Concrete calculator
Enter the size of your slab, footing, post hole or steps to see how much concrete to order in cubic yards, or how many 80, 60 or 40 lb bags to buy. A waste allowance is included, and you can add prices to compare bags with ready-mix.
Bags of concrete mix
- 80 lb bags (0.6 cu ft each)
- 62
- 60 lb bags (0.45 cu ft each)
- 82
- 50 lb bags (0.375 cu ft each)
- 98
- 40 lb bags (0.3 cu ft each)
- 123
Standard concrete mix yields. Total weight about 5,500 lb (2,495 kg).
Amounts
- Exact volume (no extra)
- 1.23 cu yd
- Cubic feet to order
- 36.7
- Ready-mix order (to the next ¼ yard)
- 1.5 cu yd
- Nothing is uploaded
- Free, no sign-up
- Works on phone and desktop
How to work out how much concrete you need
- 1
Choose what you are pouring: a slab, a footing, a round hole (post holes and tubes) or steps.
- 2
Enter the sizes the way you read a tape measure: 12' 6", 12 ft 6 in and 12.5 all work.
- 3
Set how many you are pouring and the extra for waste. 10% covers uneven ground and spills on most jobs.
- 4
Read the cubic yards to order, or the number of bags to buy. Add prices to compare bags with a ready-mix truck.
Features
Slabs, footings, holes and steps
Four shapes cover patios, sheds, footings, fence posts, Sonotube piers and poured steps, with as many of each as you need.
Yards, feet and meters
Mix feet, inches and fractions freely. Results come in cubic yards, cubic feet and cubic meters.
Every bag size
See how many 80, 60, 50 and 40 lb bags you need side by side, and what the load weighs.
Bags or ready-mix?
Enter a bag price and a price per yard to compare both ways of buying, rounded the way suppliers sell it.
The formula, with an example
Concrete is sold by volume, so every job comes down to length × width × depth. Measure in feet, turn inches into feet by dividing by 12, multiply, and divide by 27 to get cubic yards. A patio 12 ft long and 10 ft wide at 4 inches thick is 12 × 10 × 0.333 = 40 cubic feet, or 1.48 cubic yards. With 10% extra that is about 1.63 cubic yards, or 74 bags of 80 lb mix.
Round shapes use the area of a circle: π × radius × radius × depth. A post hole 10 inches across and 3 feet deep holds about 1.6 cubic feet, a little under three 80 lb bags. Fill a few holes and you will see why concrete for a fence adds up quickly.
Typical sizes
| Job | Usual thickness or size | Concrete for the example |
|---|---|---|
| Patio, 12 × 12 ft | 4 in | 1.78 cu yd |
| Shed base, 8 × 10 ft | 4 in | 0.99 cu yd |
| Sidewalk, 4 ft wide × 30 ft | 4 in | 1.48 cu yd |
| Driveway, 12 × 40 ft | 5 in | 7.41 cu yd |
| Fence post hole, 10 in × 3 ft | 3 × post width | 1.64 cu ft each |
Volumes are before the waste allowance. Your local building code and the ground conditions set the real requirements.
Bags vs ready-mix
Bagged concrete mix is easiest for anything up to about a cubic yard: you can buy exactly what you need, mix it in a wheelbarrow or a rented mixer, and pour at your own pace. Beyond that, mixing by hand gets slow, and a slab poured in batches can show seams. Ready-mix trucks deliver consistent concrete in one go, usually ordered to the nearest quarter or half yard. Many suppliers add a short-load fee for small orders, so get a quote with delivery included before you decide.
- Up to about 1 cubic yard: bags are practical and often cheaper.
- 1 to 3 cubic yards: compare a bag order, a rented mixer and a short load.
- Over 3 cubic yards: ready-mix is almost always the better choice.
Before you pour
Most slabs sit on a compacted gravel base, often about 4 inches deep. Work out that layer with the gravel calculator. Measure the area of an irregular patio with the square footage calculator first, or check any volume in the cubic yard calculator. For small pours and fence posts, the concrete bag calculator puts the number of bags first.
Frequently asked questions
How do I calculate how much concrete I need?
Multiply length × width × thickness, all in feet, to get cubic feet, then divide by 27 for cubic yards. A 10 × 10 ft slab 4 inches (0.333 ft) thick is 33.3 cubic feet, or 1.23 cubic yards. Add 5 to 10% for waste.
How many bags of concrete make a yard?
One cubic yard is 27 cubic feet. An 80 lb bag makes about 0.6 cubic feet, so you need 45 bags. With 60 lb bags (0.45 cu ft) it is 60 bags, and with 40 lb bags (0.3 cu ft) it is 90.
How thick should a concrete slab be?
Four inches is the usual thickness for patios, sidewalks and shed bases. Driveways for cars are commonly 4 to 6 inches, and thicker where heavy vehicles drive. Local building codes and the load decide the final number.
Should I buy bags or order ready-mix?
Bags suit small jobs such as fence posts, small pads and repairs. Above about 1 cubic yard (45 bags of 80 lb) a ready-mix truck is usually faster and costs about the same, but many suppliers charge extra for small loads.
How much extra concrete should I order?
Most people add 5 to 10%. Uneven or soft ground, forms that bow and spillage all use more than the drawing suggests, and running short halfway through a pour causes weak joints.
How much does a yard of concrete weigh?
Normal concrete weighs about 150 lb per cubic foot, so a cubic yard weighs roughly 4,000 lb (about 1,800 kg). Check that your trailer or truck can carry a bag order before you load it.
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