How to use the concrete calculator
- Pick the shape. Use "Slab, footing or wall" for anything rectangular: patios, sidewalks, footings, walls or steps. For a wall, enter the wall's length, its height as the "width", and its thickness as the depth. Use "Round" for post holes and cardboard tube forms.
- Enter the sizes. Plain numbers are inches. Use
10'or10' 6for feet. Enter how many identical pours you have, like 6 post holes. - Add waste. 5–10% covers spillage, uneven subgrade and over-dig. Go higher for rough excavations.
The math
Convert everything to feet, multiply, and divide by 27 to get cubic yards. There are 27 cubic feet in a cubic yard (3 × 3 × 3). For a 10' × 10' patio, 4" thick: 10 × 10 × 0.333 = 33.3 cubic feet ÷ 27 = 1.23 cubic yards. Add 10% and order 1.36, which most suppliers round to 1½ yards.
For a round hole: π × (diameter ÷ 2)² × depth. A 12" tube 4' deep is 3.14 × 0.5² × 4 = 3.14 cubic feet. Six of them is 18.8 cubic feet, about 32 bags of 80 lb mix, or 0.7 yards.
Bags or ready-mix?
- Under about 1 yard (roughly 45 × 80 lb bags): bags are usually cheaper and easier, especially for post holes.
- 1 yard and up: price ready-mix. Many plants charge a "short load" fee under 3–5 yards, and some rent tow-behind trailers for 1–2 yard pours.
- Weight adds up: concrete weighs about 145 lb per cubic foot, so a yard is nearly 2 tons. Check your truck before hauling bags.
Common thicknesses
- Sidewalks, patios, shed floors: 4"
- Driveways and garage floors: 4–6", often with thickened edges
- Footings: sized by load and soil, typically 6–12" thick and at least below the frost line. Check local code.
FAQ
How do I calculate how much concrete I need?
Multiply length × width × thickness in feet to get cubic feet, then divide by 27 for cubic yards. Add 5–10% for waste. A 10' × 10' slab 4" thick needs about 1.23 cubic yards before waste.
How many 80 lb bags of concrete make a yard?
About 45. A typical 80 lb bag yields 0.6 cubic feet, and a cubic yard is 27 cubic feet: 27 ÷ 0.6 = 45 bags. With 60 lb bags (0.45 cu ft) it takes 60, and with 40 lb bags (0.3 cu ft) it takes 90.
How many bags of concrete for a post hole?
A 10" diameter hole 3' deep holds about 1.64 cubic feet, so 3 bags of 80 lb mix. A 12" hole 4' deep holds 3.14 cubic feet, or 6 bags. Subtract a little if a post sits in the hole.
How much does a yard of concrete weigh?
Normal-weight concrete is about 145–150 lb per cubic foot, so a cubic yard weighs roughly 3,900–4,050 lb, close to 2 tons.