Ordering too little concrete leaves a cold joint in the middle of your pour; ordering too much wastes money on material you cannot return. This calculator turns slab dimensions into an order: volume in cubic yards and cubic metres, the number of premix bags, and a ready-mix quantity rounded up to the quarter yard most plants deliver in.
Formula
Volume = length × width × thickness. In imperial units, convert thickness to feet (inches ÷ 12), multiply to get cubic feet, then divide by 27 for cubic yards. In metric, convert thickness to metres and the result is already in cubic metres.
Worked example
A 12 × 10 ft patio at 4" thick: 12 × 10 × (4 ÷ 12) = 40 ft³ ÷ 27 = 1.48 yd³. Add 10% and you need about 1.63 yd³ — order 1.75 yd³, or about 74 bags of 80 lb premix (each yields roughly 0.6 ft³).
Bags or ready-mix?
Under about 1 yd³, bagged concrete mixed on site is usually cheaper. Above 1.5–2 yd³, a ready-mix truck saves hours of mixing and gives a more consistent slab. Many plants have a short-load fee for small orders, so ask before you book.
Thickness guide
| Use | Typical thickness |
|---|---|
| Walkways, patios | 4" / 100 mm |
| Shed or hot-tub pad | 4–6" / 100–150 mm |
| Residential driveway | 5–6" / 125–150 mm |
| Garage floor | 4–6" / 100–150 mm |
Before forming, square the slab with the 3-4-5 calculator and confirm both diagonals match.
Watch: how it is done
Video by MARSHALLTOWN on YouTube. More in our video library.
Frequently asked questions
How many 80 lb bags make a cubic yard?
About 45. Each 80 lb bag yields roughly 0.6 cubic feet, and a cubic yard is 27 cubic feet.
How much extra concrete should I order?
5–10% covers spillage, an uneven base and forms that bow slightly. Use the higher end for hand-dug or uneven subgrade.
Is 4 inches thick enough for a driveway?
For cars it can work on a well-compacted base, but 5–6 inches is the common recommendation for driveways, especially with heavier vehicles.
Results are planning estimates. For structural work, confirm requirements with your local building authority and a qualified professional. See our disclaimer.