Pouring a slab and not sure how many bags of cement to buy? This concrete calculator works out the exact volume you need for slabs, footings, columns, and stairs, then breaks it down into cement bags, sand, and aggregate so you don’t end up short on site or with a driveway full of leftover bags.

Enter your dimensions in either metric or imperial, pick a mix ratio, and the numbers update as you type.

Why people use this tool

• No more guessing on material orders: get exact bag counts and sand/aggregate quantities before you go to the supplier.
• Works for four common pour types: slabs, footings, round or square columns, and staircases.
• Built-in wastage allowance: add 5-15% on top so you’re not caught short mid-pour.
• Cost estimate included: enter your local material prices and see a full cost breakdown by item.
• Mix your units freely: metric and imperial inputs are both available, and results switch cleanly between the two.

How much concrete do you actually need

Concrete is sold and mixed by volume, not by area, so the thickness of a slab matters just as much as its length and width. A 4m by 3m slab poured at 10cm thick needs roughly 1.2 cubic metres of concrete, while the same footprint at 15cm thick needs 1.8. Small changes in depth add up fast once you’re paying for ready-mix by the cubic metre or cubic yard.

The same logic applies in imperial: a 13ft by 10ft slab poured 4 inches thick needs about 1.6 cubic yards of concrete. Switch between Metric and Imperial in the calculator above and the results stay consistent either way.

This is also where most DIY estimates go wrong: people measure the surface area and forget to multiply by thickness, then order too little and have to stop a pour halfway through to get more delivered.

Understanding the mix ratio

The calculator offers four standard mixes: M15 (1:2:4), M20 (1:1.5:3), M22.5 (1:1.25:2.5), and M25 (1:1:2). Each number in the ratio is cement to sand to coarse aggregate, by volume. M15 is a leaner mix suited to non-structural work like paths and light foundations. M20 is the general-purpose choice for most slabs, footings, and driveways. M25 has more cement per batch and is used where the concrete needs to bear more load, such as structural columns.

If your local supplier or engineer specifies a different ratio, switch to Custom Ratio and enter your own cement, sand, and aggregate proportions.

Reading your results: bags, sand, and aggregate

The calculator applies a 1.54 dry-volume factor before splitting the mix. This accounts for the fact that cement, sand, and gravel take up less space once mixed and compacted than they do as separate dry materials, so ordering the raw components requires more volume than the finished wet concrete.

From there:

Cement is converted to 50kg bags using a density of 1,440 kg per cubic metre. Sand and aggregate are shown in both volume and weight, using standard densities of 1,600 kg/m³ and 1,550 kg/m³. Water is calculated at a 0.5 water-cement ratio, a common starting point for workable concrete that most mixes are built around.

Estimating cost

Toggle Estimate Cost and enter what you pay locally per bag of cement and per cubic metre (or cubic yard) of sand and aggregate. The tool multiplies each by the quantities calculated above and shows a line-by-line breakdown alongside the total, so you can see exactly where the money goes rather than just a single number at the end.

If you need to convert the results into different units for a supplier quote, a Volume Converter or Weight Converter can help. For the area of an irregular slab before you get to volume, the Area Calculator is a useful first step, and the Percentage Calculator is handy if you want to work out a custom wastage margin by hand.

These estimates are for planning purposes. For structural work or anything that needs to meet local building codes, confirm quantities and mix design with a licensed contractor or engineer.