How to use the concrete calculator
- Pick a shape. Use Slab for patios, driveways, sheds and floors, Footing for strip footings and grade beams, and Column for sonotubes and post holes.
- Enter your measurements. Length and width in feet (or metres), thickness in inches (or centimetres). Add a quantity if you're pouring several identical pieces.
- Keep the waste allowance. 10% covers spillage, uneven subgrade and form bulge. Go to 15% if the ground isn't level.
- Compare bags with ready-mix. The calculator shows both costs so you can see which is cheaper for your job.
The concrete volume formula
For a rectangular slab or footing, multiply length by width by thickness. Because thickness is usually given in inches, divide it by 12 to convert to feet first, then divide the result by 27 to turn cubic feet into cubic yards.
Round column = π × (Diameter ÷ 2)² × Height
Bags = Volume (ft³) ÷ bag yield (ft³)
In metric, multiply length × width × thickness in metres to get cubic metres directly. One cubic yard is 0.7646 m³.
How many bags in a yard of concrete?
| Bag size | Yield per bag | Bags per yd³ | Bags per m³ |
|---|---|---|---|
| 40 lb (18 kg) | 0.30 ft³ | 90 | 118 |
| 60 lb (27 kg) | 0.45 ft³ | 60 | 79 |
| 80 lb (36 kg) | 0.60 ft³ | 45 | 59 |
Worked examples
A 12 × 12 ft patio poured 4 in thick: 12 × 12 × (4 ÷ 12) = 48 ft³. Divide by 27 to get 1.78 yd³. With 10% extra that's 1.96 yd³, or about 88 bags of 80 lb mix. At that size, order ready-mix.
Eight 10 in sonotubes, 4 ft deep: π × (10 ÷ 24)² × 4 = 2.18 ft³ each, so 17.5 ft³ in total. With 10% extra that's 19.2 ft³, or 32 bags of 80 lb mix.
A 3 × 2.4 m base 10 cm thick: 3 × 2.4 × 0.10 = 0.72 m³. With 10% extra that's 0.79 m³, or about 68 bags of 25 kg mix.
How thick should a concrete slab be?
| Project | Typical thickness |
|---|---|
| Walkway, patio | 4 in (10 cm) |
| Shed or garage base | 4–6 in (10–15 cm) |
| Residential driveway | 4–5 in, 6 in for heavy vehicles |
| Hot tub pad | 4–6 in (10–15 cm) |
Always check your local building code. Footings, frost depth and reinforcement requirements vary by region.
Frequently asked questions
How much does a yard of concrete cost?
Ready-mix typically runs about $150–$200 per cubic yard in the US before delivery fees. Many suppliers charge a short-load fee for orders under 4–5 yards. Change the price field to match your local quote.
Should I use bags or order a truck?
Bags make sense for small jobs under about one cubic yard, like post holes, steps and small pads. Above that, ready-mix saves a lot of labor and gives a more consistent pour.
How much waste should I allow?
Use 5–10% for formed slabs on a compacted, level base. Use 10–15% for uneven ground, trenches dug without forms, or when it's your first pour.
How much does a cubic yard of concrete weigh?
Standard concrete weighs about 4,000 lb per cubic yard (around 2,400 kg per m³). That matters for trailer loads and for anything poured on top of an existing structure.