About This Tool
Temperature conversion between Fahrenheit, Celsius, and Kelvin is one of the most common calculations in science, travel, cooking, and daily weather checks. The United States uses Fahrenheit for weather and cooking, while most of the world uses Celsius. Scientists and engineers rely on the Kelvin scale, which starts at absolute zero. This converter shows all three temperature scales simultaneously, so you never need to run the calculation twice. Enter a temperature in any scale and see the equivalent in the other two instantly. A built-in reference table shows key temperature benchmarks like the freezing and boiling points of water, normal body temperature, and absolute zero. No registration, no data collection, and results update as you type.
How Temperature Scales Work
The three main temperature scales each have different zero points and degree sizes:
- Fahrenheit (F): Developed by Daniel Gabriel Fahrenheit in 1724. Water freezes at 32 F and boils at 212 F, giving 180 degrees between the two points. Used primarily in the United States, the Bahamas, and a few other territories.
- Celsius (C): Created by Anders Celsius in 1742. Water freezes at 0 C and boils at 100 C, making it a base-10 system that pairs naturally with the metric system. Used by the vast majority of countries worldwide.
- Kelvin (K): Proposed by Lord Kelvin in 1848. Zero Kelvin (0 K) is absolute zero, the theoretical coldest possible temperature where molecular motion stops. Each Kelvin degree is the same size as a Celsius degree, so 0 C = 273.15 K. Used extensively in physics, chemistry, and engineering.
Note that Kelvin does not use the word "degrees." You say "300 Kelvin," not "300 degrees Kelvin." This is because Kelvin is an absolute scale, not a relative one.
Conversion Formulas Explained
Temperature conversions are not simple multiplication because the scales have different zero points. Here are the exact formulas:
Fahrenheit to Celsius: C = (F - 32) x 5/9. Subtract 32 to account for the offset, then multiply by 5/9 to scale the degree size. Example: 72 F = (72 - 32) x 5/9 = 40 x 0.5556 = 22.22 C.
Celsius to Fahrenheit: F = C x 9/5 + 32. Multiply by 9/5 to expand the degree size, then add 32 for the offset. Example: 25 C = 25 x 1.8 + 32 = 45 + 32 = 77 F.
Celsius to Kelvin: K = C + 273.15. Since the degree sizes are identical, you only need to add the offset. Example: 100 C = 100 + 273.15 = 373.15 K.
Kelvin to Celsius: C = K - 273.15. Reverse the addition. Example: 300 K = 300 - 273.15 = 26.85 C.
Fahrenheit to Kelvin: First convert F to C, then C to K. Example: 212 F = 100 C = 373.15 K.
When Each Scale Is Used
Knowing which scale to use depends on context:
- Daily weather: Fahrenheit in the US, Celsius everywhere else. A comfortable day is about 72 F / 22 C.
- Cooking: American recipes use Fahrenheit (350 F for baking), European recipes use Celsius (180 C). Professional kitchens often know both.
- Medicine: Normal body temperature is 98.6 F or 37 C. A fever is typically above 100.4 F / 38 C.
- Science: Kelvin for thermodynamics, gas laws, and any calculation involving absolute temperature. Celsius for chemistry lab work and biological research.
- Engineering: HVAC systems in the US use Fahrenheit. International engineering standards typically use Celsius or Kelvin depending on the application.
Fun fact: Fahrenheit and Celsius intersect at exactly -40 degrees. At that point, -40 F = -40 C.
Tips for Quick Mental Conversions
You do not always have a calculator handy. These shortcuts give reasonably close estimates:
F to C shortcut: Subtract 30, then divide by 2. Example: 86 F becomes (86 - 30) / 2 = 28 C. The actual answer is 30 C, so the error is small for everyday temperatures.
C to F shortcut: Double the Celsius value, then add 30. Example: 25 C becomes 25 x 2 + 30 = 80 F. Actual: 77 F. Close enough for deciding what to wear.
Memorize anchor points: Know that 0 C = 32 F, 100 C = 212 F, and 37 C = 98.6 F. From these anchors, you can interpolate. If 20 C = 68 F and 30 C = 86 F, then 25 C is roughly midway at 77 F.
The 1.8 multiplier: For exact conversion, remember that each Celsius degree equals 1.8 Fahrenheit degrees. Multiply C by 1.8 and add 32. This is the precise formula in its simplest form.
Frequently Asked Questions
Why does the US still use Fahrenheit?
The US adopted Fahrenheit before the metric system became widespread. While Congress passed the Metric Conversion Act in 1975, it was voluntary. Cultural inertia, the cost of changing infrastructure (road signs, thermostats, ovens), and familiarity have kept Fahrenheit in daily use. Scientific and military applications in the US do use Celsius and Kelvin.
What is absolute zero and can we reach it?
Absolute zero is 0 Kelvin (-273.15 C / -459.67 F). At this temperature, atoms have minimum possible thermal energy and stop moving in classical physics. In practice, scientists have cooled materials to within a billionth of a degree of absolute zero, but reaching exactly 0 K is considered physically impossible due to the third law of thermodynamics.
At what temperature are Fahrenheit and Celsius equal?
Fahrenheit and Celsius are equal at exactly -40 degrees. You can verify this with the formula: C = (F - 32) x 5/9. Plugging in -40: (-40 - 32) x 5/9 = -72 x 5/9 = -40. This is the only point where both scales read the same number.
How do I convert oven temperatures between F and C?
Common oven conversions to memorize: 325 F = 163 C (slow bake), 350 F = 177 C (standard bake), 375 F = 190 C (moderate), 400 F = 204 C (hot), 425 F = 218 C (very hot), 450 F = 232 C (pizza/bread). Most ovens have a margin of error of 10-25 degrees anyway, so rounding to the nearest 5 C is perfectly fine for cooking.
What is the Rankine temperature scale?
Rankine is to Fahrenheit what Kelvin is to Celsius. It uses the Fahrenheit degree size but starts at absolute zero. So 0 R = 0 K = -459.67 F. To convert Fahrenheit to Rankine, add 459.67. Rankine is occasionally used in some US engineering fields, particularly in thermodynamics involving the Fahrenheit scale, but Kelvin is far more common globally.