How to Convert Temperature (Fahrenheit, Celsius & Kelvin)

Temperature is one of those measurements where the entire world can’t agree on a single scale. The US uses Fahrenheit for weather, cooking, and thermostats. Most other countries use Celsius. Scientists worldwide use Kelvin. If you travel, cook from international recipes, or read research papers, you need to convert between these scales regularly.

This guide covers the formulas, key reference points, mental math tricks, and a cooking temperature chart you can bookmark. For instant conversions, use our Free Temperature Converter.

The Three Temperature Scales

Fahrenheit (F)

Daniel Gabriel Fahrenheit developed this scale in 1724. He set 0F as the temperature of a brine solution (salt, water, and ice) and 96F as roughly human body temperature (later corrected to 98.6F). Water freezes at 32F and boils at 212F.

Fahrenheit is the standard in the United States, the Bahamas, the Cayman Islands, Palau, and a few other territories. If you grew up in the US, you instinctively know that 72F is a comfortable room, 32F means ice, and 100F is a scorching day.

Celsius (C)

Anders Celsius proposed this scale in 1742, originally inverted (0 for boiling, 100 for freezing). It was soon flipped to its current form. Water freezes at 0C and boils at 100C at standard atmospheric pressure. The 0-100 range between these two anchor points makes Celsius intuitive for scientific work.

Every country outside the small Fahrenheit group uses Celsius for daily weather, cooking, and medicine.

Kelvin (K)

Lord Kelvin introduced the absolute temperature scale in 1848. It starts at absolute zero (0 K), the theoretical point where all molecular motion stops. There are no negative Kelvin values. Kelvin uses the same increment size as Celsius—a 1-degree change in Celsius equals a 1-degree change in Kelvin.

Kelvin is the SI standard for scientific measurement. You will encounter it in physics, chemistry, astronomy, and engineering. Note that Kelvin doesn’t use a degree symbol: it is written 300 K, not 300 degrees K.

Conversion Formulas

Celsius to Fahrenheit

$F = (C \times \frac{9}{5}) + 32$

Steps:

1. Multiply the Celsius value by 9.
2. Divide by 5.
3. Add 32.

Example: Convert 25C to Fahrenheit.

• 25 x 9 = 225
• 225 / 5 = 45
• 45 + 32 = 77F

Fahrenheit to Celsius

$C = (F - 32) \times \frac{5}{9}$

Steps:

1. Subtract 32 from the Fahrenheit value.
2. Multiply by 5.
3. Divide by 9.

Example: Convert 98.6F to Celsius.

• 98.6 - 32 = 66.6
• 66.6 x 5 = 333
• 333 / 9 = 37C

Celsius to Kelvin

$K = C + 273.15$

Example: Convert 100C (boiling water) to Kelvin.

• 100 + 273.15 = 373.15 K

Kelvin to Celsius

$C = K - 273.15$

Example: Convert 0 K (absolute zero) to Celsius.

• 0 - 273.15 = -273.15C

Fahrenheit to Kelvin

$K = (F - 32) \times \frac{5}{9} + 273.15$

Example: Convert 212F (boiling water) to Kelvin.

• 212 - 32 = 180
• 180 x 5/9 = 100
• 100 + 273.15 = 373.15 K

Key Reference Points

Event / Condition	Fahrenheit	Celsius	Kelvin
Absolute zero	-459.67F	-273.15C	0 K
Liquid nitrogen	-320.4F	-195.8C	77.4 K
Water freezes	32F	0C	273.15 K
Refrigerator	37F	2.8C	275.9 K
Cool room	68F	20C	293.15 K
Human body	98.6F	37C	310.15 K
Hot summer day	100F	37.8C	310.9 K
Water boils	212F	100C	373.15 K
Oven (baking)	350F	177C	450.15 K
Wood fire	~1,100F	~593C	~866 K

The crossover point where Fahrenheit and Celsius read the same number is -40. At -40F = -40C, the two scales intersect.

Mental Math Shortcuts

Celsius to Fahrenheit (rough): Double the Celsius value and add 30. This gives you a quick estimate that is accurate within a few degrees for everyday temperatures.

• 20C → 40 + 30 = 70F (exact: 68F)
• 30C → 60 + 30 = 90F (exact: 86F)
• 37C → 74 + 30 = 104F (exact: 98.6F—less accurate at body temp)

Fahrenheit to Celsius (rough): Subtract 30, then divide by 2.

