sheet for QUIKRETE Concrete Mix: an 80 lb bag yields about 0.60 cu ft, a 60 lb bag about 0.45 cu ft and a 40 lb bag about 0.30 cu ft. Source: QUIKRETE Concrete Mix data sheet (PDF). Other brands may differ, so check your bag.

Metric: we convert your meters and centimeters to feet and inches using 1 ft = 0.3048 m, do the same math, and show cubic meters. One cubic meter is about 1.31 cubic yards.

Worked example

A 10 x 10 ft patio slab, 4 inches thick, no extra:

  1. Volume: 10 x 10 x 4 / 12 = 33.33 cu ft
  2. Cubic yards: 33.33 / 27 = 1.23 cu yd
  3. 80 lb bags: 33.33 / 0.60 = 55.6, rounded up to 56 bags (or 75 bags of 60 lb, or 112 bags of 40 lb)

With the default 10% extra, the same slab needs 36.67 cu ft, which is 62 bags of 80 lb.

A concrete slab showing length, width and thicknessLength 10 ftWidth 10 ft4 in10 x 10 x (4/12) = 33.33 cu ft
Thickness is entered in inches and divided by 12 to get feet.

Four round post holes, 12 in across and 48 in deep: each is 3.14159 x 0.5 x 0.5 x 4 = 3.14 cu ft, so four are 12.57 cu ft (0.47 cu yd), or 21 bags of 80 lb with no extra.

Bags or a ready-mix truck?

For small jobs such as fence posts, a few steps or a small pad, premixed bags are easy to buy and mix yourself. For larger slabs, ask local ready-mix suppliers about delivery: they sell by the cubic yard, and many have a minimum order. Compare their quote with the bag count above before you decide.

Materials checklist

  • Concrete (bags or ready-mix), using the numbers above
  • Gravel or compacted base if your plans call for it
  • Form boards and stakes for slabs; tube forms for round footings if needed
  • Reinforcement (mesh or rebar) only as your plans or local code require
  • Wheelbarrow or mixer, hoe, shovel, screed board, float and edger
  • Gloves, eye protection and long sleeves: wet concrete can irritate or burn skin

FAQ

How many bags of concrete do I need for a 10×10 slab?

At 4 inches thick, a 10 x 10 ft slab is 33.33 cubic feet, which is 56 bags of 80 lb (0.60 cu ft each) before any extra. At 6 inches thick it is 50 cubic feet, or 84 bags of 80 lb.

How many cubic feet are in a cubic yard?

27. A cubic yard is a cube 3 feet on each side: 3 x 3 x 3 = 27 cubic feet.

Why round the bag count up?

You cannot buy part of a bag, and running short in the middle of a pour is a real problem. We always round up.

How much extra concrete should I order?

Ground is rarely perfectly flat and some mix is always lost. We default to 10% extra as a rule of thumb; your supplier or contractor may suggest a different amount for your job.

Does this tell me how thick my slab or footing must be?

No. Thickness, footing size and reinforcement depend on the load, soil and local building code. Check your plans or ask your local building department.

Part of our outdoor-career toolkit: From office to outdoors.

Estimates only. Results are estimates based on the measurements you enter. Check local building codes and your product labels, confirm quantities with your supplier, and buy a little extra material. This is not professional or structural advice. Concrete work may need a permit; footings, reinforcement and slab thickness are set by your plans and local code, not by this calculator. Last checked: September 27, 2026.

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