Work out how much concrete you need. This free concrete calculator finds slab or footing volume in cubic metres, yards and feet, adds a waste allowance, and estimates bags and a 1:2:4 cement/sand/aggregate mix.
Enter the slab dimensions (thickness in the same units). Add a waste allowance – 5-10% is typical – for spillage and uneven ground. “How many” repeats identical pours.
Volume = length x width x thickness, plus your waste allowance. Bag and mix figures are estimates – bag yields and mix ratios vary by product and project, so always check the bag and confirm with your supplier.
How the Concrete Calculator Works
This tool finds the volume of concrete needed for a slab, footing or pad, then converts that volume into cubic yards, cubic feet, an approximate bag count, and a from-scratch cement/sand/aggregate mix if you’d rather batch it yourself.
Formula: Volume = Length × Width × Thickness × Quantity × (1 + Waste% ÷ 100). The wet volume (in m³) converts to cubic yards (× 1.307950619) and cubic feet (× 35.314667). Bags needed = wet volume ÷ 0.011 m³ per 25 kg/60 lb bag, rounded up. For a from-scratch mix, dry volume = wet volume × 1.54, then split 1:2:4 by volume across 7 parts for cement, sand and aggregate, with cement bags = cement volume ÷ 0.0347 m³ per 50 kg bag, rounded up.
- Length — the long side of the slab, footing or pad, in metres or feet
- Width — the short side of the same pour, in the same unit as length
- Thickness — the depth of the pour, entered in the same unit (e.g. 0.1 m for a 10 cm slab)
- How many — the quantity field, for repeating identical pours (e.g. several fence-post footings)
- Waste allowance — the extra percentage added for spillage and uneven ground, typically 5–10%
Example Scenarios
| Length (m) | Width (m) | Thickness (m) | Waste | Concrete needed |
|---|---|---|---|---|
| 5 | 4 | 0.10 | 10% | 2.20 m³ |
| 10 | 8 | 0.15 | 10% | 13.20 m³ |
| 3 | 3 | 0.10 | 5% | 0.95 m³ |
| 8 | 6 | 0.20 | 15% | 11.04 m³ |
| 2 | 2 | 0.05 | 10% | 0.22 m³ |
| 12 | 6 | 0.10 | 10% | 7.92 m³ |
Concrete Calculator FAQ
How much concrete do I need for a 10×10 ft slab?
Convert the slab to length × width × thickness in matching units, then add a waste allowance. A 10 ft × 10 ft slab at 4 in (0.333 ft) thick is about 3.7 ft³ before waste — enter those three dimensions into the calculator to get the exact figure with your waste percentage included.How many 60 lb bags of concrete do I need?
The calculator estimates roughly one 25 kg/60 lb bag per 0.011 m³ of concrete, rounded up to a whole bag. Bag yield varies by brand and by how much water is added, so treat the bag count as a planning estimate and check the yield printed on your specific bag.What waste allowance should I use for a concrete pour?
5–10% is a common starting point for a level, well-formed slab. Uneven or sloped ground, hand-mixing, or a first-time pour can justify pushing toward the higher end, since spillage and formwork gaps use up extra material quickly.Can I use this for footings instead of a slab?
Yes — enter the footing’s length, width and depth just as you would a slab, and use the “how many” field to repeat the same footing dimensions across multiple identical piers or post holes instead of calculating each one separately.What does the 1:2:4 mix ratio mean?
It’s a common volume ratio for site-mixed concrete: 1 part cement, 2 parts sand, 4 parts aggregate (gravel or crushed stone). The calculator shows the dry volume of each part so you know roughly how much of each material to have on hand.Does this account for ready-mix truck minimum orders?
No — it only estimates the volume of concrete your project needs. Ready-mix suppliers often have minimum order sizes and short-load fees below a certain volume, so confirm those terms with your supplier once you have your volume figure.Related Calculators
If you’re batching cement and aggregate yourself rather than ordering ready-mix, the concrete/cement calculator can help refine bag counts by mix ratio. Ordering base material for underneath the pour is easier with the gravel calculator, and once the slab is in you can measure the finished floor area with the square footage calculator.