Celsius, Fahrenheit, and Kelvin: how to convert

Celsius, Fahrenheit, and Kelvin describe the same physical temperature with different zeros and steps. Convert with formulas (not a single length-style factor). In the unit converter temperature tab, 20 °C is 68 °F, 0 °C is 273.15 K, and 32 °F is also 273.15 K. Those three results come from the same functions the interactive tool uses.

Formulas you can trust

Write the pair you need, then plug in the number.

Celsius to Fahrenheit

°F = (°C × 9/5) + 32

Fahrenheit to Celsius

°C = (°F − 32) × 5/9

Celsius to Kelvin

K = °C + 273.15

Kelvin to Celsius

°C = K − 273.15

Fahrenheit to Kelvin (via Celsius or in one chain)

K = (°F − 32) × 5/9 + 273.15

The site library converts every temperature through kelvin, then out to the target unit. That keeps round trips honest and blocks values below absolute zero.

When to use each scale

Celsius (°C) is the everyday scale in most countries for weather, cooking, and school science. Water’s familiar teaching points are 0 °C (freezing) and 100 °C (boiling) at standard atmospheric pressure in common classroom tables.

Fahrenheit (°F) remains common for U.S. weather reports and many home thermostats. Mild indoor air near 68 °F is the same as 20 °C.

Kelvin (K) is the SI thermodynamic temperature unit, used when absolute temperature appears in equations and calibrations. Absolute zero is 0 K, equal to −273.15 °C. In modern SI style write 293.15 K, not “293.15 degrees kelvin.”

For length and mass swaps, see unit conversion basics. This page stays on temperature.

Worked ladder: one day of numbers

20 °C to Fahrenheit

(20 × 9/5) + 32 = 36 + 32 = 68. The converter shows 68 with no leftover fraction.

68 °F back to Celsius

(68 − 32) × 5/9 = 36 × 5/9 = 20. Round trip closed.

0 °C and 32 °F to Kelvin

0 + 273.15 = 273.15 K. From Fahrenheit: (32 − 32) × 5/9 + 273.15 = 273.15 K. Same ice point, two starting labels.

100 °C to Kelvin and Fahrenheit

100 + 273.15 = 373.15 K. Fahrenheit: (100 × 9/5) + 32 = 212. Classroom boiling point pairs stay aligned.

Body-temperature style check: 37 °C

(37 × 9/5) + 32 = 98.6. The reverse from 98.6 °F returns 37 °C. Use this as arithmetic practice for labels, not as medical advice.

The −40 coincidence

−40 °C converts to −40 °F, and the reverse matches. Memorize it as a landmark: below that point, Fahrenheit readings are colder-looking numbers than Celsius for the same weather story, and above it the opposite pattern appears in everyday talk.

Absolute zero edge

0 K is −273.15 °C and about −459.67 °F. The converter rejects any input that would land below 0 K, because that is not a physical temperature on these scales.

How to run the conversions in the tool

1. Open the unit converter and select Temperature.
2. Choose from and to among C, F, and K.
3. Type the value; the result updates live.
4. Read the formula line to confirm the path.
5. Use presets such as Celsius to Fahrenheit (20) or Fahrenheit to Celsius (68) when you want a known check.

Swap reverses the units. Copy places the displayed result on the clipboard. Reset clears the fields. Precision only changes display rounding; the underlying math stays full precision.

Common mistakes

Forgetting the +32 or −32. Scaling by 9/5 alone is not enough for Celsius and Fahrenheit.

Treating Kelvin like a relative degree offset from Fahrenheit. You cannot add 273.15 to a Fahrenheit reading and call it kelvin. Convert to Celsius first, or use the full Fahrenheit-to-kelvin chain.

Writing “degrees Kelvin.” Modern SI uses kelvin without the degree word. Celsius and Fahrenheit still use the degree symbol in ordinary writing.

Assuming weather apps and lab notes share a scale. A U.S. forecast in Fahrenheit next to a European packing list in Celsius needs an explicit conversion before you compare “how hot” stories.

Rounding mid-chain. Convert fully, then round once for the label or message you send.

Plain-language map

Celsius is the international daily scale, Fahrenheit is still common in the United States, and Kelvin is the absolute science scale. 20 / 68 / 293.15 is one mild indoor reading in Celsius, Fahrenheit, and Kelvin. 0 / 32 / 273.15 is one ice-point reading. Keep those anchors and most other conversions are plug-and-chug.

Try it

Set C to F with 20 and confirm 68. Set C to K with 0 and confirm 273.15. Set F to C with 98.6 and confirm 37. Then try −40 either way between Celsius and Fahrenheit.