Temperature Converter

Convert temperatures between all major scales

Temperature is measured on several different scales, and converting between them is a frequent need: weather forecasts use Celsius in most of the world and Fahrenheit in the United States; scientific work uses Kelvin; engineering and aviation sometimes use Rankine. Enter a temperature in any scale and see the equivalent in all four at once.

The four temperature scales

Celsius (°C): The most widely used scale globally. Defined by the freezing point of water (0°C) and the boiling point of water (100°C) at standard atmospheric pressure. The SI standard for everyday temperature measurement.

Fahrenheit (°F): Used primarily in the United States. Water freezes at 32°F and boils at 212°F. The scale was defined by Daniel Gabriel Fahrenheit in 1724. The 32-degree offset and 180-degree span between freezing and boiling seem arbitrary because the original calibration points were 0°F (a brine/salt/water/ice mixture) and 96°F (human body temperature, approximately).

Kelvin (K): The SI base unit for thermodynamic temperature. Kelvin starts at absolute zero (0 K = −273.15°C) and uses the same degree size as Celsius. It has no negative values. Scientists use Kelvin in physics, chemistry, and astronomy because it directly represents thermodynamic temperature without offsets.

Rankine (°R): Used in some engineering contexts in the US. Like Kelvin, it starts at absolute zero, but uses Fahrenheit-sized degrees. 0°R = 0 K = −459.67°F. It is rarely encountered outside US engineering thermodynamics.

Key reference temperatures

| Event | Celsius | Fahrenheit | Kelvin |

|---|---|---|---|

| Absolute zero | −273.15 | −459.67 | 0 |

| Water freezes | 0 | 32 | 273.15 |

| Human body | 37 | 98.6 | 310.15 |

| Water boils | 100 | 212 | 373.15 |

| Sun's surface | ~5,500 | ~9,932 | ~5,773 |

Conversion formulas

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

The −40 curiosity

At −40°, both Celsius and Fahrenheit give the same numerical reading. This is because the scales have different zero points but their values cross at this one point. For temperatures below −40°, Fahrenheit values are numerically higher than Celsius; above −40°, Celsius values are numerically higher.

How to use the converter

Type a value into any one of the four temperature fields and the other three update immediately, converted using the exact formulas relating each scale. There is no need to remember which formula applies to which pair of scales — enter a Fahrenheit oven temperature and read off the Celsius equivalent instantly, or check a Kelvin value from a science textbook against the everyday Celsius reading you are used to.

Why Fahrenheit's numbers look so odd

The 32-degree freezing point and 180-degree gap to boiling on the Fahrenheit scale seem arbitrary until you know their origin. Daniel Gabriel Fahrenheit calibrated his scale in 1724 using two reference points that made sense to him at the time: 0°F was based on the coldest stable temperature he could reliably reproduce with a mixture of ice, water and salt, and 96°F was intended to approximate human body temperature. Later refinements shifted the exact values slightly — body temperature is now measured as 98.6°F rather than 96°F — but the scale's odd-looking numbers are a direct fossil of those eighteenth-century reference points, unlike Celsius, which was designed from the start around the clean, memorable landmarks of water freezing and boiling.

Why scientists prefer Kelvin

Kelvin is not just Celsius with a different starting point for convenience — it exists because thermodynamic calculations, particularly those involving gas laws and energy, work correctly only when temperature is measured from true absolute zero, the point at which molecular motion theoretically stops entirely. Using Celsius or Fahrenheit in these formulas would require constantly adding offset corrections, whereas Kelvin removes the offset problem entirely by starting at zero where zero actually means "no thermal energy." This is why Kelvin has no negative values in normal use, and why physics and chemistry problems are formulated in Kelvin by default.

Rankine, the forgotten fourth scale

Rankine rarely comes up outside specific corners of American engineering, but it fills a real gap: it combines the zero point of Kelvin (true absolute zero) with the degree size of Fahrenheit, which is convenient for thermodynamic calculations carried out in a US engineering context where Fahrenheit is already the working unit for everyday temperatures. Because Rankine starts at absolute zero like Kelvin, formulas that require an absolute temperature scale work correctly in Rankine without needing an offset correction, while still producing numbers that feel familiar in scale to someone used to thinking in Fahrenheit. It appears mainly in US aerospace and mechanical engineering thermodynamics and is rarely seen anywhere else.

A quick way to estimate Celsius to Fahrenheit in your head

When travelling and checking a foreign weather forecast, a fast mental shortcut is to double the Celsius figure and add 30 — not mathematically exact, but close enough for deciding what to wear. Doubling 20°C gives 40, plus 30 is 70, versus the precise 68°F, a difference small enough not to matter for everyday purposes, whether deciding on a jacket or setting an oven from a foreign recipe. This calculator naturally gives the exact figure instead, but the quick mental version is worth knowing for the many moments when you do not have a device in hand.

Private and instant

All conversions run entirely in your browser, so results update instantly as you type, and no temperature values you enter are ever sent to a server, logged or shared.