• 80F → (80 - 30) / 2 = 25C (exact: 26.7C)
• 50F → (50 - 30) / 2 = 10C (exact: 10C—spot on)

These shortcuts work best in the 0-40C / 30-100F range, which covers most daily weather and cooking scenarios.

Cooking Temperature Conversions

Recipes from different countries use different scales and conventions. UK and Australian recipes often specify “Gas Mark” settings for older ovens.

Description	Fahrenheit	Celsius	Gas Mark
Very low	250F	120C	1/2
Low	300F	150C	2
Moderate	350F	177C	4
Moderately hot	375F	190C	5
Hot	400F	204C	6
Very hot	450F	232C	8
Broil / Grill	500F	260C	9

Common cooking benchmarks:

• Bread baking: 350-450F (177-232C) depending on the type
• Roasting chicken: 425F (218C) for crispy skin
• Slow roast: 275-300F (135-150C) for tough cuts of meat
• Pizza (home oven): 475-500F (246-260C), as hot as your oven goes
• Candy making (hard crack): 300F (149C)

If a European recipe says “180C,” set your American oven to 356F—or just round to 350F, which is close enough for most baking.

Where Each Scale Is Used Worldwide

Fahrenheit countries: The United States and its territories use Fahrenheit for weather forecasts, thermostats, cooking, and medical thermometers. A handful of Caribbean nations also use it.

Celsius everywhere else: Canada, Mexico, the UK, Europe, Asia, Africa, South America, and Australia all report weather in Celsius. The UK is a hybrid case—older generations sometimes still reference Fahrenheit for hot weather (“It’ll be 80 degrees this weekend”), but official forecasts are always Celsius.

Kelvin in science: Physics equations, chemistry lab work, color temperature for lighting and photography (e.g., 5600 K daylight), and astrophysics all require Kelvin. You will also see it in HVAC engineering and thermodynamics.

Worked Examples

Example 1: You are visiting Paris and the weather app says 15C. How warm is that in Fahrenheit?

• 15 x 9/5 = 27
• 27 + 32 = 59F (bring a jacket)

Example 2: A fever is considered 100.4F in the US. What is that in Celsius?

• 100.4 - 32 = 68.4
• 68.4 x 5/9 = 38C

Example 3: A chemistry experiment requires a reaction at 500 K. What oven temperature is that in Fahrenheit?

• 500 - 273.15 = 226.85C
• 226.85 x 9/5 + 32 = 440.3F

Skip the Math

Our Temperature Converter handles Fahrenheit, Celsius, and Kelvin conversions instantly. Type in any value, select your scales, and get your answer. No formulas needed.

Frequently Asked Questions

Why does the US use Fahrenheit instead of Celsius?

The US adopted Fahrenheit in its colonial era because the scale was widely used in the British Empire at the time. When most of the world switched to metric/Celsius during the 19th and 20th centuries, the US never mandated the change. Attempts were made in the 1970s (the Metric Conversion Act of 1975), but the transition was voluntary, and Fahrenheit remained embedded in American daily life, building codes, medical practice, and consumer products.

Is there a temperature where Fahrenheit and Celsius are the same?

Yes. At -40 degrees, both scales read the same value: -40F = -40C. You can prove this by plugging -40 into the conversion formula: (-40 x 9/5) + 32 = -72 + 32 = -40. It’s the only intersection point.

What is absolute zero and can we reach it?

Absolute zero is 0 K (-273.15C / -459.67F). It represents the theoretical point where atoms have minimal possible energy and all thermal motion ceases. Scientists have cooled materials to within fractions of a nanodegree above absolute zero, but reaching exactly 0 K is physically impossible under the third law of thermodynamics. The coldest temperature ever recorded in a lab was about 38 picokelvin (38 trillionths of a degree above absolute zero) at the University of Bremen in 2021.

What is a “normal” body temperature in Celsius?

The classic figure is 37C (98.6F), established by Carl Wunderlich in 1851 after measuring thousands of patients. More recent studies suggest the average has drifted down to about 36.6C (97.9F), possibly due to reduced inflammation levels in modern populations. A reading of 38C (100.4F) or above is generally considered a fever in both systems.

How do I convert Gas Mark to Fahrenheit or Celsius?

Gas Mark is a temperature scale used on older British and some Australian ovens. The formula is: Fahrenheit = (Gas Mark x 25) + 300, or Celsius = (Gas Mark x 14) + 121. Gas Mark 4 = 350F = 177C (moderate oven). Gas Mark 6 = 400F = 204C (hot oven). Most modern recipes include both Celsius and Fahrenheit, so Gas Mark conversions are mainly needed for vintage cookbooks.