The formula
How it works
Estimate how much concrete a slab, footing or path needs from its length, width and depth. The calculator gives the volume in cubic yards or cubic metres and a rough number of pre-mixed bags.
FAQ
How many bags of concrete do I need?
It depends on the bag size. A 60 lb bag makes about 0.45 cubic feet, and an 80 lb bag about 0.60. The calculator assumes 60 lb bags in imperial and 25 kg bags in metric — check your bag and adjust.
Should I order extra?
Yes. Add around 10% to your estimate for spillage, uneven ground and slight over-digging. Running out mid-pour leaves a weak cold joint, so it is better to have a little spare.
Should I order ready-mix or use bags?
For volumes above roughly 1 cubic yard (or 1 cubic metre), ready-mix delivered by truck is usually cheaper and less labour than mixing dozens of bags by hand. Smaller pours like posts or steps are often easier as bags.
What depth should I use for a driveway or footing?
A typical slab or path is 4 inches (10 cm) thick, while driveways are often 5–6 inches (13–15 cm) and footings can be deeper depending on the load. Enter the depth your project calls for rather than the default.
Does the calculator account for reinforcement or waste?
No, it only multiplies length, width and depth to get raw volume. Rebar, mesh and formwork take up negligible volume, but the suggested 10% buffer covers general waste and spillage.
Why do imperial and metric bag counts use different bag sizes?
The calculator assumes the bag size most commonly sold in each system — 60 lb bags in the US and 25 kg bags in metric regions. If your supplier sells a different size, divide the volume by that bag’s stated yield instead.
Can I use this for round or irregular shapes?
This calculator assumes a rectangular slab. For a circular pour like a post footing, calculate the volume as a cylinder instead, then adjust for any irregular edges by rounding the length and width up slightly.
About the concrete calculator
This calculator estimates the volume of concrete you need for a slab, footing, path or pad, and turns it into a rough number of bags. Concrete is sold and mixed by volume — cubic yards in the US, cubic metres elsewhere — so getting the volume right is the key to ordering the correct amount. Because concrete cannot easily be topped up once it starts to set, a good estimate up front saves both money and a lot of stress on pour day.
How to use it
Choose imperial (feet and inches) or metric (metres and centimetres), then enter the length, width and depth of the pour. Depth is usually the smallest number — a typical slab might be 4 inches or 10 cm thick. The calculator multiplies the three to give the volume, then converts it to cubic yards or cubic metres and estimates the bags needed. For example, a 10 ft by 10 ft slab 4 inches deep needs about 1.2 cubic yards.
The formula
Volume is length times width times depth, , with all three in the same unit. In imperial the depth in inches is divided by 12 to get feet first, and the cubic-foot result is divided by 27 to reach cubic yards. In metric the depth in centimetres is divided by 100 to get metres, giving the volume directly in cubic metres. Bag counts come from dividing the volume by the yield printed on a bag.
Where it is used
Builders and contractors use it to order ready-mix or bagged concrete for foundations, floors, driveways and footings, and to price a job accurately. DIY landscapers use it for fence-post footings, garden paths, shed bases and patio slabs. Because over-ordering wastes money and under-ordering ruins a pour, an accurate volume estimate is one of the most-used sums on any building site.