To calculate concrete, multiply length × width × thickness (in feet) to get cubic feet, then divide by 27 to get cubic yards. A 10 × 10 ft slab that is 4 inches thick needs about 1.23 cubic yards. The rest of this guide shows how to apply the formula to real projects and how much extra to order.
The Concrete Formula
Concrete is ordered and priced by the cubic yard. One cubic yard is 3 ft × 3 ft × 3 ft, which equals 27 cubic feet. That is why every calculation ends with a division by 27.
Measure in feet, and convert thickness from inches to feet by dividing by 12. Mixing inches and feet is the most common source of wrong totals.
Enter your dimensions and get cubic yards, bags and cost in seconds.
Use the Concrete Calculator →Worked Examples
Example 1 — Patio, 12 × 10 ft, 4 inches thick
Order about 1.6 cubic yards.
Most patios sit on a compacted gravel base. The gravel calculator shows how much stone that layer needs.
Example 2 — Driveway section, 20 × 10 ft, 4 inches thick
Order about 2.6 cubic yards.
Driveways that carry heavy vehicles are often poured thicker, so confirm the thickness with your contractor. To compare against paving, run the same area through the asphalt calculator or the driveway cost calculator.
Example 3 — Round footings, 12 in diameter, 3.5 ft deep, 6 tubes
Round footings use the cylinder formula: volume = π × radius² × height, with the radius in feet.
About 39 bags of 60 lb mix (17.32 ÷ 0.45).
What a Cubic Yard of Concrete Looks Like
One cubic yard is a block about the size of a washing machine. When a contractor says "order 3.5 yards," they mean 3.5 cubic yards, not square yards or linear yards.
| Question | Answer |
|---|---|
| Coverage at 4 in thick | About 81 sq ft per cubic yard |
| Coverage at 6 in thick | About 54 sq ft per cubic yard |
| Weight of 1 cubic yard | About 4,000 lb (about 2 tons) |
| 80 lb bags per cubic yard | About 45 (0.60 cu ft each) |
| 60 lb bags per cubic yard | About 60 (0.45 cu ft each) |
| Truck mixer capacity | 8 to 12 cubic yards |
Bag yields vary a little by brand, so check the label. The truck capacity figure comes from the National Ready Mixed Concrete Association (see Sources).
Concrete Cubic Yards by Area and Thickness
Find your area, then read across to your thickness. Values are net volume before extra.
| Area | 3 in | 4 in | 5 in | 6 in |
|---|---|---|---|---|
| 50 sq ft | 0.46 | 0.62 | 0.77 | 0.93 |
| 100 sq ft | 0.93 | 1.23 | 1.54 | 1.85 |
| 150 sq ft | 1.39 | 1.85 | 2.31 | 2.78 |
| 200 sq ft | 1.85 | 2.47 | 3.09 | 3.70 |
| 300 sq ft | 2.78 | 3.70 | 4.63 | 5.56 |
| 400 sq ft | 3.70 | 4.94 | 6.17 | 7.41 |
| 500 sq ft | 4.63 | 6.17 | 7.72 | 9.26 |
| 750 sq ft | 6.94 | 9.26 | 11.57 | 13.89 |
| 1,000 sq ft | 9.26 | 12.35 | 15.43 | 18.52 |
How to Calculate Concrete for Common Projects
Driveway
Measure the full length and width, including any apron. For an irregular shape, split it into rectangles and add the results. Thickness depends on the loads the driveway will carry, so confirm it with your contractor. Plan the compacted base too, using the gravel calculator.
Patio and walkway
Patios and walkways for foot traffic are commonly poured about 4 inches thick. A 40 ft long, 4 ft wide walkway at 4 inches is 40 × 4 × (4 ÷ 12) = 53.33 cu ft, which is 1.98 cubic yards, or about 2.1 with 5% extra.
Footings and columns
Spread footings use the rectangle formula. Round footings use the cylinder formula shown above. Footing depth is set by local frost depth and building code, so check your local requirements before you calculate.
Steps
Treat each step as a solid block: width × tread depth × total height of that step. Calculate each step separately, add them together, and add the landing if you are pouring one.
Garage floor
Garage slabs are commonly 4 to 6 inches thick, depending on the vehicles and equipment they carry. A well-compacted base matters as much as thickness, because soft or wet ground under a slab is a common cause of cracking.
Ready-Mix or Bags, and How Much Extra to Order
Ready-mix concrete is sold by the cubic yard. Bags make sense for small jobs, usually under about 1 cubic yard, such as a few footings. For larger pours a truck is faster and avoids hours of mixing. Many suppliers have a minimum order or add a short-load fee for small deliveries, so ask before you book.
The National Ready Mixed Concrete Association advises ordering about 4–10% more than your estimate. Use about 5% for a simple, well-formed slab and closer to 10% for irregular shapes or uneven ground. Suppliers usually deliver in quarter-yard steps, so round up to the next quarter yard.
Common Mistakes to Avoid
- Mixing units. Convert thickness from inches to feet before multiplying.
- Using the planned depth instead of the real one. Measure after excavation and base compaction, because low spots add volume.
- Forgetting thickened edges. If your slab has deeper edge beams, calculate them as separate sections.
- Ordering the exact number. Always add extra and round up to the supplier's increment.
- Confusing cement with concrete. Cement is one ingredient. People often say "cement yards" when they mean cubic yards of concrete, and the formula is the same.
Concrete Calculation — Common Questions
A 10 × 10 ft slab at 4 inches thick is 33.33 cubic feet, or 1.23 cubic yards. With 5% extra, order about 1.3 cubic yards. At 6 inches thick it is 1.85 cubic yards, or about 1.95 with extra.
A 20 × 20 ft pad at 4 inches thick is 133.3 cubic feet, or 4.94 cubic yards. With 5% extra, order about 5.2 cubic yards, which your supplier will round to the next increment.
A 30 × 30 ft slab at 4 inches thick is 300 cubic feet, or 11.11 cubic yards. With 5% extra, order about 11.7 cubic yards. At 6 inches it is 16.67 cubic yards before extra.
An 80 lb bag yields about 0.60 cubic feet, so one cubic yard (27 cubic feet) needs about 45 bags. A 60 lb bag yields about 0.45 cubic feet, so about 60 bags.
Split the area into rectangles, calculate each with length × width × thickness (in feet) ÷ 27, then add the totals. Use about 10% extra, because irregular edges waste more than a clean rectangle.
For rectangular footings use length × width × depth (in feet) ÷ 27. For round footings use π × radius² × depth in feet, multiply by the number of footings, divide by 27 and add extra. You can check your totals with the concrete calculator.
Sources and Further Reading
- National Ready Mixed Concrete Association, CIP 31 — Ordering Ready Mixed Concrete: concrete is sold by the cubic yard, truck mixers carry 8 to 12 cubic yards, and about 4–10% extra is recommended.
- Prices, minimum orders and delivery rules vary by supplier. Confirm thickness, mix and quantity with your local ready-mix supplier or contractor before ordering.
These figures are planning estimates. See our Disclaimer.
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