Concrete Calculator

How much concrete do I need?

Concrete is ordered and mixed by volume, so the whole job is one calculation: length × width × thickness. The only trick is that thickness is usually given in inches (or centimeters) while length and width are in feet (or meters), so the units have to be reconciled before you can read the answer in cubic yards or cubic meters — which is exactly what this tool does.

The formula

• US: volume in cubic feet = length (ft) × width (ft) × (thickness in ÷ 12). Divide by 27 to get cubic yards, the unit ready-mix is sold in.
• Metric: volume in cubic meters = length (m) × width (m) × (thickness cm ÷ 100).

Example: a 10 ft × 10 ft slab poured 4 inches thick is 10 × 10 × 0.333 = 33.3 cubic feet, or about 1.23 cubic yards before waste.

Bags vs. ready-mix

For small jobs you mix bags by hand; for anything over about half a cubic yard, ordering ready-mix by the truck is cheaper and far less work. The bag counts here use the common yields: an 80 lb bag of pre-mix makes roughly 0.60 cubic feet, a 60 lb bag about 0.45, and a 40 lb bag about 0.30. A 40 kg bag makes roughly 0.018 cubic meters. Always round up — a half-empty bag is far better than stopping a pour to run to the store.

Why add a waste allowance

Real forms are never perfectly level, the subgrade has dips, and some concrete is always lost to spillage and the truck chute. A 5–10% waste allowance is standard; for an uneven or hand-dug excavation, 10% or more is sensible. Running short mid-pour creates a cold joint — a visible, weaker seam — so it pays to over-order slightly.

FAQ

Does this work for footings, columns, and post holes?

Yes, for anything rectangular: enter the trench or footing length, width, and depth. For round columns or post holes the volume is different (π × radius² × depth); a rectangular estimate will over-state a cylinder by about 27%.

How many bags make a cubic yard?

A cubic yard is 27 cubic feet, so it takes roughly 45 of the 80 lb bags, 60 of the 60 lb bags, or 90 of the 40 lb bags. That's why bags only make sense for small jobs.

Can I enter thickness in inches and length in feet together?

Yes — that's the normal way to measure a slab, and the calculator reconciles the units for you. It converts the thickness from inches (or centimeters) and combines it with the length and width in feet (or meters) to give a clean answer in cubic yards or cubic meters.

How much waste should I add?

A 5–10% allowance is standard for a level form on good subgrade; bump it toward 10% or more for an uneven or hand-dug excavation. It's always safer to over-order slightly than to run short and create a weak cold joint mid-pour.

Does the concrete calculator stay private?

Yes. All the math runs in your browser, so the dimensions you enter are never uploaded or stored anywhere.

Is it free and does it work on mobile?

Yes to both. The calculator is free with no sign-up and runs in any modern phone or desktop browser, so you can use it on-site from your phone.