How the concrete calculator works
It works out the volume of what you're pouring, adds your waste allowance, then converts the result to cubic yards (how ready-mix is sold) and bags (how premix is sold).
Rectangle: cubic feet = length (ft) × width (ft) × thickness (in) ÷ 12
Round post hole: cubic feet = 3.1416 × (diameter (in) ÷ 24)² × depth (ft)
Cubic yards = cubic feet ÷ 27
How many bags of concrete per cubic yard?
| Bag size | Yield per bag | Bags per cubic yard |
|---|---|---|
| 80 lb | 0.60 ft³ | 45 |
| 60 lb | 0.45 ft³ | 60 |
| 40 lb | 0.30 ft³ | 90 |
Example: a 10 × 10 ft patio, 4 inches thick
10 × 10 × 4 ÷ 12 = 33.3 cubic feet. Adding 10% gives 36.7 cubic feet, or 1.36 cubic yards. That's 62 bags of 80 lb mix or 82 bags of 60 lb mix. At $175 a yard, ready-mix comes to about $238 before any delivery or short-load fee; 62 bags at $6.50 cost about $403.
Bags or a ready-mix truck?
For small jobs under about one cubic yard, such as fence posts or a small pad, bags are simpler. Above that, mixing 45 or more bags by hand is hard work and a ready-mix truck is often cheaper, even with a short-load fee. Ask your supplier about their minimum order.
Frequently asked questions
How thick should a concrete slab be?
Four inches is standard for patios, walkways and shed floors. Driveways that carry cars are usually 5 to 6 inches, and heavier loads need more plus reinforcement. Check your local code.
How many 80 lb bags make a yard of concrete?
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?
Add 5% to 10% to cover uneven ground, form bulges and spills. Running short in the middle of a pour causes a weak cold joint, so round up.
How much concrete for a fence post?
Choose post hole, then enter the hole diameter and depth. A 10 inch hole 3 feet deep takes about 1.6 cubic feet, close to three 80 lb bags with no allowance for the post itself.
Last reviewed: 2026-10-31