Therm (thm)
Modern equivalent
100,000 Btu = 29.31 kilowatt-hours
Exactly 100,000 British Thermal Units, adopted as the standard billing unit for gas supply across Britain.
Gas meters measure volume, but gas is sold by the energy it delivers, and the calorific value of the supply varies with its composition. Suppliers therefore convert the metered volume into therms using a declared calorific value, then convert therms into kilowatt-hours for the final charge. Both figures still appear on British gas bills, which is why householders encounter a Victorian-era imperial unit and a metric one on the same page.