Concrete calculator

Concrete volume, cement bags, sand and aggregate for slabs, footings and columns.

  • Free, no sign-up
  • Runs in your browser
How it is calculated

Wet volume = length × width × thickness (× count). Dry volume = wet volume × dry factor (1.54 by default, allowing for voids and bulking). Cement = dry volume × cement part ÷ total parts × 1,440 kg/m³ ÷ bag size. Sand and aggregate = dry volume × their parts ÷ total parts.

Check locally: The dry factor, cement density and bag size are common assumptions you can edit. Use a designed mix for structural work.

How to use the Concrete Calculator

Use this concrete calculator to work out how much concrete a slab, footing, beam or column needs, and the cement, sand and aggregate for a nominal mix, before you order materials or ready-mix.

  1. Choose the element: a slab, footing or beam, a rectangular column, or a round column or pier.
  2. Enter its dimensions and how many identical elements there are.
  3. Pick the mix, such as M20 (1:1.5:3), or type your own cement : sand : aggregate ratio.
  4. Set the wastage (5% is typical) and, if needed, edit the dry volume factor, cement density and bag size under Assumptions.
  5. Copy the results, download a CSV, or print the summary for site.

Worked example

A 4 m × 3 m slab, 150 mm thick, is 1.8 m³ of concrete, or 1.89 m³ with 5% wastage. For an M20 mix the dry volume is 1.89 × 1.54 = 2.91 m³, which needs about 762 kg of cement (16 bags of 50 kg), 0.79 m³ of sand and 1.59 m³ of aggregate. A ready-mix order rounds up to 2 m³.

Good to know

  • The 1.54 dry volume factor allows for voids and bulking of dry materials; 1.52 to 1.57 is common.
  • Nominal mixes suit small and non-structural work. Structural concrete should follow a designed mix from the engineer.
  • Switch to imperial to get cubic yards and feet for orders in the US.

Questions

Why multiply by 1.54?

Dry materials shrink when mixed with water and fill voids, so about 54% more dry volume is needed. You can change this factor.

Is anything I enter saved or sent?

No. The calculation runs in your browser. Nothing you type is sent anywhere. You can copy the result, print it or download it as a CSV.