Concrete Calculator

Estimate concrete volume for a slab and how many bags it takes.

Measurement units

Extra added for overpour and uneven subgrade. 5–10% is typical.

Result—

Enter your values and calculate to see the result.

Formula used

net volume (ft³)  = length × width × thickness / 12
gross volume      = net volume × pieces × (1 + waste/100)
volume (yd³)      = gross volume / 27
volume (m³)       = yd³ × 0.764555
bags              = gross volume / bag yield
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Worked example

Slab: 10 ft × 10 ft × 4 in, 1 slab   Waste: 5%

Net: 10 × 10 × 4/12 = 33.33 ft³   Gross: 33.33 × 1.05 ≈ 35 ft³

Volume: 35 / 27 ≈ 1.30 yd³ (about 0.99 m³)

Bags: 35 / 0.6 ≈ 59 × 80 lb bags (or 44 × 25 kg bags)

Frequently asked questions

How much waste should I add?

Plan for about 5–10% extra over the net volume to cover spillage, uneven subgrade, and slight overpour. This calculator adds your chosen waste percentage on top of the measured volume before reporting the total.

Should I order bags or ready-mix?

Bags are convenient for small jobs — a few slabs, posts, or patches. Beyond roughly half a cubic yard, ready-mix delivered by truck is usually cheaper and far less labour. The bag counts shown are a buying guide for the bagged route.

Do the bag counts include the waste margin?

Yes. The volume used for the 80 lb, 60 lb, and 25 kg bag counts already includes the waste percentage you entered, and each count is rounded up to a whole bag.