Tinker 
How to use the concrete calculator
Choose metric or imperial units, then enter the length and width of the area and the thickness of the pour. In metric, length and width are in metres and thickness in centimetres; in imperial, length and width are in feet and thickness in inches.
Add a waste allowance (10% is a sensible default) and pick the bag size sold near you. The calculator shows the volume in cubic metres, cubic yards and cubic feet, the number of premix bags, and the approximate weight.
The formula
Volume = length × width × thickness, with all three in the same unit. Thickness in centimetres is divided by 100 to get metres; imperial dimensions are converted to metres first (1 ft = 0.3048 m, 1 in = 0.0254 m). The volume is then multiplied by 1 + waste% / 100.
Bags are estimated assuming dry premix yields about 1 m³ of concrete per 2,160 kg of mix, so a 25 kg bag makes roughly 0.0116 m³. The weight figure uses 2,400 kg per cubic metre of cured concrete. Brands yield slightly differently, so check the bag label and treat the bag count as an estimate.
Worked example
A 4 m × 3 m patio slab 10 cm thick: 4 × 3 × 0.10 = 1.2 m³. Adding 10% waste gives 1.32 m³, or about 1.73 cubic yards. Each 25 kg bag yields 25 / 2,160 ≈ 0.01157 m³, so 1.32 / 0.01157 ≈ 114.05, rounded up to 115 bags. The finished slab weighs around 3,168 kg.
Practical tips
Above about 1 m³ (1.3 yd³), ordering ready-mixed concrete by truck is usually cheaper and much less work than mixing bags. Measure thickness at several points, since an uneven sub-base can add a lot of volume. For irregular shapes, split the area into rectangles and add the results.
Frequently asked questions
How thick should a concrete slab be?
Garden paths and patios are commonly 7.5-10 cm (3-4 in). Driveways for cars are usually 10-15 cm (4-6 in). Follow local building guidance for structural work.
How many bags of concrete make a cubic metre?
About 87 bags of 25 kg, 108 bags of 20 kg, or roughly 60 bags of 80 lb, depending on the brand's stated yield.
How much waste should I allow?
5% for neat formwork on a level base, 10% for most DIY jobs, and up to 15% for uneven ground or awkward shapes.
How do I convert cubic metres to cubic yards?
Divide cubic metres by 0.7646. For example, 1.32 m³ is about 1.73 yd³.