The forecast in your destination says it will be 10 °C colder tomorrow, so you convert 10 °C, get 50 °F, and start packing a winter coat. That conversion is wrong, and the mistake behind it trips up almost everyone who switches between Celsius and Fahrenheit. Here is the actual math, with real examples you can reuse on your next trip.
A temperature and a temperature change are two different things
When you convert a temperature, you are converting a point on a scale: 20 °C, 30 °C, 86 °F. When you convert a temperature change, you are converting a gap between two points: “10 degrees colder”, “a 5 degree rise”. These two jobs look alike, but they need different formulas.
The reason is simple. Celsius and Fahrenheit do not start at the same zero. Water freezes at 0 °C but at 32 °F, so the Fahrenheit scale is shifted by 32 degrees. That shift matters when you convert a point on the scale, and it disappears completely when you convert a gap, because it cancels out on both ends.
The two formulas you actually need
To convert a temperature (a point on the scale):
- °F = (°C × 9/5) + 32, in plain words: multiply by 1.8, then add 32
- °C = (°F − 32) × 5/9, in plain words: subtract 32, then multiply by 5/9 (about 0.56)
To convert a temperature change (a gap), drop the 32 entirely:
- a change in °F = a change in °C × 9/5, so one Celsius degree of difference equals 1.8 Fahrenheit degrees of difference
A real example: reading a forecast abroad
Say it is 25 °C today and the forecast calls for 15 °C tomorrow, a drop of 10 °C.
Convert each temperature properly: 25 °C × 1.8 + 32 = 77 °F, and 15 °C × 1.8 + 32 = 59 °F. The real drop is 77 − 59 = 18 °F.
Now apply the gap formula directly to the 10 °C drop: 10 × 9/5 = 18 °F. Same answer, one step. If you had used the full formula on the gap instead, you would have added 32 and landed on 50 °F, a number that has nothing to do with how much colder it will feel. A 10 °C drop is an 18 °F drop, not a 50 °F one.
The same logic works in reverse. A 9 °F rise in a US forecast is 9 × 5/9 = 5 °C, not (9 − 32) × 5/9, which would give you a negative number for a temperature that is going up.
Quick conversions for everyday travel
A few reference points make most forecasts easy to read:
- 0 °C = 32 °F, water freezes
- 15 °C = 59 °F, roughly the average surface temperature of the Earth
- 20 °C = 68 °F, a mild spring day (20 × 9/5 = 36, plus 32 = 68)
- 30 °C = 86 °F, a hot summer afternoon
Going the other way, a US forecast of 86 °F gives (86 − 32) × 5/9 = 30 °C.
A common mental shortcut is to double the Celsius value and add 30, which rounds 1.8 up to 2 and 32 down to 30. At 20 °C it gives 70 °F instead of 68 °F, close enough to pick an outfit. At 30 °C it gives 90 °F instead of 86 °F, and the gap keeps growing as temperatures climb, so use the exact formula when the number really matters.
The 98.6 °F myth and fever abroad
If you get sick while traveling and borrow a Celsius thermometer, the usual reference is 37 °C, which converts to exactly 98.6 °F. But that figure is not a precise modern measurement. It is the conversion of a 19th century German standard of 37 °C, and normal body temperature actually varies around 36.8 °C, give or take 0.7 °C.
In practice, that range runs from about 36.1 °C to 37.5 °C, or roughly 97 °F to 99.5 °F. A reading of 36.6 °C is not a sign that something is wrong just because it is below 37.
Mistakes to avoid
- Adding 32 when converting a change in temperature: the offset only applies to temperatures, never to differences between them
- Treating 98.6 °F as an exact universal normal: it is a rounded historical standard
- Trusting the “double and add 30” shortcut on very hot or very cold days, where its error grows
- Assuming water always boils at exactly 100 °C: modern reference values put it at 99.98 °C at sea level, a detail that only matters for precise work
Keep two rules in mind and you will never misread a foreign forecast again: temperatures get the +32, temperature changes do not.
