Concrete Calculator

Concrete Calculator works out the volume for a slab, a strip footing, a round column or a set of post holes, and converts it into both pre-mixed bags and the separate cement, sand and aggregate for mixing on site.

The bag count is the figure that changes plans. A 25 kg bag of pre-mix yields only about 0.011 cubic metres, because concrete weighs around 2,400 kg per cubic metre. A modest 4 by 3 metre slab at 100 mm thick needs over a hundred bags, which is roughly three tonnes of material to carry and mix. Above about half a cubic metre, ready-mixed delivered is almost always cheaper and very much less work.

The mix ratios are the conventional ones: 1:2:4 for general purpose work, 1:3:6 for foundations, 1:1.5:3 and 1:2:3 where more strength is wanted. The dry materials bulk down by roughly a third once mixed and compacted, which is accounted for -- ordering sand and aggregate by the finished volume leaves you short.

The page shows its working, not just its answer. Where there is a formula it is printed with your own numbers substituted in, so you can check the result rather than take it on trust -- which is the difference between a calculator you can use for something that matters and one you cannot.

Everything runs inside your browser. What you type is never uploaded, never logged and never stored, and the calculator keeps working if your connection drops after the page has loaded.

• Shows concrete needed, pre-mixed bags, cement, sand and aggregate together, so you are not running it twice
• A results table you can read across
• Updates as you type, with no button to press and no page reload
• Copy the result in one click
• Free, no sign-up, and it runs entirely in your browser

The concrete calculator is used by writers, developers, students, marketers and anyone else who needs the job done once without installing software. Common cases include:

• Working out concrete for a shed base or patio slab
• Costing post holes for a fence
• Converting a volume into bags or a ready-mix order
• Getting the cement, sand and aggregate split for mixing