Concrete Calculator
Work out exactly how much concrete you need — slabs, footings, columns, tubes, stairs and post holes, all on one page — with volume, pre-mix bag counts, ready-mix, weight and cost in both metric and imperial.
Set your units and waste at the top, fill in a shape's measurements, then press Calculate to see its volume, bag count and ready-mix. Each shape works on its own; press Clear to reset it.
Pick the unit for each measurement from its own dropdown (feet, inches, yards, metres or cm) — mix and match freely. Bag $ / Mix $ are optional. Estimates only — confirm quantities with your supplier before ordering.
Slab, wall & footing
Column, hole & Sonotube
Circular slab & tube
Stairs
Curb & gutter
Fence post holes
How to work out how much concrete you need
Every concrete estimate starts with one thing: volume. Once you know the volume of the space you are filling, everything else — bags, ready-mix, weight and cost — follows from it. The three building-block formulas are:
- Rectangular slabs, walls and footings: Volume = Length × Width × Thickness
- Round columns, holes and Sonotubes: Volume = π × (Diameter ÷ 2)² × Height
- Ring / tube shapes: Volume = π × ((Outer ÷ 2)² − (Inner ÷ 2)²) × Height
Keep every measurement in the same unit before multiplying. Because 1 cubic yard equals 27 cubic feet and 1 cubic metre is about 1.31 cubic yards, the calculator converts automatically so you can read the answer in whichever unit your supplier uses.
Turning volume into bags
Pre-mix bags are sold by their dry weight, and each size makes a known amount of finished concrete. To find the bag count, divide your total volume by the yield of one bag and round up. Typical yields are:
| Bag size | Approx. yield | Bags per yd³ / m³ |
|---|---|---|
| 40 lb | 0.30 ft³ | ~90 per yd³ |
| 50 lb | 0.375 ft³ | ~72 per yd³ |
| 60 lb | 0.45 ft³ | ~60 per yd³ |
| 80 lb | 0.60 ft³ | ~45 per yd³ |
| 20 kg | ~0.0094 m³ | ~106 per m³ |
| 25 kg | ~0.0117 m³ | ~85 per m³ |
| 30 kg | ~0.0141 m³ | ~71 per m³ |
Worked example
Say you are pouring a 10 ft × 10 ft slab, 4 inches thick. In feet that is 10 × 10 × 0.333 = 33.3 cubic feet, which is about 1.23 cubic yards. At 0.6 ft³ per bag that is roughly 56 bags of 80 lb mix. Adding a 10% waste allowance brings it to about 1.36 cubic yards — the point at which ordering ready-mix usually makes more sense than mixing bags by hand.
Estimating tips
- Always round up and add a small safety margin — being short mid-pour is far worse than a little left over.
- Break odd shapes into rectangles and circles, calculate each, then add them together using the Quantity field or separate runs.
- For a set of steps, estimate the flat landing as a slab and the steps themselves with the Stairs option.
- Compact and grade the sub-base well; a dip of even half an inch across a big slab adds real volume.
Estimates only — not professional engineering advice. Site conditions, mix design and over-excavation all affect the real quantity.
Sources & methodology
Volume is length × width × thickness for each shape (with a curved or tapered adjustment for curbs, columns, tubes and stairs), then converted to cubic yards or metres. Bag counts and weight are derived from concrete's typical density of about 2,130 kg/m³ (133 lb/ft³), and ready-mix is rounded to your chosen ordering unit.
Sources: Standard geometric volume formulas; typical concrete density of ~2,130 kg/m³ (≈133 lb/ft³).
Frequently asked questions
How much concrete do I need?
Multiply length × width × thickness for the volume, then convert to cubic yards or metres. This tool does it for any shape and also gives bag counts, ready-mix and weight, with an optional waste allowance.
How many 80 lb bags of concrete make a cubic yard?
About 45. A cubic yard is 27 ft³ and an 80 lb bag yields roughly 0.6 ft³. Sixty-pound bags need about 60 per yard, forty-pound bags about 90.
How thick should a concrete slab be?
Around 4 inches (10 cm) for residential patios, paths and floors; 5–6 inches (13–15 cm) for driveways and heavier loads.
Should I use bags or order ready-mix concrete?
Bags suit small jobs like footings and post holes. For a driveway or full patio, ready-mix delivered by the yard or cubic metre is cheaper and much less work.
How much extra should I order for waste?
Add roughly 5–10% for spillage, uneven ground and over-excavation. The waste selector adds this for you.
How much does concrete weigh?
This calculator estimates weight at about 2,130 kg per cubic metre (133 lb per cubic foot). Real concrete density varies with the mix and moisture, so use the figure as a planning estimate.