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Concrete Calculator
Cubic yards and 40, 60 and 80 lb bags of concrete for slabs, footings, columns and steps, with waste and cost.
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Ordering too little concrete stops a pour halfway; ordering too much wastes money. This concrete calculator works out exactly how much you need for a slab, patio, footing or wall, a round column or post hole, or a set of steps – in cubic yards, cubic feet and the number of 40, 60 or 80 lb bags – with an allowance for waste and an optional cost estimate.
How to Calculate Concrete for a Slab
- Multiply length × width × thickness, with every measurement in feet. Convert the thickness from inches by dividing by 12.
- Divide by 27 to turn cubic feet into cubic yards (a cubic yard is 3 ft × 3 ft × 3 ft = 27 ft³).
- Add 5–10% for uneven ground, spillage and form flex.
Example: a 10 × 10 ft patio, 4 inches thick: 10 × 10 × (4 ÷ 12) = 33.33 ft³ = 1.23 yd³. With 10% extra you need about 1.36 cubic yards – or 62 bags of 80 lb concrete.
Round Columns, Piers and Post Holes
For a cylinder, use π × radius² × height. A 12-inch-diameter hole that is 4 ft deep holds π × 0.5² × 4 = 3.14 ft³ – about six 80 lb bags. For eight fence posts, enter 8 in "How many." For posts, subtract the volume of the post itself if you want a closer figure, or keep it as extra.
Concrete Steps
Steps are calculated as stacked blocks: the bottom step is as deep as all the treads together, the next one tread shorter, and so on. Three 4-ft-wide steps with a 7-inch rise and 11-inch run need about 12.8 ft³ (0.48 yd³) before waste. A landing at the top should be added separately as a slab.
How Many Bags of Concrete Do I Need?
Premixed bags yield roughly:
| Bag size | Yield | Bags per cubic yard |
|---|---|---|
| 80 lb | 0.60 ft³ | 45 |
| 60 lb | 0.45 ft³ | 60 |
| 40 lb | 0.30 ft³ | 90 |
Check the yield printed on the bag you buy – it varies slightly by brand and mix. Bags make sense for small jobs such as post holes, footings and repairs. Above about one cubic yard, a ready-mix truck is usually cheaper and much faster: 45 bags weigh 3,600 lb and have to be mixed by hand or with a rented mixer. Most suppliers have a minimum order or charge a short-load fee for less than a few yards.
Recommended Slab Thickness
| Project | Typical thickness |
|---|---|
| Sidewalks, patios, shed floors | 4 in |
| Residential driveways (cars) | 4–5 in |
| Driveways for trucks, RVs, heavy loads | 6 in |
| Garage floors | 4–6 in |
| Footings | at least 6–8 in, below the frost line |
A 24 × 24 ft two-car driveway at 4 inches needs about 7.1 yd³; at 6 inches it needs 10.7 yd³ before waste. Local building codes set the minimum depth for footings and the frost line in your area, so check with your building department before pouring structural work.
Tips for Ordering Concrete
- Measure the actual depth of your forms in several places – a slab that is ½ inch deeper than planned uses 12% more concrete on a 4-inch slab.
- Round up to the nearest quarter or half yard when ordering ready-mix.
- Use the percentage calculator to compare quotes, and the date calculator to plan curing time – concrete reaches most of its strength in about 28 days.
Sources
Last reviewed: October 2026 by the ShrinkBee team. Results are estimates. Follow local building codes and manufacturer instructions.
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