Example: a 10 × 10 ft patio, 4 in thick
- Convert the thickness to feet: 4 in ÷ 12 = 0.333 ft.
- Volume: 10 × 10 × 0.333 = 33.3 cu ft. Divide by 27 for cubic yards: 1.23 cu yd.
- Add 10% for spillage and uneven ground: 36.7 cu ft, or 1.36 cu yd.
- Bags: 36.7 cu ft ÷ 0.6 cu ft per 80-lb bag = 61.1, so 62 bags — or order 1.5 cu yd of ready-mix.
How it's calculated
- Slab or footing: length × width × thickness, with every measurement in the same unit.
- Post hole or column: π × (diameter ÷ 2)² × depth, times the number of holes.
- Cubic feet ÷ 27 = cubic yards. In metric, meters × meters × meters is already m³.
- Add the extra percentage, divide by what one bag makes, and round up to whole bags.
Assumptions
- An 80-lb bag of pre-mixed concrete makes about 0.60 cu ft; a 60-lb bag about 0.45 cu ft. Metric bags use the same yield per kilogram — about 0.0117 m³ per 25 kg bag.
- Ready-mix is rounded up to the next ¼ cu yd (or ¼ m³), the way most plants take orders.
- 10% extra by default: excavations are never perfectly flat, and running short mid-pour is far worse than having some left over.
- Rebar, mesh or a post inside the hole isn't subtracted, so the answer is slightly generous.
Frequently asked questions
How many 80-lb bags of concrete make a cubic yard?
About 45. Each 80-lb bag makes roughly 0.6 cu ft of concrete, and a cubic yard is 27 cu ft (27 ÷ 0.6 = 45). With 60-lb bags you'd need 60, and with 40-lb bags 90.
How thick should a concrete slab be?
4 inches is standard for patios, walkways and shed floors. Driveways for cars are usually 5–6 inches. Anything carrying heavy loads — RVs, trucks or structural footings — should follow local code or an engineer's spec.
Is it better to use bags or order ready-mix?
For small jobs up to about 1 cubic yard (45 × 80-lb bags), bags are simplest. Beyond that, ready-mix delivery or a rental mixer saves hours of heavy mixing. Many suppliers charge a short-load fee for small orders, so compare the total price.
How much extra concrete should I order?
Plan on 5–10% more than the exact volume. The ground is rarely perfectly level, forms bow slightly, and some concrete is always lost. Running short in the middle of a pour leaves a weak joint, so a little extra is cheap insurance.
How much concrete do I need for a fence post?
Choose Post hole / column and enter the hole diameter, depth and number of posts. A 10-inch hole 3 ft deep holds about 1.6 cu ft — roughly three 80-lb bags with a small margin. The post itself takes up some room (a 4×4 fills about 15% of a 10-inch hole), so you may have a little left over.
How much does a cubic yard of concrete weigh?
About 4,000 lb (2 tons) once mixed. That's why bags add up fast: 45 bags of 80 lb is 3,600 lb of dry mix — more than most half-ton pickups can safely carry in one trip.