How we calculate
Every calculator shows its formula and a worked example on the page. This page brings together the fixed figures they use and the assumptions we make.
Sourced figures
| Figure | Value | Source |
|---|---|---|
| Concrete per 20kg bag | 0.009 m³ | Derived from Tarmac’s coverage for Blue Circle Postmix |
| Postmix per fence post | 1 × 20kg bag | Tarmac |
| Driveway sub-base | 150mm | Marshalls |
| Paving laying course | 30mm compacted | Marshalls |
| Bricks per m², one skin | 60 | Wienerberger, 10mm joint |
| Multi-Finish per 25kg bag | 10 m² at 2mm | British Gypsum, via Buildworld |
| Plain tiles per m² | 60 at 100mm gauge | Marley |
| Composite joist centres | 300mm | UltraDecking, domestic |
| Gravel density | 1.7 t/m³ | Beesley & Fildes |
| Electricity unit rate | 26.32p per kWh | Ofgem price cap, 1 October to 31 December 2026 |
| EV charging losses | 10% | Low end of E.ON Next’s 10 to 25% range |
Our own working assumptions
Some figures vary by supplier and we couldn’t find one published figure to rely on. We use these, and each page that uses them says so:
- Concrete weighs about 2.4 tonnes per m³.
- Compacted MOT Type 1 weighs about 2.1 tonnes per m³, and sharp sand about 1.6.
- Semi-dry sand and cement screed needs about 2,000kg of dry material per m³.
- Mortar for brickwork is a rough guide of about 0.02 m³ per m² of half-brick wall.
- Window U-values in the radiator calculator are typical figures, not measurements.
Rounding
Anything you buy whole (bags, blocks, panels, boards) is rounded up, because you can’t buy part of one. Volumes and weights are shown to two decimal places.
How often we check
Energy prices are updated each time Ofgem changes the price cap: 1 January, 1 April, 1 July and 1 October. Product coverage figures are rechecked at least every six months, and sooner if a manufacturer changes a product. Figures were last checked on 25 October 2026.
What the calculators don’t do
They estimate quantities and running costs. They don’t design structures, check building regulations or replace a heat loss survey. See our disclaimer.