Calorie (th) to British Thermal Units
cal (th)
BTU
Conversion History
| Conversion | Reuse | Delete |
|---|---|---|
1 cal (th) (Calorie (th)) → 0.0039656668314660937 BTU (British Thermal Units) Just now |
Quick Reference Table (Calorie (th) to British Thermal Units)
| Calorie (th) (cal (th)) | British Thermal Units (BTU) |
|---|---|
| 1 | 0.0039656668314660937 |
| 100 | 0.39656668314660937032 |
| 1,000 | 3.96566683146609370318 |
| 4,184 | 16.5923500228541360541 |
| 10,000 | 39.6566683146609370318 |
| 100,000 | 396.56668314660937031799 |
About Calorie (th) (cal (th))
The thermochemical calorie (cal th) is defined as exactly 4.184 joules — the amount of heat needed to raise one gram of water by one degree Celsius under controlled conditions. It was standardized in 1935 by the US National Bureau of Standards for use in thermochemical measurements. The thermochemical calorie differs slightly from the International Table calorie (4.1868 J) and the 15°C calorie (4.18580 J). It is primarily used in chemistry for reporting heats of reaction and combustion.
One thermochemical calorie is the energy needed to warm 1 mL of water by 1 °C. The heat of combustion of glucose is about 670 kcal (th) per mole.
About British Thermal Units (BTU)
The British thermal unit (BTU) is the amount of heat required to raise one pound of water by one degree Fahrenheit at its maximum density (~39°F). One BTU equals approximately 1,055 joules. It remains the dominant unit for heating and cooling equipment in the United States — air conditioners, furnaces, heat pumps, and water heaters are all rated in BTU or BTU/hour. Natural gas prices in the US are quoted in dollars per million BTU (MMBtu).
A standard residential air conditioner is rated at 10,000–24,000 BTU/hour. Burning one kitchen match releases roughly 1 BTU of heat.
Etymology: Developed in the 19th century alongside the rise of steam engineering in Britain and the US, standardized as the energy needed to raise one pound of water by one degree Fahrenheit. The "British" name stuck even as the UK adopted SI units.
Calorie (th) – Frequently Asked Questions
What is the difference between a thermochemical calorie and an International Table calorie?
The thermochemical calorie (cal th) is defined as exactly 4.184 joules; the International Table calorie (cal IT) is exactly 4.1868 joules — a difference of 0.066%. The thermochemical value was fixed by the US National Bureau of Standards in 1935 for chemistry; the IT value was adopted for steam tables. In nutritional contexts, the difference is irrelevant, but in precise calorimetry it can matter.
Why do chemists still use thermochemical calories instead of joules?
Decades of published thermochemical data — heats of formation, bond energies, combustion enthalpies — are recorded in cal th and kcal th. Converting every reference table to joules would be error-prone and disruptive. Biochemistry textbooks still quote ATP hydrolysis at ~7.3 kcal/mol and glucose oxidation at ~686 kcal/mol. The convention persists because the existing literature is too vast to rewrite.
How does a bomb calorimeter actually measure the calories in food?
A dried, weighed food sample is sealed in a steel vessel filled with pure oxygen, submerged in a known mass of water. An electric spark ignites the sample, which burns completely. The temperature rise of the surrounding water — measured to 0.001°C — gives the total heat released. One degree rise per gram of water equals one calorie. Corrections for the heat capacity of the bomb itself, the ignition wire, and acid formation give results accurate to ±0.1%. Atwater then applied digestibility factors to convert bomb values to usable food energy.
What is the heat of combustion of common fuels in thermochemical calories?
Hydrogen releases about 34,000 cal th per gram; methane about 13,300 cal th/g; ethanol about 7,100 cal th/g; and glucose about 3,720 cal th/g. These values appear throughout chemistry textbooks as standard reference data. The higher the cal/g value, the more energy-dense the fuel — which is why hydrogen is attractive despite being hard to store.
Why did the 1935 NBS definition fix the thermochemical calorie at exactly 4.184 joules?
Before 1935, the calorie was defined by water's heat capacity, which varies with temperature — the 15°C calorie, 20°C calorie, and mean calorie all differed slightly. The US National Bureau of Standards ended the ambiguity by defining the thermochemical calorie as exactly 4.184 J, a round value close to all the experimental variants. This gave chemists a fixed, reproducible conversion factor independent of water's quirky temperature-dependent heat capacity.
British Thermal Units – Frequently Asked Questions
Why are air conditioners rated in BTU instead of watts?
US HVAC manufacturers adopted BTU/hour because heating and cooling equipment historically measured heat removal or addition, not electrical input. A 12,000 BTU/h window unit removes 12,000 BTU of heat per hour from a room — that figure directly tells you the cooling capacity. Watts measure electrical power consumed, which is less due to the efficiency (EER) of the unit. The convention stuck because the entire US supply chain uses it.
How many BTU does it take to heat a room?
A rough rule of thumb is 20 BTU per square foot of living space in a temperate climate. A 300 sq ft bedroom needs about 6,000 BTU/h; a 1,500 sq ft open-plan living area needs roughly 30,000 BTU/h. Actual requirements vary with insulation, ceiling height, climate zone, and window area. Poorly insulated older homes may need 30–40 BTU per square foot.
What is the difference between BTU and BTU/h?
BTU is a unit of energy (heat); BTU/h is a unit of power (rate of heat flow). When an air conditioner is labelled "12,000 BTU," the industry shorthand actually means 12,000 BTU per hour. Technically one BTU equals about 1,055 joules of energy, while 1 BTU/h equals about 0.293 watts. The distinction matters for energy calculations but is routinely blurred in product marketing.
How does the BTU relate to natural gas pricing in the US?
US natural gas is priced in dollars per million BTU (MMBtu) at the wholesale level and dollars per therm (100,000 BTU) on residential bills. One cubic foot of pipeline gas contains roughly 1,020 BTU. The Henry Hub benchmark price of $2.50/MMBtu means each therm costs about $0.25 wholesale — residential prices are higher after delivery and utility markups.
Why does the UK no longer use British thermal units despite the name?
The UK metricated energy units in the 1970s–1990s, switching gas billing from therms (100,000 BTU) to kilowatt-hours and scientific work to joules. The "British" in BTU reflects 19th-century British steam engineering origins, not current usage. Today the BTU is almost exclusively an American unit, used for HVAC, gas pricing, and appliance ratings across the US.