How to use the concrete calculator
Concrete is sold by volume, but most of the work on site is measured as length, width and thickness. Getting the conversion right — and adding enough waste — is what separates a clean pour from a frantic mid-afternoon call to the ready-mix plant.
- Choose the shape: slab (floor or patio), footing/pad, or column/pier.
- Set length units (feet or metres) and thickness units (inches or centimetres). Column diameter uses the thickness unit so you can enter 12 inches or 30 cm naturally.
- Enter the dimensions and a waste percentage (10% is a sensible default).
Results show volume in cubic metres and cubic yards, the volume with waste, and estimated bag counts for common US (40/60/80 lb) and metric (20/25/40 kg) bag sizes based on typical yields.
Concrete volume formulas
Slab / footing = Length × Width × Thickness
Rectangular column = Length × Width × Height
Round column = π × (Diameter ÷ 2)² × Height
yd³ = ft³ ÷ 27 · m³ = ft³ × 0.0283168
Order volume = Volume × (1 + Waste % ÷ 100)
Keep thickness in the same base unit before multiplying. Four inches is 4 ÷ 12 = 0.333 ft; 100 mm is 0.10 m. Ready-mix yards are ordered to 0.25 or 0.5 yd³ increments in many markets — round up to what your supplier will deliver.
| Bag size | Typical yield | Bags per yd³ (approx.) |
|---|---|---|
| 40 lb (US) | 0.30 ft³ | ~90 |
| 60 lb (US) | 0.45 ft³ | ~60 |
| 80 lb (US) | 0.60 ft³ | ~45 |
| 20 kg | ~0.009–0.010 m³ | ~100–110 per m³ |
| 25 kg | ~0.012–0.014 m³ | ~75–85 per m³ |
| 40 kg | ~0.018–0.020 m³ | ~50–55 per m³ |
Worked examples
A 20 × 10 ft patio slab, 4 inches thick
Volume = 20 × 10 × (4 ÷ 12) = 66.67 ft³ = 2.47 yd³ (1.89 m³). With 10% waste, order 2.72 yd³. In 80 lb bags at 0.60 ft³ each that is about ⌈66.67 × 1.1 ÷ 0.60⌉ = 123 bags — which is why a ready-mix truck is the usual choice above about a cubic yard.
A strip footing
A 40 ft long footing, 2 ft wide and 12 inches deep: 40 × 2 × 1 = 80 ft³ = 2.96 yd³. With 10% waste, plan on 3.26 yd³. Ask the engineer for the rebar schedule separately; volume alone does not size reinforcement.
Metric round piers
Six piers, each 300 mm diameter and 1.2 m deep. One pier is π × 0.15² × 1.2 ≈ 0.085 m³; six are 0.51 m³. With 10% waste, 0.56 m³ — roughly 40–45 of the 25 kg bags, or a small ready-mix order if the site allows.
Who this calculator helps
- Architects checking early foundation and slab volumes for cost plans and coordinating with structural engineers.
- Builders and concrete contractors estimating ready-mix orders and bag counts for small works.
- Landscape designers sizing patio, path and pier pours.
- Architecture and engineering students converting design dimensions into volumes for studio and lab work.
- Homeowners planning a shed base, fence posts or a DIY patio before hiring a mixer.
Pour day tips
- Confirm formwork is square and braced; volume assumes the forms hold their dimensions under pressure.
- Have the waste allowance on site — short loads on a hot day set before a second truck arrives.
- Match mix strength (for example 25 MPa / 3500 psi) to the specification; volume calculators do not choose the mix.
- Measure the slab area with the square footage calculator, and for roof structures check pitch with the roof pitch calculator.
Exploring a concrete finish, board-formed walls or a polished slab look on the real building? Upload a photo and try it with Exterior Redesign.
