Boring Tools

Concrete Volume Calculator

Open tool

Estimate the volume and bags of concrete needed for a slab or footing.

Estimate how much concrete a rectangular slab, footing, or pad needs. Enter the length, width and thickness to get the volume — and, in imperial units, an approximate count of 80 lb bags of mix.

How it works

The volume of a rectangular slab is length × width × thickness, with the thickness converted to the same unit as the other two. In imperial units the tool gives cubic feet (length in ft × width in ft × thickness in in ÷ 12) and cubic yards (cubic feet ÷ 27). In metric it gives cubic metres (length in m × width in m × thickness in cm ÷ 100).

In imperial only, the bag count assumes an 80 lb bag of ready-mix yields about 0.6 cubic feet, and is rounded up to a whole number of bags.

How to use it

  1. Choose Metric or Imperial with the toggle.
  2. Enter the Length, Width and Thickness of the area to fill.
  3. Read the volume (and bag estimate, in imperial).
  4. Add a margin for waste before ordering — see the guidelines below.

Options & controls

Units

Metric / Imperial toggle
Switches between metres/centimetres and feet/inches. It is a site-wide setting shared with the other measurement tools and remembered in this browser.

Imperial inputs

Length (ft)
Default: 10.
Width (ft)
Default: 10.
Thickness (in)
The depth of the slab, in inches. Default: 4.

Metric inputs

Length (m)
Default: 3.
Width (m)
Default: 3.
Thickness (cm)
The depth of the slab, in centimetres. Default: 10.

Buttons

Copy
Copies the result as text, for example “100 cu ft (3.7 cu yd) — 167 80lb bags (approx.)”, or “0.9 cubic meters” in metric.
Reset
In the title row. Restores the sample values for both unit systems.

Reading the results

Imperial results

Cubic feet
The volume of concrete needed.
Cubic yards
The same volume in cubic yards, the unit ready-mix trucks usually sell by.
80lb bags (approx.)
The number of 80 lb bags of dry mix needed, rounded up.

Metric result

Cubic meters
The volume of concrete needed. No bag count is given in metric, because bag sizes vary by country.

Tips & guidelines

  • Order a little extra — a common rule of thumb is 5–10% more — to cover uneven ground, spillage and slightly over-thick pours.
  • Each unit system keeps its own set of boxes. After switching units, enter the measurements again.
  • For a large pour, ready-mix delivery is usually easier than bags. Use the cubic yards or cubic metres figure when you ask for a quote.
  • For footings and slabs of different depths, work out each section separately and add the volumes together.
  • Check the yield printed on your bag. Different brands and sizes give different volumes per bag.

Limits

  • Rectangular shapes only. Circles, L-shapes, sloped or stepped slabs must be split into rectangles and added up by hand.
  • There is no waste allowance built in, and no calculation of cement, sand and gravel separately.
  • The bag estimate is imperial only, and assumes 0.6 cubic feet per 80 lb bag.
  • This is an estimate for planning, not a structural design.

Saved in your browser

Your measurements are not saved. Only the Metric / Imperial choice is remembered in this browser, and it is shared with other tools.

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