Celsius to Fahrenheit: The Formula, the Chart, and the Mental Math Trick
Reviewed by Jerry Croteau, Founder & Editor
Table of Contents
The United States, Liberia, and the Cayman Islands are the only places in the world that primarily use Fahrenheit. For the rest of the world — and for anyone who travels, follows international weather, reads scientific content, or cooks from European recipes — Celsius to Fahrenheit conversion is a regular necessity.
Our converts instantly in either direction. This guide covers the exact formula and the shortcuts that make mental conversion fast.
The exact conversion formulas
Celsius to Fahrenheit: F = (C x 9/5) + 32
Fahrenheit to Celsius: C = (F - 32) x 5/9
Worked examples: Celsius to Fahrenheit
| Situation | Celsius | Calculation | Fahrenheit |
|---|---|---|---|
| Water boils | 100°C | (100 x 1.8) + 32 | 212°F |
| Body temperature | 37°C | (37 x 1.8) + 32 | 98.6°F |
| Comfortable room | 22°C | (22 x 1.8) + 32 | 71.6°F |
| Hot summer day | 35°C | (35 x 1.8) + 32 | 95°F |
| Cold winter day | -10°C | (-10 x 1.8) + 32 | 14°F |
| Water freezes | 0°C | (0 x 1.8) + 32 | 32°F |
Worked examples: Fahrenheit to Celsius
| Situation | Fahrenheit | Calculation | Celsius |
|---|---|---|---|
| Oven (medium) | 350°F | (350 - 32) x 5/9 | 177°C |
| Fever threshold | 100.4°F | (100.4 - 32) x 5/9 | 38°C |
| Nice spring day | 68°F | (68 - 32) x 5/9 | 20°C |
| Freezer temp | 0°F | (0 - 32) x 5/9 | -17.8°C |
The mental math shortcuts
Quick Celsius to Fahrenheit
For everyday weather temperatures, this approximation is accurate within 1-2 degrees:
- Double the Celsius temperature
- Add 30
Example: 20°C → 20 x 2 = 40, + 30 = 70°F (exact: 68°F)
Example: 30°C → 30 x 2 = 60, + 30 = 90°F (exact: 86°F)
Quick Fahrenheit to Celsius
- Subtract 30
- Divide by 2
Example: 80°F → 80 - 30 = 50, / 2 = 25°C (exact: 26.7°C)
This approximation works well between -10°C and 45°C (14°F to 113°F) — the range covering most real-world weather. For medical temperatures, oven temperatures, or scientific work, use the exact formula.
Temperature reference chart: key values
| °C | °F | Context |
|---|---|---|
| -40 | -40 | The scales meet — same value in both systems |
| -20 | -4 | Extreme cold, northern winters |
| -10 | 14 | Cold winter day |
| 0 | 32 | Water freezes / ice melts |
| 10 | 50 | Cool autumn day |
| 16 | 61 | Mild spring weather |
| 20 | 68 | Comfortable room temperature |
| 25 | 77 | Warm but pleasant |
| 30 | 86 | Hot summer day |
| 37 | 98.6 | Normal human body temperature |
| 40 | 104 | Dangerous heat, high fever |
| 100 | 212 | Water boils at sea level |
Why -40 is the same in both scales
The two scales cross at -40 degrees, the one temperature with the same value in both Celsius and Fahrenheit. This can be verified: F = (-40 x 9/5) + 32 = -72 + 32 = -40°F. The scales diverge in opposite directions from that point: each 1°C increase equals a 1.8°F increase, so Fahrenheit numbers grow faster above -40 and fall faster below it.
Kelvin: the third scale
Kelvin is the SI unit of temperature, used in science. Zero Kelvin (absolute zero) is the lowest possible temperature — the point where molecular motion stops. The scale uses the same degree size as Celsius:
K = °C + 273.15
0°C = 273.15 K. 100°C = 373.15 K. -273.15°C = 0 K (absolute zero).
For everyday conversions in either direction, use the .
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