Concrete Footing Calculator — Foundation Concrete Estimator UK

The concrete footing calculator does the heavy lifting, but knowing the formula helps you sense-check the result. Multiply length × width × depth, all converted to metres, and you get your volume in cubic metres. That is the number you need whether you are ordering ready-mix or buying bags.

For a strip footing running 8 metres, 600mm wide and 450mm deep: 8 × 0.6 × 0.45 = 2.16m³. At 84 bags of 25kg per cubic metre, that is 182 bags, well past the point where ready-mix saves you time and money. For a single pad footing at 1m × 1m × 0.3m deep, the volume is just 0.3m³, which is about 25 bags. That kind of job is easy to do with bagged concrete.

Always add 10% to your calculated total. Trench bases are never perfectly flat, and some concrete sticks to the mixer and barrow. Running short halfway through a foundation pour means a cold joint, which weakens the whole structure. Use this concrete footing calculator to get the base number, then round up before ordering.

The concrete mix ratio for foundations matters more than it does for paths or patios. Footings carry real loads, so the mix needs to be right from the start.

For most domestic strip and pad footings, a 1:2:3 ratio (cement:sand:gravel) by volume gives you C25 strength. That is the minimum most structural engineers specify for house foundations in the UK. A 1:2:4 concrete mix for foundations gives C20, which is fine for garden walls, fence post pads, and light structures, but not for load-bearing house footings.

If you are buying pre-mixed bags, look for ones labelled "Foundation Mix" or "C25". These already have the concrete mix for foundation footings built in. For larger jobs, specify GEN3 or ST2 from a ready-mix supplier. Both meet the concrete mixture for foundation requirements under BS 8500. The concrete mix ratio for foundations on clay soil or near trees may need upgrading to C30; check with Building Control if you are unsure. The concrete bag calculator can help you work out how many bags you need once you know your volume.

Strip footings are the most common type on UK residential sites. The standard size for a single-storey extension is 600mm wide and 450mm deep. For a two-storey structure, that usually goes to 750mm wide and 600mm deep, though your structural engineer or Building Control will confirm the exact dimensions based on soil type and load.

Here are the concrete volumes for common strip footing runs. These figures include 10% waste. Use the concrete footing calculator above to check volumes for your exact dimensions:

Once you pass about 1.5m³, ready-mix concrete is almost always cheaper and faster than bagging by hand. A half-load of ready-mix (4m³ minimum with most suppliers) works out at roughly £100-£140 per cubic metre delivered. Bags cost around £350-£560 per cubic metre when you buy individually. The concrete slab calculator covers slab volumes if you need to work out floor slabs on top of your footings.

UK Building Regulations set minimum footing sizes based on what is being built and the ground underneath. These are the typical dimensions for standard soil conditions:

On clay soil, depths increase, often to 750mm or 900mm to get below the zone where the soil swells and shrinks with moisture. Trees within 5 metres of footings usually require 1.5m-3m depth. Your footings calculator gives you the concrete volume; confirm the depth with Building Control before you pour. Concrete for foundations on problem ground often needs C30 or above, plus sulphate-resisting cement.