Two scales, two different jobs
Celsius is anchored to water: 0° for freezing, 100° for boiling. It is tidy, which is why science and most of the world adopted it. Fahrenheit was anchored to something else entirely — roughly the range of human weather — putting freezing at 32° and boiling at 212°.
That awkward-looking spread has one practical advantage: it gives about 1.8 Fahrenheit degrees for every Celsius degree, so everyday air temperatures divide into finer whole numbers. The difference between 71°F and 73°F is one people notice; in Celsius both round to 22°.
Kelvin sidesteps both conventions by starting at absolute zero, the point where thermal motion stops. It shares Celsius’s degree size, so conversion is just addition — and it never goes negative, which is why physics uses it.
The conversions in practice
Enter a temperature, choose the scale it is in, and the equivalent values in the other two scales appear instantly. Whether you are reading a foreign weather forecast, following a recipe, or checking a science figure, the conversion is immediate and runs entirely in your browser.
The three scales
Celsius is used across most of the world and is anchored to water: 0° for freezing, 100° for boiling. Fahrenheit, common in the United States, places freezing at 32° and boiling at 212°, giving finer whole-number gradations for everyday air temperatures. Kelvin shares Celsius’s degree size but starts at absolute zero, which makes it the scale of choice in science because it never goes negative.
Remembering the formulas
The two you will use most are °F = °C × 9/5 + 32 and its reverse, °C = (°F − 32) × 5/9. Kelvin is the easiest: just add or subtract 273.15 from Celsius. A useful landmark is that the two everyday scales meet at −40°, and body temperature sits near 37°C or 98.6°F. This converter applies all of these exactly, so you never have to trust mental arithmetic.