Temperature Converter Free Online
Convert temperature between Celsius (°C), Fahrenheit (°F), Kelvin (K), and Rankine (°R) instantly. Includes quick presets for freezing, boiling, body, and absolute-zero temperatures.
All Scales
Pro — bulk batch conversion, API access, history & favorites
API access · Priority queue · Team workspace
How It Works
Enter Temperature
Type any temperature value into the From input. Decimals and negative values are supported (Celsius and Fahrenheit go below zero, Kelvin and Rankine cannot — values below absolute zero will still display the math but are physically meaningless). The conversion runs as you type so you can hold a key or paste a value.
Pick Source & Target Scale
Choose your source scale (Celsius, Fahrenheit, Kelvin, or Rankine) and the target scale. The All Scales panel below shows the same temperature converted into every scale at once, so you can pick the scale you need without switching dropdowns.
Use Quick Presets
The four preset buttons (Freezing 0°C, Boiling 100°C, Body 37°C, Absolute Zero −273.15°C) load common reference temperatures so you can instantly see them in any scale. Useful for cooking, science class, or sanity-checking weather forecasts when traveling between countries.
Temperature Converter Features
Celsius (°C)
The international standard scale used in science and in every country except the United States, Bahamas, Cayman Islands, Liberia, and Palau. Defined so water freezes at 0°C and boils at 100°C at standard atmospheric pressure. Used universally in physics, chemistry, weather, cooking, and medicine.
Fahrenheit (°F)
The legacy scale used in everyday US weather forecasts, cooking, and HVAC. Water freezes at 32°F and boils at 212°F. The 180-degree gap between freezing and boiling means each Fahrenheit degree is smaller (more granular) than a Celsius degree, which some people prefer for ambient air temperature.
Kelvin (K)
The SI absolute thermodynamic scale used in physics, astronomy, and engineering. 0 K is absolute zero (no thermal motion), and each Kelvin is the same size as a Celsius degree. Room temperature is about 293 K. There is no degree symbol — Kelvin temperatures are written as 295 K, not 295°K.
Rankine (°R)
An absolute scale based on Fahrenheit, used in some US engineering disciplines (especially thermodynamics and aerospace). 0°R is absolute zero, and each Rankine degree is the same size as a Fahrenheit degree. Room temperature is about 530°R. Conversion: °R = (°F + 459.67).
Live & Bidirectional
The result updates instantly as you type. Use the swap button to flip the From and To scales — useful when you want to verify a conversion round-trips cleanly, or when you realize you have entered the source and target the wrong way around.
Exact Conversion Math
Conversion uses the exact formulas defined by the BIPM and ISO standards. C → K = C + 273.15, F → C = (F − 32) × 5/9, R → K = R × 5/9. JavaScript double-precision floats give 15+ significant digits — far more than is meaningful for any real-world temperature measurement.
Free vs Pro
| Feature | Free | Pro |
|---|---|---|
| All 4 temperature scales | ||
| Live conversion | ||
| Quick presets | ||
| Bulk CSV / Excel conversion | — | |
| REST API access | — | |
| Conversion history & favorites | — |
Frequently Asked Questions
Absolute zero is the lowest temperature physically possible — the point at which all molecular thermal motion stops. It is exactly 0 K, equivalent to −273.15°C, −459.67°F, or 0°R. The third law of thermodynamics says it cannot actually be reached by any finite process, but laboratory experiments have come within a billionth of a degree.
Daniel Fahrenheit chose his zero point as the freezing temperature of a brine of water, ice, and ammonium chloride (the coldest he could reliably reproduce in 1724) and 96°F as human body temperature. Later, the scale was redefined so water freezes at exactly 32°F and boils at 212°F (a 180-degree gap), which is what gives the conversion factor of 5/9 to and from Celsius.
Use Kelvin for any calculation that involves a temperature ratio or absolute thermodynamic property — gas laws (PV = nRT), Stefan–Boltzmann radiation, color temperature of light, or anything where a "0" needs to mean genuinely zero thermal energy. Use Celsius for everyday measurements and for differences in temperature, where the choice of zero point does not matter.
Yes. The math is exact (using the defined SI conversion factors) and JavaScript carries 15 significant digits. The limitation is your input thermometer accuracy, not this converter. For high-precision metrology (better than 0.001°C), refer to ITS-90 fixed points instead of just numerical conversion.
Color temperature is measured in Kelvin and describes the color of light a black-body radiator would emit at that temperature. Warm white bulbs are ~2700 K (yellowish), neutral white is ~4000 K, daylight is ~5500 K, and overcast sky is ~6500 K. Convert to Celsius if you need to compare to physical filament temperatures (an incandescent filament glows at ~2700°C, much hotter than 2700 K perceived color).