Concrete calculator

Order too little concrete and a pour grinds to a halt half-finished; order too much and you've paid for a heap you have to break up and cart away. This calculator works out how much you need for a slab, base or set of post holes, in cubic metres and bags, with a waste allowance built in. Here's how it works.

How to work out concrete for a slab

For a rectangular slab or base, the volume is simply length times width times thickness, with everything in metres.

Volume = length × width × thickness.

A 4m by 3m slab at 100mm thick is 4 × 3 × 0.1 = 1.2 cubic metres. The only trick is the thickness: 100mm is 0.1m, so convert millimetres to metres by dividing by 1,000. The calculator handles that for you, and adds your waste allowance on top.

How to work out concrete for post holes

Post holes are round, so the volume of each is pi times the radius squared, times the depth, and you multiply by the number of holes.

Volume = π × radius² × depth × number of holes.

For four holes at 200mm across and 600mm deep, each hole holds about 0.019 cubic metres, so four come to roughly 0.075 cubic metres before waste. Switch the calculator to "Post holes", enter the diameter, depth and how many, and it does the maths.

Bags or ready-mixed?

Concrete is heavy, and it takes a surprising number of bags to fill even a small volume. Roughly 108 bags of 20kg pre-mixed concrete, or about 86 bags of 25kg, make up a single cubic metre.

• Bags suit small jobs: a few post holes, a small pad, a repair.
• Ready-mixed, delivered by the cubic metre, is normally cheaper, faster and more consistent for a slab or anything over about half a cubic metre.

The calculator shows the volume in cubic metres, which is how ready-mix is ordered, plus an approximate bag count for both common sizes so you can weigh it up.

Always allow for waste

Never order the exact calculated volume. Ground is rarely level, holes get dug a little wide, and some concrete is always lost to spillage and what sticks in the barrow. A waste allowance of 5% to 10% covers it. The calculator adds 10% by default, and you can change it. Running a fraction short part-way through a pour is a far bigger problem than a little left over.

A note on the mix and the numbers

Bag yields vary a little between products and mixes, so treat the bag counts as a close guide rather than a precise figure, and check the coverage printed on the bag you're buying. The volume itself is exact geometry, so that's the number to trust when you're ordering ready-mix.

From materials to a priced job

Knowing the quantity is half the job; the other half is turning it into a price that pays. Once you've got your materials worked out, drop them into a clean, itemised quote with the quote generator, and make sure the whole job is priced to profit with how to price a job so you actually make a profit. For groundwork specifically, there's more in how much to charge to lay a patio.

This tool is for general estimating and gives approximate quantities. Bag yields, mixes and ground conditions vary, so check the product coverage and your own measurements, and order a sensible amount over rather than exactly to the calculation.