What is the formula for Celsius to Fahrenheit?

°F = °C × 9/5 + 32. The quick mental approximation is to double the Celsius value and add 30 (accurate to within a few degrees for everyday temperatures).

Why does Kelvin not use the degree symbol?

By SI convention, the kelvin is a base unit (like the metre or second) and is written without the degree sign. The temperature 300 K is "three hundred kelvin" not "three hundred degrees kelvin."

When would I use Rankine?

Rankine is used in some US engineering thermodynamics calculations, particularly in older aerospace and steam engineering literature, where absolute temperatures are required but engineers were accustomed to Fahrenheit.

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How to Convert Temperature Units Free — Celsius, Fahrenheit, Kelvin (2026)

By Rui Barreira · Last updated: 18 June 2026

Temperature is one of the most commonly converted quantities — recipes, weather reports, science, and engineering all use different scales. This converter handles all four common temperature scales at once: Celsius, Fahrenheit, Kelvin, and Rankine.

How to Use

Enter a temperature value.
Select the input unit (°C, °F, K, or °R).
All four scales update instantly in the result table.
Click Show formulas to reveal the exact conversion equations for the selected input unit.

How It Works

All conversions route through Celsius as an intermediate. The input is first converted to °C using the appropriate inverse formula, then Celsius is mapped to each of the three remaining scales. Kelvin and Rankine use an absolute zero offset; Fahrenheit uses a scale factor and offset.

The Four Temperature Scales

Celsius (°C) sets 0 at the freezing point of water and 100 at boiling (at 1 atm). It is the standard in science and most of the world. Fahrenheit (°F) is used in the United States for everyday weather and cooking; water freezes at 32°F and boils at 212°F. Kelvin (K) is the SI base unit of temperature, starting at absolute zero (−273.15°C). It has no degree symbol. Rankine (°R) is an absolute scale like Kelvin but uses Fahrenheit-sized degrees; absolute zero is 0°R = −459.67°F.

Key Reference Points

Absolute zero: 0 K = −273.15°C = −459.67°F = 0°R. Water freezes: 0°C = 32°F = 273.15 K = 491.67°R. Body temperature: 37°C = 98.6°F = 310.15 K. Water boils: 100°C = 212°F = 373.15 K.

Frequently Asked Questions

What is the formula for Celsius to Fahrenheit?: °F = °C × 9/5 + 32. The quick mental approximation is to double the Celsius value and add 30 (accurate to within a few degrees for everyday temperatures).
Why does Kelvin not use the degree symbol?: By SI convention, the kelvin is a base unit (like the metre or second) and is written without the degree sign. The temperature 300 K is “three hundred kelvin” not “three hundred degrees kelvin.”
When would I use Rankine?: Rankine is used in some US engineering thermodynamics calculations, particularly in older aerospace and steam engineering literature, where absolute temperatures are required but engineers were accustomed to Fahrenheit.
Is this free?: Yes, entirely free with no signup required.

Frequently Asked Questions

What is the formula for Celsius to Fahrenheit?: °F = °C × 9/5 + 32. The quick mental approximation is to double the Celsius value and add 30 (accurate to within a few degrees for everyday temperatures).
Why does Kelvin not use the degree symbol?: By SI convention, the kelvin is a base unit (like the metre or second) and is written without the degree sign. The temperature 300 K is "three hundred kelvin" not "three hundred degrees kelvin."
When would I use Rankine?: Rankine is used in some US engineering thermodynamics calculations, particularly in older aerospace and steam engineering literature, where absolute temperatures are required but engineers were accustomed to Fahrenheit.