🏗️ Concrete Volume Calculator
Calculate concrete volume needed for slabs, footings, and patios in cubic yards and pre-mixed bags.
Working out how much concrete to order
Concrete is sold by volume, and every ordering mistake comes from one of three things: mixing units, forgetting that slab thickness is measured in inches while everything else is in feet, and ordering the exact calculated volume with no allowance for waste. Running short mid-pour is far more expensive than ordering a little extra.
The basic volumes
| Shape | Formula | Notes |
|---|---|---|
| Slab or footing | length × width × thickness | Convert thickness to the same unit first |
| Column or post hole | π × radius² × height | Radius is half the diameter |
| Stairs | sum of each step as a slab | Treat each tread as a separate block |
| Curb | cross-sectional area × length | Split an L-shape into two rectangles |
A cubic yard is 27 cubic feet, which is the conversion that governs US ordering. A cubic metre is 1,000 litres, and one cubic yard is 0.7646 cubic metres.
A slab, step by step
A patio 12 ft by 16 ft at 4 inches thick. Convert the thickness: 4 inches is 4/12 = 0.333 ft.
- Volume in cubic feet: 12 × 16 × 0.333 = 64 cubic feet.
- Convert to cubic yards: 64 / 27 = 2.37 cubic yards.
- Add 10% for waste, spillage and uneven subgrade: 2.61 cubic yards.
- Order 2.75 or 3 cubic yards, since most suppliers sell in quarter-yard increments.
The thickness conversion is where most errors happen. Multiplying 12 × 16 × 4 gives 768, and dividing that by 27 gives 28.4 cubic yards, which is twelve times too much. Always convert inches to feet before multiplying.
How much waste to allow
| Situation | Allowance |
|---|---|
| Flat slab on well-prepared subgrade | 5–8% |
| Slab on uneven or soft ground | 10% |
| Footings and trenches | 10–15% |
| Post holes in loose soil | 15–20% |
Footings over-pour more than slabs because trench walls rarely hold their shape. Post holes in sandy soil can take substantially more than the calculated volume as the sides slump.
Bags versus ready-mix
| Bag size | Yield | Bags per cubic yard |
|---|---|---|
| 40 lb | 0.011 yd³ | 90 |
| 50 lb | 0.0138 yd³ | 72 |
| 60 lb | 0.017 yd³ | 60 |
| 80 lb | 0.022 yd³ | 45 |
| 25 kg | 0.0125 m³ | 80 per m³ |
Bags make sense below about half a cubic yard. Above roughly one cubic yard, 45 eighty-pound bags means 1.6 tonnes of material to carry and mix, and the result is less consistent than ready-mix. Most suppliers have a minimum delivery of one cubic yard and charge a short-load fee below three or four.
Strength and reinforcement
Concrete strength is specified in psi or MPa at 28 days. A domestic patio or path typically calls for 2,500 to 3,000 psi (17–20 MPa), a driveway for 3,500 to 4,000 psi (25–28 MPa), and structural work for more. Reinforcement with mesh or rebar does not change the volume you order but does change the slab's behaviour: concrete is strong in compression and weak in tension, and steel supplies what it lacks.
A worked example
A driveway 20 ft by 10 ft at 5 inches, plus four post holes 10 inches in diameter and 3 ft deep. Slab: 20 × 10 × 0.4167 = 83.3 cubic feet, so 3.09 cubic yards. Holes: π × 0.4167² × 3 = 1.64 cubic feet each, 6.54 in total, so 0.24 cubic yards. Combined 3.33, plus 10% waste is 3.66. Order 3.75 cubic yards.
Frequently Asked Questions (FAQs)
How many cubic feet are in a cubic yard?
Twenty-seven, because a yard is three feet and 3 × 3 × 3 = 27. This is the single most useful conversion in concrete ordering and the one most often forgotten.
How many 80 lb bags make a cubic yard?
About 45. A 60 lb bag yields roughly 0.017 cubic yards so you need about 60, and a 40 lb bag needs about 90. Above one cubic yard, ready-mix is almost always cheaper and better.
How thick should a concrete slab be?
Four inches for patios, walkways and shed bases. Five to six inches for driveways carrying cars, and more for heavy vehicles. Thicker is not automatically better: a well-compacted subgrade under four inches outperforms six inches over soft ground.
How much extra should I order?
Ten per cent for most jobs, more for footings and post holes where the ground is loose. Running short mid-pour creates a cold joint that is permanently weaker than a continuous pour, and a second delivery costs more than the extra material would have.
How long does concrete take to cure?
It is walkable in 24 to 48 hours and ready for vehicles after about a week, but it reaches its specified strength at 28 days and continues gaining slowly for years. Keeping it damp for the first week significantly improves the final strength.
What is the difference between concrete and cement?
Cement is the binding powder. Concrete is the finished material: cement plus sand, gravel and water. Asking for a cement driveway is like asking for a flour cake. A typical mix is roughly 1 part cement to 2 sand to 3 gravel by volume.