Calories (IT) to British Thermal Units (cal (IT) to BTU)

1 calorie (IT) equals 0.003968320719 British thermal units (rounded).

1 calorie (IT) = 0.003968320719 British thermal units

How to convert calories (IT) to British thermal units

To convert calories (IT) to British thermal units, multiply the number of calories (IT) by 0.003968320719. The factor is rounded to 10 significant digits. To go the other way, multiply British thermal units by 251.9957611 (rounded).

British thermal units = calories (IT) × 0.003968320719

Example: 10 calories (IT) = 0.03968320719 British thermal units.

The 'calorie' on food labels is really a kilocalorie. The thermochemical calorie (4.184 J) is used here unless marked IT.

About these units

Calorie (IT) (cal (IT))

ScientificHistorical unit (no longer in standard use)

The International Table calorie: exactly 4.1868 joules, slightly larger than the thermochemical calorie. Used in steam tables.

Steam engineers needed a calorie tied to the joule rather than to the behaviour of water, and the Fifth International Conference on the Properties of Steam, held in London in July 1956, fixed one. The result, the International Table calorie, is exactly 4.1868 J, defined as 1.163 milliwatt-hours. An earlier International Steam Table calorie of 1929 was about 4.1860 J, so the 1956 choice slightly adjusted the older figure.

Compared with the thermochemical calorie of 4.184 J, the International Table version is larger by 0.0028 J, or roughly 0.07 percent. That is negligible for cooking but can matter in steam tables and engineering handbooks, where the calorie and its kilocalorie multiple of 4,186.8 J are used. NIST lists both calories as exact values and flags them separately for that reason. The figure used here for this unit is 4.1868 J. Anyone converting data should check which calorie a source assumes before comparing numbers.

1 calorie (IT) = 4.1868 joules (exactly).

Sources: Wikipedia: Calorie; NIST SP 811, Appendix B.8 (energy conversion factors); Wikipedia: Conversion of units of energy

British Thermal Unit (BTU)

US & ImperialCurrent unit

The International Table BTU, exactly 1,055.05585262 joules. The energy to heat one pound of water by one degree Fahrenheit; used for heating, cooling and gas appliances in the US.

Few energy units come in as many versions as the British thermal unit. It was defined as the heat needed to raise one pound of liquid water by one degree Fahrenheit, but because water's heat capacity shifts slightly with temperature, definitions differ by up to 0.5 percent: about 1,054.35 J for the thermochemical Btu, 1,054.80 J at 59 degrees Fahrenheit (used for American natural gas pricing), 1,054.68 J at 60 degrees (mainly Canadian), and 1,059.67 J at 39 degrees. A 1942 note suggests the term apparently arose in the United States and was later adopted in Britain.

The version on this site is the International Table Btu, about 1,055.06 J, which comes from the international steam table conferences that tied heat units to the joule. Joule's own 1850 experiments gave 772.692 foot-pounds of work per Btu, the number that first linked heat to mechanical effort.

Today the Btu survives in natural gas pricing (MMBtu means a million Btu), US building energy reporting, and equipment ratings in Btu per hour. One ton of air-conditioning is 12,000 Btu/h, or 3.52 kW. A single wooden kitchen match burning releases about one Btu.

1 British thermal unit = 1,055.055853 joules (approximately).

Sources: Wikipedia: British thermal unit; Wikipedia: James Prescott Joule

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