Concrete Calculator
How much concrete do you need? Enter the dimensions of your slab, footing, column or stairs and get the volume, the bags and the cost.
- Slab, column, stairs
- Yards or m³
- Bag counts
- Private
Cost (optional)
Leave at 0 to skip the cost.
How to calculate concrete
Shape, dimensions, waste.
- 1
Choose what you are pouring
Slab or footing, round column, tube or stairs — in US or metric units.
- 2
Enter the dimensions
Length and width in feet or metres, thickness in inches or centimetres; set the quantity for identical pours.
- 3
Add a waste allowance
Keep the suggested 10 % and read the volume to order, the equivalent in bags and the cost.
Order the right amount the first time
Too little concrete ruins a pour; too much is money in the skip.
Four pour types
Rectangular slabs and footings, round columns and post holes, tubes and stairs.
Waste included
Uneven ground, spillage and over-excavation are covered by an allowance you control.
Bags or ready-mix
See the cubic yards for a truck order and the number of pre-mixed bags side by side.
Weight
Know what the cured concrete weighs before you load a trailer or a floor.
Cost estimate
Enter the price per cubic yard or cubic metre to budget the pour.
Link to your calculation
Copy a link that reopens the calculator with your numbers filled in — handy for sharing or saving a scenario.
Made for phones
Large fields, the right keyboard for numbers and a layout that fits small screens.
Private by design
Everything is calculated in your browser. Your numbers are not sent to a server.
How much concrete do I need?
Concrete is ordered by volume: length × width × thickness. In US units, convert the thickness to feet and divide by 27 to get cubic yards. A 12 × 10 ft patio slab that is 4 in thick is 12 × 10 × 0.333 = 40 ft³, or 1.48 yd³. In metric units a 4 × 3 m slab 10 cm thick is 4 × 3 × 0.10 = 1.2 m³.
Always add a waste allowance of 5–10 %. Sub-bases are never perfectly level, forms bulge and some concrete stays in the mixer or pump. Running short in the middle of a pour creates a cold joint that weakens the slab, so rounding up is cheap insurance: the example patio needs about 1.6 yd³ ordered.
For small jobs, pre-mixed bags are practical: an 80 lb bag makes about 0.6 ft³, so one cubic yard takes 45 bags. Above roughly one cubic yard (or one cubic metre), ready-mix delivered by truck is usually less work and often cheaper. Typical thicknesses are 4 in (10 cm) for paths and patios and 5–6 in (12–15 cm) for driveways.
Yield of pre-mixed concrete bags
| Bag | Makes about | Bags per cubic yard |
|---|---|---|
| 40 lb | 0.30 ft³ | 90 |
| 50 lb | 0.375 ft³ | 72 |
| 60 lb | 0.45 ft³ | 60 |
| 80 lb | 0.60 ft³ | 45 |
| 25 kg | 0.41 ft³ (11.7 L) | 66 (86 per m³) |
Tips
- Need a compacted gravel base under the slab? Work it out with the Gravel Calculator.
- For tanks, planters and odd shapes, the Volume Calculator covers 12 solids.
- Finishing the floor afterwards? See the Tile Calculator and the Flooring Calculator.
- Ready-mix suppliers often have a minimum order and charge a “short load” fee below it — ask before deciding between bags and a truck.
Frequently asked questions
Can’t find your answer? Contact us — we reply quickly.
How do I calculate cubic yards of concrete?
Multiply length × width × thickness in feet and divide by 27. For inches of thickness, divide by 12 first: a 4 in slab is 0.333 ft thick.
How many 80 lb bags of concrete make a cubic yard?
45 bags, because each 80 lb bag yields about 0.6 cubic feet and a cubic yard is 27 cubic feet. With 60 lb bags you need 60.
How thick should a concrete slab be?
4 inches (10 cm) is standard for patios, paths and shed floors; driveways and garage floors are usually 5 to 6 inches (12–15 cm). Local building codes and soil conditions take priority.
How much does concrete weigh?
About 150 lb per cubic foot, 4,050 lb per cubic yard or 2,400 kg per cubic metre once cured.
How much extra concrete should I order?
5–10 % for simple, well-formed pours; closer to 10 % on uneven ground or with complex shapes.
Are my numbers stored or sent anywhere?
No. The calculation runs in your browser. Your last inputs are remembered on your own device only, so the form is still filled in when you come back; use Reset to clear them.
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