The therm is a unit of energy you will meet mainly on natural-gas bills. It exists for convenience in billing, not because it corresponds to anything fundamental in physics — it is a convention layered on top of the British thermal unit.
The definition
The tool uses the US therm, defined as:
1 therm = 100,000 BTU_IT≈ 1.0550559 × 10^8 J(that is10^5 × 1055.05585262 J)= 0.1 MMBTU(one tenth of a million BTU)
Because the therm is built directly on the IT BTU, converting a therm to BTU, MMBTU or joules is exact — the number is fixed by convention, not measured.
A billing unit, not a physical constant
The therm rounds off to a tidy hundred-thousand BTU precisely because it was designed as a billing unit. That tidiness is a matter of convention, not physics: there is nothing in nature that prefers 100,000 BTU. The tool labels the therm as a defined convention. Both the US and the UK/EC therm are defined through the same chain — 100,000 British thermal units on the International Table definition — so this catalog claims no divergence between them; an earlier version of this page asserted one at the 10^-5 level, which nothing in the source ledger supports.
Why a therm is not the same as "a therm of gas"
A therm is an amount of energy. Your gas meter, however, measures volume (cubic metres or cubic feet), and the supplier multiplies that volume by a calorific value to arrive at the energy you are billed for. So converting the gas volume on your meter into therms is not the exact operation that converting therms to BTU is — it depends on the gas composition and on reference conditions, and it can never reproduce your bill exactly. See natural gas: m³ to kWh for why that volume-to-energy step is always an estimate, and the therm unit page for the exact energy-unit conversions.