Convert Fahrenheit to Celsius

Fahrenheit to Celsius Conversion

The formula is:

°C = (°F - 32) × 5/9

Or equivalently: subtract 32, then divide by 1.8.

Key Reference Points

°F	°C	Significance
-40	-40	Where both scales meet
0	-17.78	Very cold winter
32	0	Water freezes
50	10	Cool weather
68	20	Room temperature
77	25	Comfortable weather
86	30	Hot day
98.6	37	Normal body temperature
104	40	Dangerous fever
212	100	Water boils

Body Temperature and Fever

Understanding fever thresholds is one of the most important reasons to convert F to C:

°F	°C	Status
97–99	36.1–37.2	Normal
99–100.3	37.2–37.9	Low-grade fever
100.4+	38+	Fever
103+	39.4+	High fever
105+	40.6+	Seek medical help

Most of the world uses Celsius for medical temperatures, but US hospitals and home thermometers typically display Fahrenheit.

The "Subtract 30, Divide by 2" Trick

For quick estimates: subtract 30, then divide by 2.

• 72°F → (72 - 30) ÷ 2 = 21°C (exact: 22.2°C)
• 90°F → (90 - 30) ÷ 2 = 30°C (exact: 32.2°C)

This is less accurate than the exact formula but useful for quick mental estimates.

Science and Engineering

The Celsius scale is used internationally in science because it aligns with the Kelvin scale (K = °C + 273.15). Fahrenheit is rarely used in scientific publications.