Celsius and Fahrenheit are both still in everyday use worldwide, and while the conversion formula is simple to write down, it's genuinely easy to misremember or misapply under pressure.
Where the two scales came from
Celsius (originally proposed by Anders Celsius in 1742) sets 0° at water's freezing point and 100° at its boiling point, at standard atmospheric pressure — a clean, decimal-friendly design. Fahrenheit, developed earlier by Daniel Gabriel Fahrenheit in the early 1700s, was originally based on a different reference system entirely (involving a brine mixture and, per some historical accounts, human body temperature), which is why its freezing (32°) and boiling (212°) points for water look far less intuitive.
The actual conversion formula
Fahrenheit to Celsius: subtract 32, then multiply by 5/9. Celsius to Fahrenheit: multiply by 9/5, then add 32. The most common mistake is doing the multiplication and the addition/subtraction in the wrong order — the 32 has to be handled correctly relative to the multiplication step in each direction, not just tacked on anywhere in the calculation.
A rough mental-math shortcut
For a quick, approximate Celsius-to-Fahrenheit estimate, doubling the Celsius figure and adding 30 gets you reasonably close for everyday weather-range temperatures — it's not exact, but it's fast enough for a "roughly how hot is that" gut check.
Get the exact figure
A temperature converter applies the precise formula every time, useful when the mental shortcut isn't accurate enough — cooking temperatures and medical contexts, for instance, where precision actually matters.
The bottom line
The formula itself is simple, but the order of operations is exactly where manual conversion most often goes wrong — worth double-checking rather than trusting a half-remembered version.
Try it yourself
Put this into practice with our temperature converter.