UK Concrete Calculator

Getting a concrete order wrong is expensive in both directions: order too little and you are left with an unfinished pour and a second delivery charge, order too much and you have paid for concrete you never used. This calculator works out how much concrete you need for a slab, footing, post hole or column, in cubic metres, weight, and the number of ready-mix bags.

How to use the concrete calculator

Choose what you are pouring: a slab or footing uses length, width and depth, while a post hole or column uses diameter and depth or height instead, since these are typically round. Enter your dimensions in metres or feet, and for post holes or columns, add how many you need if you are doing several at once, such as a run of fence posts. Enter your bag size, commonly 20kg, and a price per bag if you know it, to see an estimated cost alongside the quantities.

How the calculation works

For a slab or footing, the calculator multiplies length by width by depth to get the volume in cubic metres. For a post hole or column, it uses the volume of a cylinder, based on the radius (half the diameter) and the depth or height. The volume is then multiplied by a standard concrete density of around 2,400kg per cubic metre to estimate the total weight.

For bags, the calculator uses a widely cited builders' merchant rule of thumb: a 20kg bag of ready-mixed concrete yields approximately 0.01 cubic metres once mixed with water, scaled up or down for other bag sizes. This figure is rounded up to a whole number of bags, since you cannot buy part of a bag and it is always safer to have a small amount spare than to run out mid-pour.

Worked example

Take a garden patio slab 3 metres by 3 metres, at a 100mm (0.1m) depth.

The volume is 3 × 3 × 0.1 = 0.9 cubic metres, weighing around 2,160kg. Using 20kg bags, that works out at 90 bags. At this volume, most people compare the cost and effort of 90 bags against a small ready-mix delivery, since many suppliers can deliver volumes from around 1 cubic metre upwards.

Now try a single fence post hole in the calculator above: 300mm diameter and 600mm deep. The volume comes to around 0.042 cubic metres, needing roughly 5 bags at 20kg each, a far more manageable DIY job than the patio slab. If you are setting several fence posts at once, increase the quantity field to see the total bags needed across all of them in one go.

Recommended mix ratios by job type

A general purpose 1:2:4 mix (cement, sand, aggregate, by volume) suits most everyday DIY jobs: patios, garden paths, sheds bases and simple footings. Post holes are often filled with a proprietary quick-set post mix instead of a traditional wet mix, since it is designed to be poured dry around the post and left to absorb ground moisture, setting in around an hour without any separate mixing. Structural columns and other load-bearing work typically need a stronger mix, such as 1:1.5:3, and should always follow a structural engineer's specification rather than a general DIY guideline.

Weather and timing considerations

Concrete cures through a chemical reaction with water, not simply by drying out, which is why weather matters more than many first-time DIYers expect. Pouring in very hot weather can cause the surface to cure too quickly, leading to cracking, while pouring in cold weather, particularly close to or below freezing, can stop the curing reaction almost entirely and seriously weaken the finished concrete. Most UK guidance suggests avoiding pours when temperatures are forecast to fall below around 5°C within the first 48 hours.

Rain shortly after pouring is another common problem, since it can wash cement paste from the surface before it has set, leaving a weak, dusty finish. Covering a fresh pour with plastic sheeting is a simple, cheap way to protect it from both rain and excessive moisture loss in hot weather, and is worth doing as standard practice rather than only when rain looks likely.

Common mistakes DIYers make

A common mistake is underestimating depth, particularly for footings and foundations, where building regulations or a structural engineer's specification may require a specific minimum depth that a general DIY guide cannot safely assume for your specific ground conditions and building type. Always check the actual requirement for your project rather than guessing a reasonable-looking figure.

It is also easy to forget that concrete needs curing time before it can bear weight or load, often several days for basic strength and considerably longer, sometimes weeks, for full strength. Loading a slab or setting a post too early is a common cause of cracking and failure.

Finally, ordering exactly the calculated amount with no buffer is risky, since ground conditions are rarely perfectly even and slight over-digging is common. Many builders add a small contingency, often an extra 5% to 10%, to avoid running short partway through a pour. For anything structural, load-bearing, or larger than a simple garden job, get quotes from qualified tradespeople before starting work.

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