Temperature Converter | 5-Scale Tool
Instantly convert Celsius, Fahrenheit, Kelvin, Rankine, and Réaumur. Accurate, simple, and perfect for science, cooking, or travel.
Convert Temperature
What is temperature?
At its core, temperature is a measure of the average kinetic energy of the molecules in a substance. When things get hotter, their molecules move faster; when they cool down, molecular motion slows. While this physical reality is the same everywhere in the universe, humans have invented different "languages" (scales) to measure it, depending on where they live or what work they do.
Why do we have so many temperature scales?
- Celsius: Invented in Sweden (1742). It was designed around the properties of water — freezing at 0° and boiling at 100° — making it incredibly intuitive for everyday life.
- Fahrenheit: Proposed in 1724 (by Daniel Gabriel Fahrenheit, who invented the alcohol thermometer in 1709). He used a brine solution to set his 0°F and human body temperature for the upper range. It offers finer granularity for weather reporting.
- Kelvin: The scientific standard (1848). It starts at "absolute zero," the theoretical point where molecular motion stops. It has no negative numbers, which simplifies physics equations.
- Rankine: An engineering favorite (1859). It combines the absolute zero starting point of Kelvin with the smaller degree increments of Fahrenheit.
Celsius vs. Fahrenheit: which should I use?
| Scale | Used in | Best for |
|---|---|---|
| Celsius (°C) | Most of the world, science | Standard global communication, water-based activities |
| Fahrenheit (°F) | USA, Caribbean | Weather forecasting (finer detail), baking in US ovens |
The US sticks to Fahrenheit largely due to historical precedent and the feeling that it offers more precision for air temperature without using decimals.
Understanding Kelvin: the absolute scale
Scientists prefer Kelvin because it is an "absolute" scale. It doesn't start at an arbitrary point like freezing water; it starts at Absolute Zero (0K), the lowest possible temperature in the universe.
Example: The surface of the Sun is approximately 5,778K. Liquid nitrogen boils at a chilly 77K.
Key reference temperatures
| Reference | Celsius | Fahrenheit | Kelvin |
|---|---|---|---|
| Absolute Zero | −273.15°C | −459.67°F | 0K |
| Water freezes | 0°C | 32°F | 273.15K |
| Room temperature | 20°C | 68°F | 293.15K |
| Body temperature | 37°C | 98.6°F | 310.15K |
| Water boils | 100°C | 212°F | 373.15K |
| Parity point | −40°C | −40°F | 233.15K |
Conversion formulas explained
Celsius ↔ Fahrenheit. Multiply by 1.8 (9/5) and add 32. The ratio 9/5 exists because Fahrenheit has 180 degrees between freezing and boiling, while Celsius has only 100.
°F = (°C × 9/5) + 32
°C = (°F − 32) × 5/9
Celsius ↔ Kelvin. The simplest conversion. The "step" size is identical; you just shift the starting point by 273.15 degrees.
K = °C + 273.15
°C = K − 273.15
Why is conversion so hard?
It feels difficult because the scales don't share a zero point (0°C ≠ 0°F) and they grow at different rates (1°C change is nearly double a 1°F change).
Mental math trick (F to C): Need a rough estimate? Subtract 30 and divide by 2. Example: 86°F. 86 − 30 = 56. 56 / 2 = 28°C. (Actual is 30°C — close enough for deciding what to wear.)
Frequently asked questions
What is absolute zero?
Absolute zero (−273.15°C) is the theoretical limit where all molecular motion ceases. You cannot get colder than this.
Why are −40°C and −40°F the same?
This is the "parity point" where the different starting points and growth rates of the two scales intersect mathematically.
Is a fever of 39°C dangerous?
39°C equals 102.2°F. This is a significant fever indicating the body is fighting infection. Consult a medical professional if it persists.
What oven setting is 180°C?
180°C converts to 356°F. Most US recipes round this to 350°F.