Question 1

What is the formula for Celsius to Fahrenheit?

Accepted Answer

Multiply by 9/5 and add 32, so °F = °C × 9/5 + 32. Going the other way you subtract first: °C = (°F − 32) × 5/9. The order matters, and it is the step most often done backwards when converting by hand. The converter shows the right formula for whichever pair of scales you are looking at.

Question 2

Why does a 10 °C rise come out as 18 °F and not 50 °F?

Accepted Answer

Because 50 °F is what a thermometer reading of 10 °C converts to, and a rise is not a reading. The +32 in the formula moves the zero point of the scale, and a difference between two temperatures has no zero point, so only the 9/5 applies. Switch to "A change in temperature" for forecasts that say "5 degrees warmer", climate targets, oven tolerances and lab specifications written as ±.

Question 3

Why is it 300 K and not 300 °K?

Accepted Answer

The kelvin is an SI base unit, and since 1967 it has been written without a degree sign, the same way you write 300 m rather than 300 °m. Celsius and Fahrenheit keep the sign because they are scales measured from an arbitrary zero. The converter still accepts °K if you type it, and points out the convention.

Question 4

At what temperature are Celsius and Fahrenheit the same?

Accepted Answer

At −40. Setting C equal to C × 9/5 + 32 and solving gives −40, so −40 °C and −40 °F are the same temperature. It is the only point where the two scales meet, and it is listed in the reference table for that reason.

Question 5

What are Rankine and Réaumur used for?

Accepted Answer

Rankine is Fahrenheit-sized degrees counted from absolute zero, the way kelvin is Celsius-sized degrees from absolute zero. American engineers still use it in thermodynamics and gas calculations where the inputs are in Fahrenheit. Réaumur divides the gap between freezing and boiling water into 80 degrees instead of 100. It was common in 18th and 19th century Europe and turns up today mainly in old recipes and historical documents.

Question 6

Why can't I enter a temperature below absolute zero?

Accepted Answer

Absolute zero, 0 K or −273.15 °C or −459.67 °F, is the floor of the thermodynamic scale: the state of lowest possible energy, which nothing can be cooled past. A number like −500 °F is usually a typo or a drop in temperature entered in the wrong mode, so the converter says which and leaves the result empty rather than printing a value that cannot exist.

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