The concrete formula
Concrete is ordered by volume. For a flat slab that's length times width times thickness, all in feet. Thickness is usually given in inches, so the calculator divides it by 12 first. The result in cubic feet is then divided by 27 to get cubic yards, which is how ready-mix plants sell it.
For round holes and columns, the area of a circle replaces length times width: pi times the radius squared, times the depth.
Bags are counted by yield. An 80 lb bag of standard mix makes about 0.6 cubic feet of concrete, a 60 lb bag about 0.45, and a 40 lb bag about 0.3. Fast-setting and high-strength mixes can vary a little, so glance at the label.
Worked example: a 10 × 10 patio, 4 inches thick
- 10 × 10 × (4 ÷ 12) = 33.3 cubic feet
- 33.3 ÷ 27 = 1.23 cubic yards
- Add 10% for uneven ground and spillage: 1.36 cubic yards
- That's about 62 bags of 80 lb mix
Sixty-two bags is a lot of mixing by hand. Past about a cubic yard, most people find a ready-mix truck or a rental mixer worth it. We go through that choice in bags or ready-mix.
How thick should it be?
- Walkways and patios: 4 inches is the usual standard.
- Driveways for cars: 4 to 5 inches, and 6 if heavier vehicles will park there.
- Shed pads: 4 inches works for most garden sheds.
- Footings and foundations: sized by your local building code. Check before you dig.
Why we add a little extra
Ground is never perfectly flat. A low spot half an inch deep across a whole patio can add several bags. Forms bow a bit when they're full, some concrete stays in the wheelbarrow, and running short halfway through a pour leaves a cold joint that can crack later. Ten percent extra is cheap insurance.
Putting a gravel base under the slab? Work that out with the gravel calculator.
Frequently asked questions
How many bags of concrete are in a cubic yard?
About 45 bags of 80 lb mix, 60 bags of 60 lb mix, or 90 bags of 40 lb mix. A cubic yard is 27 cubic feet.
How much concrete do I need for a fence post?
A 4 by 4 post in a 10 inch wide hole that's 2 feet deep takes a little under one cubic foot of concrete once you subtract the post. That's usually two to three 50 lb bags of fast-setting mix.
How thick should a concrete patio be?
Four inches is standard for patios and walkways with foot traffic. Go thicker if you plan to put a hot tub or another heavy load on it, and follow the maker's guidance for that load.
When should I order ready-mix instead of bags?
For anything over about a cubic yard. Mixing 45 or more bags by hand is slow, and the first batches start to set before the last ones are placed. Many suppliers have a minimum order, so ask about short-load fees.
Results are estimates for planning. Product coverage, site conditions and installation methods vary, so check manufacturer guidance before you buy.