Celsius to Fahrenheit Myths That Lead to Wrong Answers

To convert Celsius to Fahrenheit, multiply by 9/5 (1.8) and add 32: 20 °C = 68 °F. To go back, subtract 32 and multiply by 5/9: 350 °F ≈ 176.7 °C. Kelvin is simply Celsius plus 273.15. Temperature is unusual among units because the scales have different zero points, and that is the source of almost every conversion mistake.

Here are the most common myths and what is actually true.

The exact formulas

Conversion	Formula
°C → °F	°F = °C × 9/5 + 32
°F → °C	°C = (°F − 32) × 5/9
°C → K	K = °C + 273.15
K → °C	°C = K − 273.15
°C → °R (Rankine)	°R = (°C + 273.15) × 9/5

Myth 1: "Just double it and add 30"

The shortcut gets you in the right neighborhood, but it is only exact at 10 °C.

°C	Shortcut	Actual °F	Error
0	30	32	−2
10	50	50	0
20	70	68	+2
30	90	86	+4
100	230	212	+18

Fine for deciding whether to bring a jacket; not fine for cooking, medicine or lab work, where the error grows quickly at higher temperatures.

Myth 2: "A 10-degree rise is the same conversion as 10 degrees"

This trips up a lot of people. 10 °C as a temperature is 50 °F. But a change of 10 °C is a change of 18 °F, because the +32 offset only applies to points on the scale, not to differences between them.

So if a forecast says "5 °C warmer than yesterday", that is 9 °F warmer, not 41 °F. To convert a temperature difference, multiply by 1.8 (or divide by 1.8 going the other way) and skip the 32. A converter that works on temperatures, like this one, will give the wrong answer for differences, so do those by hand.

Myth 3: "Water boils at 100 degrees, so 200 is twice as hot"

Celsius and Fahrenheit zeros are arbitrary points: the freezing point of water and a cold brine mixture, respectively. You can only say something is "twice as hot" on an absolute scale that starts at absolute zero, like Kelvin or Rankine.

20 °C is 293.15 K and 40 °C is 313.15 K, so going from 20 °C to 40 °C increases absolute temperature by under 7%, not 100%.

Myth 4: "Kelvin is measured in degrees"

It isn't. The unit is simply the kelvin (symbol K, no degree sign): "water freezes at 273.15 K". The degree sign was dropped in 1967. Celsius, Fahrenheit and Rankine still use degrees.

Myth 5: "−40 is a trick question"

It isn't a trick, it is the one point where the two everyday scales meet: −40 °C = −40 °F. Plug it in: −40 × 1.8 + 32 = −72 + 32 = −40.

Useful reference points

What	°C	°F	K
Absolute zero	−273.15	−459.67	0
Water freezes	0	32	273.15
Room temperature	20	68	293.15
Body temperature	37	98.6	310.15
Water boils (sea level)	100	212	373.15
Moderate oven	180	356	453.15

Recipes often round oven settings: 350 °F is about 176.7 °C, usually written as 175 or 180 °C, and 200 °C is 392 °F, often written as 400 °F.

How to use the Temperature Converter

The Temperature Converter handles Celsius, Fahrenheit, Kelvin and Rankine:

1. Type the temperature in Value. Negative numbers are fine.
2. Choose the From and To scales, or hit ⇄ to swap them.
3. Read the result, and check the table below it to see the same temperature on all four scales at once.

Results are shown to up to 10 significant digits, so 350 °F appears as 176.6666667 °C. Round to what makes sense for your use.

Where is Rankine used?

Rankine is to Fahrenheit what Kelvin is to Celsius: an absolute scale with Fahrenheit-sized degrees. Water freezes at 491.67 °R. It mostly shows up in older US engineering and thermodynamics work. For energy calculations, such as heating, the Energy Converter handles joules, BTU and kilowatt-hours.

FAQ

Is 98.6 °F really normal body temperature?

98.6 °F is the exact conversion of 37 °C. Normal body temperature varies from person to person and through the day, so treat it as a typical value rather than a precise target.

Why does my thermostat round differently?

Most thermostats display whole or half degrees and convert internally, so switching units may move the reading slightly. 21 °C is 69.8 °F, which a thermostat may show as 70.

Can I convert below absolute zero?

The converter will calculate it, but temperatures below 0 K (−273.15 °C) don't exist physically. Treat any such result as an input error.

Convert any temperature across all four scales with the Temperature Converter.