What Is a BTU? British Thermal Unit Explained
The BTU is the energy needed to raise one pound of water by one degree Fahrenheit. Learn how it sizes furnaces and AC units, and how it links to kWh and joules.
What a BTU Measures
A BTU (British thermal unit) is a unit of energy, not power. It is defined as the energy needed to raise the temperature of one pound of water by one degree Fahrenheit. Despite the "British" name, it is the everyday energy unit for heating and cooling equipment in the United States.
Our BTU to kWh converter links it to the metric energy unit, and our hp to kW converter covers the related power rating.
BTU in Everyday Numbers
How BTU compares to other units
1 BTU ≈ 1,055 joules (about 1.055 kJ).
1 kWh = 3,412.14 BTU.
A single kitchen match releases roughly 1 BTU when burned.
Where You See BTU Ratings
Furnaces, air conditioners, and water heaters are sized in BTU per hour (BTU/h). A typical room air conditioner might be 5,000-12,000 BTU/h, while a whole-home furnace can exceed 100,000 BTU/h. The number tells you how much heating or cooling energy the device moves each hour.
Because electricity is billed in kilowatt-hours, the BTU-to-kWh link matters when you estimate running cost. See our US customary vs metric guide for how US and metric units sit side by side.
Frequently Asked Questions
What does BTU stand for?
British thermal unit, the energy to raise one pound of water by one degree Fahrenheit.
How many joules are in a BTU?
About 1,055 joules (1.055 kJ).
How many BTU are in a kWh?
Exactly 3,412.14 BTU make 1 kilowatt-hour.
Why are furnaces rated in BTU?
BTU per hour describes how much heating or cooling energy the equipment delivers each hour, which is how US HVAC is sized.
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Quick Summary
A BTU is the energy to raise one pound of water by one degree Fahrenheit. It equals about 1,055 joules, and 1 kWh = 3,412.14 BTU. US heating and cooling equipment is sized in BTU per hour.
Related Guide
US Customary vs Metric: A Complete Guide to Both Systems
Understand the US customary and metric systems, why the US uses both, and the exact conversion factors that link inches, pounds, and Fahrenheit to their metric equivalents.
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Kilowatts to Horsepower
Convert kilowatts to horsepower with the exact factor (1 kW = 1.34102 hp), a quick reference table, and common engine and motor ratings.
kWh to BTU
Convert kilowatt-hours to BTU with the exact factor (1 kWh = 3412.14 BTU), a quick reference table, and common energy-use examples.
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Disclaimer
Unit conversions on this site use internationally recognized, fixed conversion factors (for example, 1 inch = 2.54 cm, 1 pound = 0.45359237 kg, 1 °F = (°C × 9/5) + 32). They are provided for general, informational, and educational use. For applications where accuracy is critical — such as medical dosing, engineering, scientific research, trade, or legal compliance — always verify results against authoritative sources such as the U.S. National Institute of Standards and Technology (NIST) and the official International System of Units (SI) brochure, and follow any applicable local standards.
Exact Constants & Authoritative Sources
Every conversion on this page uses the fixed, internationally agreed constants below — drawn from NIST and the SI Brochure. No values are estimated or rounded as if exact. See the full list on our Sources & Standards page.
- EXACT1 mechanical horsepower (hp) = 745.6998715823 watts (exact, 550 ft·lbf/s)
- EXACT1 British thermal unit, IT (Btu) = 1055.05585262 joules (exact)
- EXACT1 Btu/h = 0.293071070172 watts (exact, Btu ÷ 3600 s)
- EXACT1 watt (W) = 1 joule per second (definition)
Common Pitfalls & How to Avoid Them
Mechanical hp vs. metric hp
Mechanical hp = 745.7 W. Metric (PS) hp = 735.5 W and electrical hp = 746 W. Engine specs may use any of these; always check which.
Btu IT vs. thermochemical
UnitFig uses the IT Btu (1055.05585262 J). The thermochemical Btu (1054.35 J) is ~0.07% smaller and used in some chemistry contexts — keep them separate.
A unit-conversion reference site has no manufacturer-specific or device calibration parameters, so "layered tuning" does not apply here. The only real error-handling is input validation: the converter rejects non-numeric, empty, or out-of-range entries and shows a clear message, then falls back to exact constants above. No estimated or rounded figures are ever presented as exact.
Authoritative sources for this page
- NIST (U.S. National Institute of Standards and Technology) — NIST SP 330 — The International System of Units (SI), 2019 edition
- BIPM (International Bureau of Weights and Measures) — SI Brochure — The International System of Units (9th edition)
Last Updated 2026 | This article was reviewed for accuracy