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Concrete Calculator

Cubic yards of concrete for a slab, footing or column, and how many bags that would be.

Enter your dimensions to see cubic yards and how many bags that is.

4 inches is standard for a residential slab.

Identical footings or columns.

10% is usual — sub-grade is never perfectly level.

Bags and cost

Optional

Exact calculation. Given your inputs, this is the answer — no estimation involved.

How this calculator works

Volume by shape, in cubic feet, converted to cubic yards at the exact definition of 27 cubic feet per yard. Thickness is entered in inches because that is how slabs are specified, and divided by twelve internally.

Bag counts use the manufacturer’s published yields: an 80 lb bag of standard concrete mix yields about 0.60 cubic feet, a 60 lb bag about 0.45, and a 40 lb bag about 0.30. Yields vary slightly between products, so bag counts are an estimate — for ready-mix, order by the cubic yard figure.

A waste allowance is applied on top, because sub-grade is never perfectly level and concrete cannot be topped up once it starts to set. Ready-mix is normally sold in quarter-yard increments, so the order quantity is rounded up accordingly.

The formula

Exactly what happens to your numbers, step by step.

  1. Slab or footing

    cubic feet = length (ft) × width (ft) × thickness (in) ÷ 12

  2. Round column or tube

    cubic feet = π × (diameter ÷ 24)² × height (ft)

    Diameter in inches is halved and converted to feet, hence dividing by 24.

  3. Ordering

    with waste  = cubic feet × (1 + waste %)
cubic yards = with waste ÷ 27
bags        = ⌈with waste ÷ bag yield⌉

A worked example

A 10 × 10 foot patio slab, 4 inches thick, with a 10% waste allowance.

What you enter

Length × width
10 ft × 10 ft
Thickness
4 inches
Waste
10%
Bag size
80 lb (0.60 ft³)

The working

Volume
10 × 10 × 4 ÷ 12 = 33.33 ft³
In cubic yards
1.235 yd³
With 10% waste
36.67 ft³
Ready-mix order
1.50 yd³ (rounded to the quarter yard)
Or bags
⌈36.67 ÷ 0.60⌉ = 62

1.5 cubic yards of ready-mix, or 62 bags of 80 lb mix

62 bags is around 5,000 lb of material to mix by hand. Past roughly one cubic yard, ready-mix delivery is usually cheaper, faster and more consistent.

Assumptions

Every result here rests on these. Change your inputs and the result changes with them.

  • The dimensions are the finished dimensions of the pour.
  • Bag yields are the manufacturer’s published figures for standard concrete mix; other products differ.
  • The waste allowance covers uneven sub-grade and spillage, not a change of plan.
  • Ready-mix quantities are rounded up to the next quarter yard, as suppliers sell it.

What this cannot tell you

  • It does not size reinforcement, specify a mix strength, or account for a gravel sub-base.
  • It does not include forms, rebar, mesh, delivery charges or short-load fees.
  • Footing dimensions are usually set by code and soil conditions, which this cannot assess.

Questions people ask

How many cubic feet are in a cubic yard?

Exactly 27 — a yard is three feet, and 3 × 3 × 3 = 27. Concrete is sold by the cubic yard, so this is the conversion that matters.

How thick should a concrete slab be?

Four inches is standard for a patio, shed base or walkway. Five to six inches is typical where vehicles will drive or park. Under three inches is prone to cracking under any real load.

When should I switch from bags to ready-mix?

Around one cubic yard, which is roughly 45 to 60 bags depending on size. Beyond that, mixing by hand is slow enough that the pour can start setting before you finish, which creates cold joints.

Why order extra?

Because sub-grade is never perfectly level, forms bow slightly, and running short mid-pour leaves a visible seam you cannot fix. Ten percent is a normal allowance.

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Sources and review

This calculator uses standard arithmetic with no external rules or published rates, so there is nothing to cite beyond the formulas shown above.

Methodology version 1.0.0 · Last reviewed