How to calculate gas boiler running costs
There are two useful starting points. If you know the useful heat your home needs, divide it by boiler efficiency to estimate the gas energy required. If you already know your gas consumption, multiply that consumption by the matching unit price to calculate the variable part of the bill.
At a heat demand of 12,000 kWh and a boiler efficiency of 90%, the boiler needs about 13,333 kWh of gas. A gas tariff of 0.09 per kWh gives an annual energy cost of 1,200. Change the currency label and use your own price; no exchange-rate conversion is applied.
Gas meter units are not always billing units
A meter may measure volume while the supplier bills in kWh. If your bill already lists consumption and price in kWh, use those values directly. That avoids estimating a conversion factor.
For a volumetric tariff, select m³ or ft³ and enter the effective kWh per m³ from your bill. The calculator converts ft³ to m³ internally. The gas-energy input should already reflect any billing correction you need. Do not treat the default conversion as a national standard.
UK imperial meters can record hundreds of cubic feet. This tool’s ft³ option means a single cubic foot, not a hundred. Convert your reading difference before entering it. One therm is treated here as 29.307107 kWh.
Efficiency and heating schedules
Enter efficiency as a percentage: 90 means that 90% of fuel energy becomes useful heat within your chosen system boundary. Use an efficiency stated on the same gross or net calorific basis as the fuel-energy input. This tool accepts efficiencies up to 100% and expects a consistent basis.
Boiler maximum capacity is not average consumption. A 24 kW boiler can modulate or cycle, so “two hours switched on” does not necessarily mean two hours at full power. Measured gas use is a better starting point when estimating the cost of a particular heating routine.
Add relevant annual standing charges or maintenance in advanced settings. The monthly figure is annual cost divided by 12, not a prediction of the higher winter bills.
Reference: GOV.UK: gas meter readings and bill calculation.