Concrete Calculator

Calculate how much concrete you need for a slab, patio, footing or round column. Results are in cubic yards (for ready-mix trucks) and in premixed bags.

Your measurements

Length
Width
in

4 in for patios and walkways, 5–6 in for driveways.

%

5–10% is typical.

* Required. Use feet, inches or both. Decimals and fractions like 6 1/2 are fine.

Results

Enter your measurements — results appear here instantly.

How the concrete calculator works

  1. Slab volume = length × width × (thickness in inches ÷ 12), in cubic feet.
  2. Column volume = π × radius² × height, with the diameter converted from inches to feet.
  3. Add extra for spillage, uneven ground and over-excavation: volume × (1 + extra %).
  4. Cubic yards = cubic feet ÷ 27. Ready-mix suppliers sell by the cubic yard.
  5. Bags = cubic feet ÷ bag yield, rounded up. An 80 lb bag yields about 0.60 cu ft, a 60 lb bag 0.45 cu ft and a 40 lb bag 0.30 cu ft.

Assumptions

  • Bag yields are the typical figures printed on standard premixed concrete bags. Check your brand, since high-strength and fast-setting mixes can differ slightly.
  • The default 10% extra covers uneven subgrade and spillage. Use more for rough excavations.
  • Gravel base, rebar and forms are not included. Use the gravel calculator for the base layer.

Worked example

A 10 ft × 10 ft patio slab, 4 in thick, with 10% extra:

  • Volume: 10 × 10 × (4 ÷ 12) = 33.33 cu ft
  • With 10% extra: 33.33 × 1.10 = 36.67 cu ft
  • Cubic yards: 36.67 ÷ 27 = 1.36 yd³
  • 80 lb bags: 36.67 ÷ 0.60 = 61.1, which rounds up to 62 bags

At more than a cubic yard, a ready-mix delivery or a rental mixer is usually worth pricing against 62 bags.

Tips before you buy

  • Mixing more than about 1 cubic yard by hand is hard work. Compare the price of a short-load ready-mix delivery.
  • Put down a compacted gravel base of 4 in under slabs to help drainage and reduce cracking.
  • Have a few extra bags on site. Running short mid-pour leaves a weak cold joint.

Frequently asked questions

How many 80 lb bags of concrete are in a yard?

About 45. One cubic yard is 27 cu ft, and each 80 lb bag yields about 0.60 cu ft, so 27 ÷ 0.60 = 45 bags.

How thick should a concrete slab be?

Walkways and patios are usually 4 in. Driveways for passenger cars are 4–5 in, and 6 in for heavier vehicles. Always follow local building codes for structural work.

How do I calculate concrete for fence posts?

Choose "Round column or post hole", enter the hole diameter and depth, then set the quantity to the number of posts. For a closer figure, subtract the volume of the post itself.