Foundation Concrete Calculator

Working out how much concrete your extension foundations need comes down to three measurements: the length of trench around the outside walls, the trench width, and the depth. Multiply them together and you have the volume in cubic metres. Ready-mix concrete delivered to a UK site typically costs £90 – £115 per m³, so that same volume figure is also what sets your bill.

This calculator turns those three measurements into a delivery plan. Enter your trench perimeter, width, and depth, and it returns the volume, how many mixer loads to order, and the likely delivered cost. It also adds a 10% over-order buffer, because a trench that runs short part-way through a pour is an expensive problem to fix.

How the volume is worked out

The base volume is simply perimeter times width times depth. Most modern extensions use a trench-fill design, where the trench is filled almost entirely with concrete rather than built up in blockwork below ground, so the poured volume matches the trench volume closely. A 0.6m trench width is the common default, though your structural engineer may specify wider.

Depth is the figure that moves the volume most. The working baseline for a domestic extension on stable ground is 1m to firm strata, but clay, made ground, or nearby tree roots can push the required depth well beyond that. The final depth is set by your structural engineer and confirmed by building control at the trench inspection before any concrete is poured. Because that inspection can call for a deeper dig, the calculator recommends holding volume back as contingency.

On top of the base volume, add the 10% buffer. Soft spots, over-dig, and small variations in trench width all soak up extra concrete, and a shortfall mid-pour risks a cold joint in the foundation as well as a costly return trip.

What it costs and how many loads to order

Standard-grade ready-mix for domestic foundations runs £90 – £115 per m³ nationally, rising to £110 – £135 per m³ across London and the South East. If your structural engineer specifies high-strength C35 or above for aggressive ground, expect roughly 15% on top of the standard rate.

Delivery logistics matter as much as the per-metre price. A full-size UK barrel truck holds 6–8 m³, so the calculator divides your volume by that capacity to show whether the pour fits one load or needs two scheduled back to back. Order below 4 m³ and most suppliers add a short-load surcharge, which is why combining small pours or using a volumetric mixer sometimes works out cheaper.

A worked example

Take a 4m by 5m single-storey rear extension. Three new external walls sit on fresh trench, which comes to roughly 18m of perimeter once the side against the existing party wall is left out. At a 0.6m trench width and the 1m building control baseline depth, multiplying the three figures gives the base volume the calculator shows above.

Add the 10% buffer and that is the number to put on your order. At £90 – £115 per m³ for standard grade, the delivered cost lands in the range the tool displays, before your groundworker's labour and any pump hire. The calculator also converts that volume into mixer loads against the 6–8 m³ truck capacity, so you know in advance whether one delivery covers the pour or a second load needs booking. Because the volume sits above the 4 m³ short-load threshold, the order avoids the surcharge that catches out smaller pours.