Concrete Calculator

Work out how many cubic yards of concrete — or how many 40, 60 or 80 lb bags — you need for a slab, footing or round column.

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Concrete needed
1.36

Over a cubic yard: ready-mix delivery is usually cheaper and easier than mixing bags.

Bags needed by bag sizebags
02040608010012014080 lb — Bags: 62 bags80 lb60 lb — Bags: 82 bags60 lb40 lb — Bags: 123 bags40 lb
80 lb bags
62
60 lb bags
82
40 lb bags
123
Show the working
  1. One pour = 33.333 ft³
  2. × 1 × (1 + 10%) = 36.67 ft³ = 1.358 yd³ = 1.038 m³
  3. 80 lb bags = 36.67 ÷ 0.60 → 62

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Formula

Slab / footing: V = length × width × thicknessRound column: V = π × (diameter ÷ 2)² × height1 yd³ = 27 ft³ = 0.7646 m³Bags = V (ft³) ÷ bag yield, rounded up

How it works

Concrete is ordered by volume — cubic yards in the US, cubic metres elsewhere. Multiply the three dimensions in the same unit and convert: a 10 × 10 ft patio slab 4 inches thick is 10 × 10 × ⅓ = 33.3 ft³, or about 1.23 yd³.

Always order a little extra. Uneven sub-grade, form bulges and spillage easily eat 5–10%, and running short in the middle of a pour leaves a weak cold joint. The waste box adds that margin before bags are counted.

Premixed bags yield roughly 0.30 ft³ (40 lb), 0.45 ft³ (60 lb) and 0.60 ft³ (80 lb) of placed concrete; check the bag for the exact figure. For anything over about a cubic yard, a ready-mix truck is usually cheaper and far less work than mixing 45+ bags by hand.

Frequently asked questions

How thick should a concrete slab be?

Four inches is standard for patios, walkways and shed floors. Driveways and garage floors carrying cars are usually 4–6 inches, and heavier loads need an engineer’s spec.

How many 80 lb bags make a cubic yard?

About 45, because a cubic yard is 27 ft³ and each bag yields about 0.6 ft³.

What about a footing?

A continuous footing is just a long, narrow slab: enter its length, width and depth. For round post holes or Sonotubes, use the column option.

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