Concrete Calculator
Choose the job, enter its size, and see how much concrete to order, how many premixed bags to buy, and the cement, sand, gravel and water if you mix it yourself. Use the slab option for footings and patios. Two extra tabs estimate floor tiles and wall paint for the same project.
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How to use it
- Choose what you are building: a slab or footing, a round column, steps, floor tiles or wall paint.
- Pick feet and inches or metric, then enter the sizes.
- Keep about 10% extra for waste, spills and uneven ground, and choose your bag size.
- Read the volume to order and the number of bags. Open the extra section for the mix and cost.
Why use this tool
Slabs, footings, columns, steps
Covers the shapes most home projects need, with a quantity for several footings or posts.
Bags or ready-mix
Shows cubic yards or cubic metres to order and the number of 40, 50, 60 or 80 lb (or 20 to 40 kg) bags.
Mix it yourself
Cement, sand, gravel and water for four common mix ratios, with the number of cement bags.
Cost compare
Enter the bag price and the ready-mix price to see which is cheaper for your job.
Tiles and paint
Two extra tabs count floor tiles and boxes, and litres or gallons of wall paint.
Private
Everything is worked out in your browser. Nothing you type is sent or saved.
How the volume is worked out
A slab or footing is length × width × thickness. A 10 × 10 ft patio 4 inches thick is 10 × 10 × 0.333 = 33.3 cubic feet, or 1.23 cubic yards (divide cubic feet by 27).
A round column is π × radius² × height. A 12 inch post hole 4 ft deep holds 3.14 cubic feet.
Solid steps are added up one by one: the first step is one riser high, the second two risers, and so on. The total is width × rise × depth × n × (n + 1) ÷ 2 for n steps.
How many bags of concrete?
Premixed bags make about 0.011 m³ per 25 kg, or about 0.45 cubic feet per 60 lb bag and 0.6 cubic feet per 80 lb bag. One cubic yard needs about 45 bags of 80 lb or 60 bags of 60 lb.
| Bag size | Concrete per bag | Bags per cubic yard | 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 |
| 20 kg | 0.0094 m³ | 82 | 107 |
| 25 kg | 0.0117 m³ | 66 | 86 |
| 40 kg | 0.0187 m³ | 41 | 54 |
Mixing concrete yourself
Dry materials shrink when water is added, so a mix starts from about 1.54 times the wet volume. That dry volume is split by the ratio. For a 1 : 2 : 3 mix, one sixth is cement, two sixths sand and three sixths gravel.
Cement weighs about 1,440 kg per cubic metre, and water is about half the weight of the cement. One cubic metre of 1 : 2 : 3 needs about 370 kg of cement, 0.51 m³ of sand, 0.77 m³ of gravel and 185 litres of water.
| Mix | Use | Cement per m³ | Cement per yd³ |
|---|---|---|---|
| 1 : 1.5 : 3 | Strong slabs, driveways | 403 kg | 680 lb |
| 1 : 2 : 3 | General use, patios, footings | 370 kg | 623 lb |
| 1 : 2 : 4 | Paths, light floors | 317 kg | 534 lb |
| 1 : 3 : 6 | Blinding, fill under footings | 222 kg | 374 lb |
Typical thickness
Use these as a starting point and follow your local building code:
- Patios and paths: 4 inches (10 cm).
- Driveways for cars: 4 to 6 inches (10 to 15 cm).
- Shed and garage floors: 4 to 6 inches (10 to 15 cm).
- Strip footings: at least 8 inches (20 cm) deep and twice the wall width.
- Fence and deck post holes: about 3 times the post width and one third of the post height.
Tile and paint estimates
Tiles: floor area ÷ area of one tile, plus 10% for cuts (15% for diagonal layouts). Paint: wall perimeter × height, minus about 1.9 m² (20 ft²) for each door and 1.5 m² (16 ft²) for each window, times the coats, divided by the coverage on the can.
Frequently asked questions
How much concrete do I need for a 10 × 10 slab?
At 4 inches thick, a 10 × 10 ft slab is 33.3 cubic feet, or 1.23 cubic yards. With 10% extra that is 1.36 cubic yards, or 62 bags of 80 lb.
How many 80 lb bags make a cubic yard?
About 45. Each 80 lb bag makes about 0.6 cubic feet, and a cubic yard is 27 cubic feet.
How much extra should I order?
Add 5 to 10% for a level, well formed slab and 10 to 15% for uneven ground or hand mixing. Running short in the middle of a pour leaves a weak joint.
Is it cheaper to buy bags or ready-mix?
Bags are usually cheaper below about one cubic yard (0.75 m³). Above that, a ready-mix truck often costs less and saves hours of mixing. Enter both prices to compare.
How do I work out concrete for a footing?
Use the slab option: length × width × depth of the trench. For several separate pad footings, enter one and set how many.
What does 1 : 2 : 3 mean?
One part cement, two parts sand and three parts gravel by volume. It is a good general purpose mix for patios, paths and footings.
Is my data sent anywhere?
No. Everything is worked out in your browser. The page only remembers the job type and units you last used, on this device.
How to plan and pour a small concrete slab
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