Concrete Mix Ratio Calculator

This calculator works out how much cement, sand, aggregate and water you need to mix a given volume of concrete to a chosen ratio. Concrete is made by combining cement, sand, coarse aggregate (gravel or stone) and water in set proportions, and getting those proportions right is what gives the finished concrete its strength and durability. Common mixes are written as a ratio of cement to sand to aggregate, such as 1 to 2 to 4 for general use or 1 to 1.5 to 3 for stronger structural work, and the water is added in proportion to the cement. Working out the actual quantities for a real volume, allowing for the fact that the dry materials pack down when mixed and wetted, is fiddly to do by hand, which is where this tool helps. You enter the finished volume of concrete you need, the mix ratio parts for cement, sand and aggregate, and the water-to-cement ratio, and the calculator returns the number of cement bags, the volume of sand and the volume of aggregate, and the litres of water. It applies the standard dry-volume allowance, since a cubic metre of wet concrete needs about 1.54 cubic metres of dry materials because they consolidate as they combine. The results update as you type. Use it to order materials for a slab, path, foundation or post, to estimate costs, or to get the proportions right for a small mix. A note on the numbers: cement bags are based on a density of about 1,440 kilograms per cubic metre and a 25 kilogram bag, common in New Zealand, and quantities are rounded up where it makes sense, since you cannot buy a fraction of a bag. Always allow a little extra for waste and spillage, and follow the specification for structural work.

Uses a 1.54 dry-volume factor; cement density ~1440 kg/m³, 25 kg bags. Bags rounded up. Allow extra for waste. Follow the specification for structural work.