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Concrete Calculator

Enter a slab's length, width and thickness, and this works out the volume in cubic yards for a ready-mix order, how many pre-mixed bags that takes, and the total cost if you enter a price.

Slab
ft
ft
in
$
On-screen number pad
Tap to type into the highlighted field
Concrete needed
1.23
cubic yards
Cubic feet
33.33
60 lb bags
75

Step-by-step proofCheck by hand
This is a planning estimate, not an engineering calculation. It does not account for sub-base preparation, waste, spillage, an uneven or sloped surface, or reinforcement (rebar or mesh). Concrete suppliers commonly recommend ordering 5–10% extra to cover these. For a structural pour — a foundation, a load-bearing slab — check the required thickness and mix with a structural engineer or your local building code before ordering.

How the concrete volume is calculated

Length and width (in feet) are multiplied by the thickness (converted from inches to feet, dividing by 12) to get the volume in cubic feet. Dividing by 27 converts that to cubic yards, the unit ready-mix concrete is ordered and priced in. For a smaller pour, the cubic-foot figure is instead divided by a bag's published yield and rounded up to a whole number of bags.

The formula

cubic feet = length × width × (thickness ÷ 12) · cubic yards = cubic feet ÷ 27

Worked example

A 10 ft × 10 ft slab, 4 inches thick, is 10 × 10 × (4 ÷ 12) = 33.33 cubic feet — about 1.23 cubic yards, or 75 sixty-pound bags rounded up from 74.1.

Frequently asked questions

Why does the bag count round up to a whole number?
Bags are sold whole — a partial bag still has to be bought, and mixing an exact fraction of a bag isn't practical on site — so the bag count is always rounded up to the next whole bag.
Where do the bag yield figures come from?
The cubic-foot yield per bag (roughly 0.30 for a 40lb bag, 0.45 for a 60lb bag, and 0.60 for an 80lb bag) is the figure published by major pre-mixed concrete brands for their standard concrete mix. Always check the actual yield printed on the bag you're buying, since it can vary slightly by brand and product.
Should I order extra beyond the calculated volume?
Most concrete suppliers and contractors recommend ordering 5–10% more than the calculated volume, to cover an uneven sub-base, spillage, and minor measurement error — running short partway through a pour is a much bigger problem than a small surplus.
Sources and method
  • Concrete volume formula and cubic-feet-to-cubic-yard conversion — general published mathematics.
  • Pre-mixed bag yield figures (40lb/60lb/80lb) — standard figures published by major pre-mixed concrete manufacturers for their general-purpose concrete mix.
Last reviewed: 20 Sep 2026 Sources last verified: 20 Sep 2026 Results use the assumptions explained on this page. Report an error How we check calculations

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