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Concrete Calculator

Calculate the concrete required for a rectangular slab.

Slab Dimensions

Estimate only. Consider adding 5–10% for waste, spillage, and uneven subgrade.

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Date Dimensions Volume Bags (80 lb)
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How Concrete Volume Is Calculated

Concrete volume for a slab is simply length × width × thickness, converted into a single unit before multiplying. This calculator converts all three dimensions to meters internally, multiplies them to get cubic meters, then converts that figure into cubic feet and cubic yards — the units most US concrete suppliers quote by.

The bag estimate assumes a standard 80 lb bag yields about 0.6 cubic feet of mixed concrete, which is the manufacturer-stated figure for most pre-mixed bags sold at hardware and building supply stores. For anything beyond a small patch or footing, ready-mix delivery by the cubic yard is almost always more cost-effective than bagged concrete.

Worked Example

A 10 ft × 12 ft patio slab, poured 4 inches thick: 10 × 12 × (4 ÷ 12) = 40 cubic feet, which converts to 1.48 cubic yards.

At 0.6 cu ft per 80 lb bag, that's 67 bags — but most ready-mix suppliers sell by the quarter-yard above a 1-yard minimum, so at this size a small ready-mix delivery is worth pricing against a pallet of bagged concrete before deciding which way to go.

Common Questions

Should I order extra concrete beyond the calculated volume?

Most contractors add 5–10% to the calculated volume to account for slab thickness variation, subgrade unevenness, and minor spillage — running short mid-pour is a much bigger problem than a small amount of leftover mix.

Does this account for rebar or wire mesh volume?

No — reinforcement steel takes up a negligible fraction of a slab's volume and is not subtracted here. If you're also scheduling the rebar itself, our Bar Bending Schedule calculator handles cut lengths, bend allowances, and cutting-stock optimization separately.

Why cubic yards instead of cubic feet for ordering?

Ready-mix trucks and batch plants price and deliver by the cubic yard in the US, so that's the number to give a supplier — cubic feet is mainly useful for cross-checking a bagged-concrete estimate.

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Last updated: September 21, 2026