How to use the concrete calculator
- Choose the shape: a slab (patios, driveways, shed bases, floors), a footing or wall (long and narrow), or a round column (fence and deck post holes, tube forms).
- Enter the dimensions. Length and width are in feet (or meters); thickness, footing size and diameter are in inches (or centimeters), because that's how they're usually measured on site.
- Set how many identical pieces you're pouring — handy for a row of post holes.
- Keep a waste allowance of 5–10%. Forms bow, subgrade is never perfectly flat, and some concrete always stays in the wheelbarrow.
Concrete volume formulas
Concrete is ordered by volume, so every shape comes down to "area × depth". The key is to convert everything to feet before multiplying, then divide cubic feet by 27 to get cubic yards.
Footing or wall: length (ft) × width (in) ÷ 12 × depth (in) ÷ 12 = cubic feet
Column: π × (diameter (in) ÷ 24)² × height (ft) = cubic feet
Cubic yards = cubic feet ÷ 27
Worked example: a 10 × 10 ft patio, 4 inches thick
10 × 10 × (4 ÷ 12) = 33.3 cubic feet. Divided by 27, that's 1.23 cubic yards. Adding 10% for waste gives about 1.36 cubic yards, or 36.7 cubic feet.
An 80 lb bag of premix yields roughly 0.6 cubic feet, so you'd need 36.7 ÷ 0.6 = 61.1, which rounds up to 62 bags.
Worked example: a 40 ft wall footing, 16 × 8 inches
A garden wall needs a continuous footing 40 ft long, 16 in wide and 8 in deep. Choose Footing or wall, enter 40 ft, a width of 16 in and a depth of 8 in: 40 × (16 ÷ 12) × (8 ÷ 12) = 35.6 cubic feet, or 1.32 cubic yards. With 10% for waste that's 39.1 cubic feet, or 1.45 cubic yards — 66 bags of 80 lb premix weighing nearly 3 tons in total, so a ready-mix delivery is worth pricing. For a poured wall, use the same shape: enter the wall's thickness as the width and its height as the depth.
Worked example: 12 fence post holes
A fence with 12 posts set in 10 in diameter holes, 3 ft deep. One hole holds 3.1416 × (10 ÷ 24)² × 3 = 1.64 cubic feet, so 12 holes hold 19.6 cubic feet (0.73 cubic yards). With 10% for waste that's 21.6 cubic feet, or 0.80 cubic yards — 36 bags of 80 lb premix (or 48 bags of 60 lb). That works out to three 80 lb bags per hole, which is a handy figure to stage beside each hole before you start mixing. The post itself displaces a little concrete, which adds to your margin.
How many bags of concrete per cubic yard?
Bag yields below are typical of standard premixed concrete (cement, sand and gravel). High-strength and fast-setting mixes can differ slightly, so check the bag.
| Bag size | Yield per bag | Bags per cubic yard | Bags per m³ |
|---|---|---|---|
| 80 lb | 0.60 cu ft | 45 | 59 |
| 60 lb | 0.45 cu ft | 60 | 79 |
| 40 lb | 0.30 cu ft | 90 | 118 |
| 25 kg | ≈0.012 m³ | 66 | 86 |
Bags or ready-mix truck?
Bagged concrete is convenient for post holes, small pads and repairs. Beyond about one cubic yard (45 bags of 80 lb, or more than 1.6 tonnes to lift and mix by hand), a ready-mix delivery is usually faster and produces a more consistent slab. Many suppliers have a minimum order or charge a "short load" fee for small deliveries, so ask when you compare prices.
Ready-mix is sold in increments (often ¼ or ½ cubic yard). Round your order up — running short mid-pour leaves a cold joint that weakens the slab.
Typical thickness
| Project | Typical thickness |
|---|---|
| Sidewalk, patio, shed base | 4 in (10 cm) |
| Driveway, cars and light trucks | 4–5 in (10–13 cm) |
| Driveway, heavier vehicles or RVs | 5–6 in (13–15 cm) |
| Garage or basement floor | 4 in (10 cm), often more for heavy use |
Footing sizes and depths are set by your local building code and frost depth, so confirm them with your building department before digging.
Tips for accurate concrete estimates
- Measure the excavation, not the plan. Over-digging by an inch across a large slab adds a lot of volume.
- Split odd shapes into rectangles. Calculate an L-shaped patio as two slabs and add them together.
- Remember the gravel base. Most slabs sit on 4 in or more of compacted gravel — use the gravel calculator for that layer.
Frequently asked questions
How many 80 lb bags of concrete make a yard?
About 45. A cubic yard is 27 cubic feet and an 80 lb bag yields roughly 0.6 cubic feet, so 27 ÷ 0.6 = 45 bags.
How much concrete do I need for a 10×10 slab?
At 4 inches thick, a 10×10 ft slab needs about 1.23 cubic yards, or roughly 1.36 cubic yards with 10% extra — about 62 bags of 80 lb premix.
How much concrete for a fence post hole?
A 10-inch-diameter hole 3 feet deep holds about 1.6 cubic feet, which is about three 80 lb bags. The post itself takes up a little room, so this estimate includes a small safety margin.
How much does a cubic yard of concrete weigh?
Normal-weight concrete is about 145–150 lb per cubic foot, so a cubic yard weighs roughly 4,000 lb (about 1.8 tonnes).
Should I add extra concrete for waste?
Yes. Order 5–10% more than the calculated volume to cover uneven subgrade, form movement and spillage. Running short during a pour is far worse than having a little left over.