Temperature Converter: Celsius, Fahrenheit, Kelvin, and Rankine
Temperature scales were developed independently in different eras and countries, which is why converting between them requires a formula rather than a simple ratio. Celsius (formerly centigrade) anchors its scale to water's behavior: 0°C for freezing and 100°C for boiling at standard pressure. Fahrenheit was calibrated differently, placing the freezing point at 32°F and boiling at 212°F, giving it 180 degrees of range across the same span. Kelvin starts at absolute zero — the coldest physically possible temperature — and uses the same step size as Celsius, making it essential for scientific calculations. Rankine does the same for Fahrenheit-based engineering.
Kelvin has no degree symbol — it's an absolute scale. 0 K (absolute zero) = −273.15°C = −459.67°F.
The Four Temperature Scales Explained
Celsius is the world's most widely used scale. Water freezes at 0°C and boils at 100°C (at sea level). It's part of the SI metric system and is used for weather, cooking, and medicine worldwide.
Fahrenheit is the primary scale in the United States. Water freezes at 32°F and boils at 212°F. The 180-degree spread between these points gives finer resolution for everyday temperature ranges than Celsius.
Kelvin is the absolute temperature scale used in physics, chemistry, and engineering. There are no negative Kelvin values — 0 K is absolute zero. The step size is identical to Celsius: a change of 1 K equals a change of 1°C.
Rankine is the Fahrenheit-based absolute scale used in US aerospace and thermodynamic engineering. Like Kelvin to Celsius, Rankine is Fahrenheit shifted to start at absolute zero. 0°R = absolute zero; 491.67°R = 32°F.
Why Science Uses Kelvin
The ideal gas law (PV = nRT) and many other physical equations require temperature on an absolute scale. If you plug in a negative Celsius or Fahrenheit temperature, the math breaks down — you'd get negative volumes or pressures. Kelvin eliminates this problem because it starts at the true physical zero of thermal energy. A gas at 200 K has exactly twice the thermal energy of a gas at 100 K — a proportional relationship that doesn't hold with Celsius.
Body Temperature and Medical References
Normal human body temperature is 37°C / 98.6°F. A temperature above 38°C (100.4°F) is generally classified as a fever. Hypothermia is typically defined as core body temperature below 35°C (95°F). In clinical settings globally, Celsius is the standard; the US still primarily uses Fahrenheit in patient-facing contexts.
Temperature in Everyday Life
Cooking: Ovens in the US use Fahrenheit (350°F = 177°C for baking). Most European recipes use Celsius. Weather: A comfortable summer day is around 25°C (77°F). Extreme heat is considered above 40°C (104°F). HVAC: Air conditioning systems in the US set thermostats in Fahrenheit; most global standards use Celsius. Understanding both scales prevents costly unit-conversion errors in international contexts.
Quick Reference
- 0°C = 32°F — water freezing point
- 20°C = 68°F — comfortable room temperature
- 37°C = 98.6°F — normal body temperature
- 100°C = 212°F — water boiling point (sea level)
- −40°C = −40°F — the only temperature equal on both scales
- 0 K = −273.15°C = −459.67°F — absolute zero
Frequently Asked Questions
How do I convert Celsius to Fahrenheit?
Multiply the Celsius value by 9/5 (or 1.8), then add 32. For example, 25°C × 1.8 + 32 = 77°F. The reverse formula is °C = (°F − 32) × 5/9.
What is Kelvin and why is it used in science?
Kelvin is the SI base unit of thermodynamic temperature. Unlike Celsius and Fahrenheit, it starts at absolute zero (the coldest possible temperature), which makes it ideal for physics and chemistry equations like the ideal gas law where negative temperatures would break the math.
What is absolute zero?
Absolute zero is 0 K = −273.15°C = −459.67°F — the theoretical minimum temperature where all molecular motion stops. It has never been achieved in practice, though scientists have come within billionths of a degree.
What is the only temperature that is the same in Celsius and Fahrenheit?
−40°. At −40°C = −40°F, both scales coincide. This is a useful anchor point: if you're ever at −40, no conversion is needed.
What is the difference between Fahrenheit and Celsius?
Celsius is based on water: 0°C freezes, 100°C boils (at sea level). Fahrenheit was calibrated differently: 32°F freezes, 212°F boils. A 1°C change equals a 1.8°F change, so Fahrenheit has finer gradations for everyday temperatures.
What is the Rankine scale?
Rankine (°R) is an absolute temperature scale like Kelvin, but based on Fahrenheit degrees. 0°R = absolute zero, and the scale increments by 1°F per degree. It is mainly used in US aerospace and thermodynamic engineering. 1 K = 1.8°R.
How is temperature measured?
Thermometers measure temperature through thermal expansion (mercury/alcohol), electrical resistance (RTDs), or voltage changes (thermocouples). Digital infrared thermometers measure thermal radiation emitted by objects without contact.