Concrete Calculator
Calculate how many cubic meters and bags of ready-mix concrete you need for a foundation, slab or footing — or flip it: how large an area you can pour with the bags you already have.
How to use the concrete calculator
- Enter the length and width of the area you're pouring.
- Enter the desired thickness — common for a deck footing is 8–10 cm, garage floors often 10–15 cm.
- The calculator shows volume in cubic meters, the number of 25 kg bags of ready-mix concrete, and total weight.
How the concrete amount is calculated
The volume is simply length × width × thickness, once the thickness is converted from centimeters to meters:
A 25 kg bag of ready-mix concrete yields roughly 12.5 liters (0.0125 m³) of poured concrete. Divide the volume by this number, rounded up, to find the bag count. Weight is calculated using a typical density of 2,400 kg per cubic meter.
Already bought a set number of bags and want to know how far they'll stretch? The "How large an area can I cover?" tab flips the math: the bags give you a fixed total volume, so the thinner you pour, the larger the area it covers.
Order ready-mix or mix it yourself?
The threshold sits somewhere around a cubic metre. Below that, bags and a mixer are usually both cheaper and more practical, particularly if the pour can be split into stages. Above it, hand mixing becomes heavy, time-consuming and hard to keep consistent — and inconsistent mixing means inconsistent strength through the pour.
Ready-mix has requirements of its own in return. The truck has to be able to reach the site, the concrete has to be discharged within a short window, and the whole pour should happen continuously to avoid cold joints where one layer has begun to set before the next arrives. If you order ready-mix, formwork, reinforcement and enough people should be in place before the truck arrives — and it is better to order slightly too much than to run out halfway.
Frequently asked questions
How thick should a foundation be?
It depends on what will stand on it. A deck footing often works with 8–10 cm, while a load-bearing garage or house foundation typically needs 10–15 cm or more, and should be assessed by a professional.
Should I use bagged mix or order ready-mix delivered by truck?
For smaller amounts (under about 1–1.5 m³), bags from a building supply store are most practical. For larger pours, it's often worth ordering ready-mix concrete delivered by truck.
Do I need reinforcement?
Most foundations and slabs should be reinforced with mesh or steel to prevent cracking and increase load-bearing capacity. This calculator only computes concrete volume, not reinforcement needs.
How large an area will the bags I bought cover?
Use the "How large an area can I cover?" tab — enter the number of 25 kg bags and your desired thickness, and the calculator works out the maximum area you can pour. The thinner the slab, the further the same number of bags will go.