The British thermal unit (BTU) is an energy unit rooted in the heat needed to warm water. It is small and old-fashioned by SI standards, but it remains deeply embedded in US energy and natural-gas markets, so the tool supports it and its common multiples.
The definitions the tool uses
There is more than one BTU in circulation, differing slightly in how the underlying calorie is defined. The tool fixes the International Table (IT) BTU:
1 BTU_IT = 1055.05585262 J1 MMBTU = 10^6 BTU_IT(one million BTU)
These are treated as fixed by convention: the number is exact by fiat, and the tool labels the BTU basis on the unit's detail page so there is no ambiguity about which BTU is meant.
What "MM" means
The "MM" in MMBTU is a frequent source of confusion. It does not mean "milli-milli" or anything metric. It is the Roman-numeral convention where M is one thousand, so MM is a thousand thousands — one million. An MMBTU is therefore 1,000,000 BTU, the standard wholesale trading unit for natural gas and other fuels in the United States. The kBTU (a thousand BTU) is common in building-energy contexts.
IT versus thermochemical BTU
The main alternative is the thermochemical BTU, which is very slightly smaller than the IT BTU. The tool deliberately does not mix the two: it uses the IT BTU throughout so that the BTU, the therm and the quad all cohere and chained conversions never pick up incoherent factors. The difference between the two BTU definitions is tiny — comparable to the roughly 0.07% gap between the IT and thermochemical calorie — but silently mixing them would still produce subtly wrong results, so a single labeled convention is chosen instead.
Bigger multiples
Built on the IT BTU are the larger units used in gas billing and national energy statistics: the therm (100,000 BTU), the MMBTU (a million BTU, = 10 therms), and the quad (10^15 BTU_IT) used for whole-country energy budgets. Because all of these share the same BTU definition, converting among them is exact and consistent.