Enter either the BTUs or the temperature to determine the unknown variable. This calculator uses a simplified relation assuming 1 pound of water as the medium.
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Why BTU and Fahrenheit Are Not Directly Convertible
BTU (British Thermal Unit) is a unit of energy. Fahrenheit is a unit of temperature. These measure fundamentally different physical quantities, so there is no direct conversion factor between them. However, they are linked through the definition of the BTU itself: one BTU is the amount of heat energy required to raise the temperature of exactly one pound of liquid water by one degree Fahrenheit at its maximum density (approximately 39 °F). This calculator uses that foundational relationship, assuming 1 lb of water as the working medium.
The Thermodynamic Formula
The general heat transfer equation that relates energy to temperature change is:
Q = m \times c_p \times \Delta T
Where Q is heat energy in BTU, m is mass in pounds, c_p is specific heat capacity in BTU/(lb·°F), and ΔT is the temperature change in °F. For 1 pound of water (c_p = 1.0 BTU/lb·°F), this simplifies to Q = ΔT, meaning 1 BTU produces a 1 °F rise. This calculator uses an equivalent form of this relationship.
Specific Heat of Common Materials (BTU/lb·°F)
The BTU required to change temperature depends entirely on what substance is being heated. The table below shows how dramatically specific heat varies across materials. A substance with a lower specific heat requires fewer BTUs per pound to achieve the same temperature change.
| Material | c_p (BTU/lb·°F) | BTU to Raise 10 lbs by 50 °F |
|---|---|---|
| Water (liquid) | 1.000 | 500 |
| Ethanol | 0.580 | 290 |
| Ice (32 °F) | 0.502 | 251 |
| Aluminum | 0.214 | 107 |
| Air (dry, 1 atm) | 0.240 | 120 |
| Glass | 0.197 | 98.5 |
| Concrete | 0.156 | 78 |
| Steel (mild) | 0.120 | 60 |
| Cast Iron | 0.110 | 55 |
| Copper | 0.092 | 46 |
| Brass | 0.089 | 44.5 |
| Lead | 0.031 | 15.5 |
| Values at approximately 68 °F (20 °C). The third column applies Q = m x c_p x ΔT for m = 10 lb, ΔT = 50 °F. | ||
Water’s specific heat of 1.0 BTU/lb·°F is the highest of any common substance. This is why water is so widely used as a heat transfer fluid in HVAC systems, engine cooling, and industrial processes. It absorbs and releases large amounts of energy with relatively small temperature changes.
BTU Values of Common Fuels and Appliances
Understanding BTU in practical terms is essential for sizing heating and cooling equipment. Below are reference BTU values for fuels and typical residential appliance ratings.
| Fuel | BTU per Unit | Unit |
|---|---|---|
| Natural Gas | 1,036 | per cubic foot |
| Propane | 91,500 | per gallon |
| Heating Oil (#2) | 138,500 | per gallon |
| Gasoline | 120,286 | per gallon |
| Kerosene | 135,000 | per gallon |
| Wood (hardwood, air dried) | ~24,000,000 | per cord |
| Coal (bituminous) | ~24,000,000 | per short ton |
| Electricity | 3,412 | per kWh |
| Approximate values. Actual BTU content varies by grade and source. | ||
| Appliance | Typical BTU/hr |
|---|---|
| Window AC (small room) | 5,000 to 8,000 |
| Window AC (large room) | 10,000 to 14,000 |
| Central AC (whole home) | 24,000 to 60,000 |
| Gas Furnace | 60,000 to 120,000 |
| Tankless Water Heater | 150,000 to 200,000 |
| Tank Water Heater (gas) | 30,000 to 50,000 |
| Gas Fireplace | 20,000 to 40,000 |
| Outdoor Gas Grill | 25,000 to 60,000 |
| Ratings are BTU per hour (BTU/hr). 1 ton of cooling = 12,000 BTU/hr. | |
BTU Unit Equivalents
The BTU has several precise definitions depending on the reference temperature of water used in the measurement. The most commonly used variant in engineering is the International Table BTU (BTU_IT), defined as exactly 1,055.05585 joules. The thermochemical BTU is slightly different at approximately 1,054.35 joules. For most practical HVAC and energy calculations, the difference is negligible.
Key equivalencies: 1 BTU = 1,055.06 joules = 252.0 calories = 0.0002931 kWh. In the other direction, 1 kWh = 3,412 BTU and 1 therm = 100,000 BTU. One ton of refrigeration (used in commercial HVAC sizing) equals 12,000 BTU/hr, which is the energy needed to melt one short ton of ice in 24 hours.
HVAC Sizing Rules of Thumb
For residential cooling, the general guideline is 20 BTU per square foot of living space. A 1,500 sq ft home in a temperate climate typically needs a 30,000 BTU (2.5 ton) central air conditioning unit. For heating, requirements are higher and more climate-dependent: mild climates need roughly 25 to 30 BTU per square foot, moderate climates 35 to 40 BTU/sq ft, and cold climates 50 to 60 BTU/sq ft. These figures assume standard 8-foot ceilings, moderate insulation, and average window area. Homes with vaulted ceilings, large window surfaces, or poor insulation may need 20% to 40% more capacity. Oversizing an HVAC unit is as problematic as undersizing because an oversized AC short-cycles, failing to adequately dehumidify the air.
Btu to Fahrenheit Conversion Table
The table below shows the temperature that results from applying various BTU amounts to 1 pound of water starting at 32 °F (the freezing point). Since water has a specific heat of 1.0 BTU/lb·°F, each BTU added raises the temperature by exactly 1 °F.
| Energy Added (BTU) | Final Temperature (°F) | Temperature Rise (°F) |
|---|---|---|
| 1 | 33 | 1 |
| 5 | 37 | 5 |
| 10 | 42 | 10 |
| 25 | 57 | 25 |
| 50 | 82 | 50 |
| 75 | 107 | 75 |
| 100 | 132 | 100 |
| 120 | 152 | 120 |
| 148 | 180 | 148 |
| 180 | 212 | 180 |
| Assumes liquid water, constant c_p of 1.0 BTU/lb·°F. At 212 °F, additional energy goes to phase change (latent heat of vaporization: 970.3 BTU/lb) rather than further temperature increase. | ||
Note that once water reaches 212 °F (100 °C at standard atmospheric pressure), additional BTU input does not raise the temperature further. Instead, 970.3 BTU per pound is required to convert the liquid water to steam at the same temperature. This latent heat of vaporization is a critical factor in boiler and steam system design.
