Celsius to Fahrenheit

The formulas

°F = (°C × 9/5) + 32
°C = (°F − 32) × 5/9

The 9/5 ratio exists because a Celsius degree is 1.8 times larger than a Fahrenheit degree - there are 100 Celsius degrees between the freezing and boiling points of water, and 180 Fahrenheit degrees across the same range. The +32 shifts for the different zero points.

Converting to Kelvin is simpler, because Kelvin shares the size of the Celsius degree:

K = °C + 273.15

Reference chart

°C	°F	What it is
−40	−40	The scales cross
−18	0	Typical home freezer
0	32	Water freezes
10	50	Cool spring day
16	61	Cool room
20	68	Comfortable room
25	77	Warm day
30	86	Hot day
37	98.6	Human body temperature
38	100.4	Fever threshold
40	104	Dangerously hot day
100	212	Water boils at sea level
180	356	Moderate oven
220	428	Hot oven

Body temperature

Normal human body temperature is conventionally 37°C or 98.6°F. That precision is misleading: the figure comes from Carl Wunderlich’s 1868 study, and modern research puts the average closer to 36.6°C (97.9°F), with substantial variation by individual, time of day and measurement site.

Fever is generally defined as 38°C (100.4°F) or above measured orally. Because the Fahrenheit degree is smaller, it happens to give finer resolution without decimals - a genuine if minor reason it persists in American clinical settings.

Cooking temperatures

Purpose	°C	°F
Low / slow oven	140–150	285–300
Moderate oven	180	356
Hot oven	220	428
Deep frying	175–190	350–375
Poultry safe internal	74	165
Beef medium-rare	57	135

Recipes usually round to convenient numbers rather than converting exactly - a recipe saying 350°F and one saying 180°C are treated as the same oven setting, even though 350°F is 176.7°C.

The three scales

Celsius anchors at the freezing and boiling points of water, 0 and 100. Standard for everyday use in almost every country and for scientific work below the point where Kelvin becomes convenient.

Fahrenheit anchors at a brine freezing point (0) and an approximation of body temperature (96 in the original scheme). The reference points look arbitrary now because they were chosen for reproducibility with 1720s equipment.

Kelvin starts at absolute zero, the point where thermal motion is at its minimum. It uses the same degree size as Celsius, so conversions are a simple offset. It is the SI base unit for temperature and is written without a degree symbol - 300 K, not 300°K.