One British thermal unit is about 1,055.06 joules, so a joule is 0.000947817 BTU. Despite the name, the BTU is barely used in Britain any more — it is the standard heat unit of the American HVAC industry.
About the units
A BTU is the energy needed to raise one pound of water by one degree Fahrenheit. Like the calorie, that definition depends on the starting temperature, so several BTUs exist: the International Table BTU at 1,055.05585 J, the thermochemical BTU at 1,054.35 J, and others defined at specific temperatures. The difference between them is under a tenth of a per cent. Britain adopted the joule and the watt for heating calculations decades ago, but the United States retained the BTU throughout, and it is the unit in which every American furnace, boiler and air conditioner is rated.
The exact factor
The International Table value of 1,055.05585262 J is the one most commonly used. Related quantities: a therm is 100,000 BTU, and one BTU per hour is 0.293071 watts. Air conditioning capacity is quoted in BTU per hour, so a 12,000 BTU/h unit — one ton of refrigeration — is 3.517 kilowatts. That last figure is the conversion that matters most when comparing an American air conditioner to a European heat pump, since the two are rated in different units for the same job.
Where you meet this conversion
Heating, ventilation and air conditioning in the United States, where furnace output, boiler capacity and air conditioner size are all in BTU per hour. Natural gas is billed in therms or in hundreds of cubic feet with a BTU content stated. Appliance efficiency ratings such as SEER and EER are expressed as BTU per watt-hour, which mixes the two systems inside a single figure. Anyone specifying equipment across the Atlantic converts constantly, and the kilowatt figure is the one that makes comparison possible.