Guide · Published September 15, 2026 · By the site maintainer

Temperature scales explained: Celsius, Fahrenheit, and Kelvin

Celsius, Fahrenheit, and Kelvin describe the same physical condition with different numerical scales. They disagree for two reasons: their zero points are placed differently, and a Fahrenheit degree is smaller than a Celsius degree or a kelvin. A correct conversion must account for both the starting point and the size of each step.

This makes temperature unlike a simple length conversion. Converting meters to feet only changes scale, so multiplication is enough. Converting Celsius to Fahrenheit changes scale and offset, so the calculation needs multiplication and addition. Forgetting the offset is the most common reason a temperature result looks plausible but is wrong.

The three scales at a glance

The water values are familiar reference points, but they need context. Freezing and boiling behavior depends on pressure and other conditions. The table uses the conventional comparison near standard atmospheric pressure; it is not a claim that every pot of water boils at one fixed temperature regardless of altitude or composition.

What Celsius measures

The Celsius scale is arranged so that the freezing and boiling points of water under stated reference conditions are about 0 and 100 degrees. Weather forecasts, building controls, cooking instructions, and clinical measurements in much of the world use degrees Celsius. The written unit is “degree Celsius,” and its symbol is °C.

A change of one degree Celsius has the same size as a change of one kelvin. The scales differ by an offset: a Celsius temperature is the corresponding kelvin temperature minus 273.15. Therefore 20 °C equals 293.15 K, while an increase from 20 °C to 25 °C is a five-degree Celsius change and also a five-kelvin change.

What Fahrenheit measures

Fahrenheit remains common in daily weather, household thermostats, and ovens in the United States. Water freezes near 32 °F and boils near 212 °F under the same conventional conditions used for the familiar Celsius comparison. That creates 180 Fahrenheit degrees across a span covered by 100 Celsius degrees.

One Fahrenheit degree is therefore five ninths the size of one Celsius degree. A 9 °F temperature increase equals a 5 °C increase. This is why the Fahrenheit-to-Celsius formula subtracts 32 and then multiplies by 5/9. Subtracting 32 aligns the zero points; multiplying changes the step size.

What Kelvin measures

Kelvin is the SI unit of thermodynamic temperature. Its zero is absolute zero, the lower boundary of the thermodynamic scale. The BIPM's official kelvin definition ties the unit to a fixed numerical value of the Boltzmann constant. Modern SI definitions use physical constants rather than preserving the unit through one particular thermometer or water sample.

The symbol is K, not °K. Say “kelvin” rather than “degree kelvin.” A value is written 300 K, with a space between the number and symbol. Because the kelvin and degree Celsius have equal interval sizes, their conversion needs only the exact offset 273.15:

• Celsius to kelvin: K = °C + 273.15
• Kelvin to Celsius: °C = K − 273.15

Exact conversion formulas

NIST describes unit conversion as a process that includes choosing a factor, carrying the calculation through, and rounding to an appropriate number of digits. Its unit conversion resources include temperature conversion references and guidance on significant digits. The Celsius and Fahrenheit converter applies the two everyday formulas in either direction.

A temperature reading is not an interval

The offset belongs to a scale reading, not to the size of a change. Suppose a room warms from 18 °C to 23 °C. The change is 5 °C. Multiply that interval by 9/5 to find a 9 °F change. Do not add 32 to an interval. Adding 32 is part of converting an actual Celsius reading to its Fahrenheit reading.

This distinction matters in laboratory tolerances, equipment specifications, and statements such as “increase the oven setting by ten degrees.” A ten-degree Celsius increase is an eighteen-degree Fahrenheit increase. Yet a reading of 10 °C is 50 °F because the conversion of the reading also includes the offset.

Worked examples

Convert 25 °C to Fahrenheit

Multiply 25 by 9/5 to get 45, then add 32. The result is 77 °F. A mental estimate that doubles Celsius and adds 30 gives 80 °F, which may be adequate for choosing clothing but is not the exact conversion.

Convert −4 °F to Celsius

Subtract 32 from −4 to get −36. Multiply −36 by 5/9 to get −20. Therefore −4 °F equals −20 °C. Use parentheses when entering negative values so the subtraction happens before multiplication.

Convert 310.15 K to Celsius

Subtract 273.15 from 310.15. The result is 37 °C. This arithmetic shows the scale relationship; it should not be used by itself to diagnose health. Measurement method, instrument accuracy, site, and individual variation all matter when interpreting a body-temperature reading.

Useful comparison points

Reference points are helpful for checking a result. If a mild 20 °C room converts to 293.15 °F, the kelvin offset was probably added without changing the unit. If 32 °F converts to 17.8 °C, the calculation probably multiplied before subtracting 32.

How to write temperatures clearly

Keep the number and unit together and preserve the sign. NIST's SI writing guide specifies “degree Celsius” rather than “centigrade” in formal metric usage and explains symbol spacing and capitalization. Celsius uses an uppercase C because the unit is named for Anders Celsius. Kelvin uses an uppercase K and no degree sign.

State the measurement conditions when they affect the meaning. “Water boiled at 96 °C at this location” is more informative than presenting 100 °C as an unavoidable observation. For equipment, record whether a value is a measured temperature, set point, tolerance, or maximum. A converted number cannot repair an ambiguous source statement.

Choose precision to match the source

A weather reading reported as 21 °C usually should not become 69.8000 °F. Those extra digits suggest precision the original value did not provide. For general weather, a whole Fahrenheit degree may be appropriate. A calibrated technical measurement may justify more digits, but its uncertainty and method matter more than the converter's display length.

Round once, after completing the calculation. Rounding the multiplication and offset separately can shift the final value. Keep a few guard digits during the work, then report a result suited to the source measurement.

Common temperature-conversion errors

• Multiplying Celsius by 9/5 but forgetting to add 32.
• Multiplying Fahrenheit by 5/9 before subtracting 32.
• Adding 32 to a temperature interval rather than a scale reading.
• Writing °K instead of K.
• Dropping a negative sign when using a calculator.
• Reporting more decimal places than the measurement supports.
• Treating a water reference point as independent of pressure and conditions.

For quick estimates, the mental conversion guide explains “double and add 30” and shows where it drifts. Use the exact formula for cooking thresholds, technical specifications, or any decision sensitive to a degree or two. The recipe conversion guide includes common oven equivalents, while the conversion chart collects reference values across several measurement categories.

The core idea is to separate scale from offset. Celsius and kelvin use equal-sized steps with different zero points. Fahrenheit uses a different zero point and a different step size. Once you identify whether you are converting a reading or an interval, the correct formula follows.