How much concrete do I need?
Working out how much concrete you need comes down to one thing: volume. Concrete is sold by volume — cubic yards in the United States, cubic meters almost everywhere else — so once you know the length, width, and thickness of your pour, you know how much to order. The hard part is converting those measurements correctly and remembering to add a little extra for waste. This concrete calculator handles both automatically, and also tells you how many 40, 60, and 80 lb bags that volume works out to if you plan to mix by hand.
How to use this calculator
- Choose Imperial (feet and inches) or Metric (meters and centimeters).
- Enter the length and width of the slab, and the thickness of the pour.
- Set quantity if you are pouring several identical slabs, footings, or pads.
- Leave a waste allowance of 5–10% so you don't come up short mid-pour.
The result updates instantly — no button to press — showing volume in both cubic yards and cubic meters, plus a bag count for each common bag size.
The concrete volume formula
The math is a simple volume calculation, with a unit conversion at the end:
Volume = Length × Width × Thickness, then divide by 27 to convert cubic feet to cubic yards (because one cubic yard = 27 cubic feet).
Worked example: a slab 10 ft long × 10 ft wide × 4 in thick. First convert the thickness to feet: 4 in ÷ 12 = 0.333 ft. Then 10 × 10 × 0.333 = 33.3 cubic feet, which is 33.3 ÷ 27 ≈ 1.23 cubic yards. Adding a 10% waste allowance brings it to about 1.36 cubic yards — roughly 62 bags of 80 lb concrete mix.
How thick should a concrete slab be?
Thickness depends on the load the slab will carry:
- 4 inches (10 cm) — patios, walkways, garden shed floors, and general foot traffic.
- 5–6 inches (13–15 cm) — driveways, garage floors, and anything that carries vehicles.
- 6 inches or more — heavy equipment pads and commercial loads (check local code).
A compacted gravel sub-base under the slab improves drainage and helps prevent cracking.
Bags of concrete vs. ready-mix delivery
Mixing bags by hand is fine for small jobs — fence posts, small pads, repairs — up to roughly one cubic yard. Beyond that, the number of bags (and the effort) climbs fast, and ordering ready-mix delivery is usually cheaper, faster, and gives a stronger, more consistent result because the whole pour cures as one. As a rule of thumb, one cubic yard is about 45 bags of 80 lb, 60 bags of 60 lb, or 90 bags of 40 lb mix.