Concrete Calculator

Start by choosing the shape of your pour: a flat slab such as a shed base or path, a footing such as a strip foundation, or a round column such as a post hole or pier.

Pick your units, either metres or feet, then enter the dimensions. For a slab or footing you enter length, width, and depth. For a column the fields change to diameter and height, because a column is round rather than rectangular.

Set a waste allowance. Ten percent is a sensible default that covers spillage and uneven ground. Lower it for clean, well-formed pours, or raise it for rough excavation.

Check the bag yield. The default of 0.011 cubic metres per bag matches a standard 25kg pre-mix bag. If your bags are a different size, read the yield on the packaging and update the field. Finally, enter a price per bag to see a rough cost. The result updates live as you type, so there is no submit button to press.

For a slab or footing, the volume is simply length multiplied by width multiplied by depth. If you entered feet, each dimension is first converted to metres by multiplying by 0.3048, so the volume comes out in cubic metres.

For a column, the calculator uses the volume of a cylinder: pi multiplied by the radius squared multiplied by the height. The radius is half of the diameter you entered.

Next, the waste allowance is applied by multiplying the volume by one plus the waste percentage divided by 100. A 1.2 cubic metre slab with a 10 percent allowance becomes 1.32 cubic metres. That figure is divided by the bag yield and rounded up to a whole number, because you cannot buy part of a bag. With a 0.011 cubic metre yield, 1.32 cubic metres needs 120 bags. The cubic yard figure is the volume with waste multiplied by 1.30795, and the cost is the bag count multiplied by the price per bag.

The headline number is the bags needed. It already includes your waste allowance and is rounded up, so it is the quantity to buy rather than the exact amount the pour consumes. Keeping a spare bag or two on site is wise for any pour.

The volume in cubic metres is the raw size of the pour before waste. Use it to compare against ready-mix quotes, which are priced per cubic metre. The cubic yards figure is the same volume with waste in imperial units, useful if your supplier quotes in yards.

The cost is a bags-only estimate based on the price you entered. It does not include reinforcement mesh, formwork timber, a sub-base, delivery, or labour, so treat it as a starting point for budgeting. If any dimension is left at zero the calculator shows a prompt rather than a misleading number, so you always know the figures are based on a complete set of measurements.