Metric Temperature Conversion: Celsius, Kelvin and Fahrenheit

Metric temperature conversion means changing a temperature between Celsius (°C), the everyday metric scale, and kelvin (K), the metric unit used in science; Fahrenheit (°F) is also included here for comparisons. Enter a temperature in any one of these units to calculate its value in the other two.

Formula

The standard conversion formulas are:

• From Celsius to kelvin: K = °C + 273.15
• From kelvin to Celsius: °C = K − 273.15
• From Celsius to Fahrenheit: °F = (°C × 9/5) + 32
• From Fahrenheit to Celsius: °C = (°F − 32) × 5/9
• From Fahrenheit to kelvin: K = (°F − 32) × 5/9 + 273.15
• From kelvin to Fahrenheit: °F = (K − 273.15) × 9/5 + 32

The calculator accepts three temperatures and averages their equivalent Celsius values for its first output, then calculates the corresponding kelvin and Fahrenheit values from that Celsius result. Here, c is the entered Celsius temperature in degrees Celsius, k is the entered kelvin temperature in kelvins, and f is the entered Fahrenheit temperature in degrees Fahrenheit. The first output is therefore an average across the three entries after conversion to the same scale; the other outputs convert that average. For a direct conversion, use the relevant two-scale formula above rather than treating unrelated readings as one temperature.

Worked Example

With the defaults, the entries are 20 °C, 293.15 K, and 68 °F. Convert the kelvin entry to Celsius: 293.15 − 273.15 = 20 °C. Convert Fahrenheit to Celsius: (68 − 32) × 5/9 = 20 °C. The Celsius entry is already 20 °C.

Average the three Celsius values: (20 + 20 + 20) ÷ 3 = 20.00 °C. Convert that average to kelvin: 20 + 273.15 = 293.15 K. Convert it to Fahrenheit: (20 × 9/5) + 32 = 68.00 °F. These match the calculator outputs rounded to two decimal places.

What Celsius and Kelvin Measure

Celsius is named for the scale commonly used in weather reports, household settings, and everyday measurements in most countries. Water freezes at 0 °C and boils at 100 °C at standard atmospheric pressure. The boiling point can shift with pressure, so those reference points do not apply unchanged at every altitude or in every system.

Kelvin is the SI base unit of thermodynamic temperature. Its increments are the same size as Celsius degrees, but its zero is absolute zero, the theoretical lower limit of thermodynamic temperature. Absolute zero is 0 K, equivalent to −273.15 °C. Kelvin is written without a degree symbol: 300 K, not 300 °K.

Fahrenheit is not a metric unit, but conversions are useful because it remains familiar in some contexts, including everyday weather and cooking references. A Fahrenheit degree is smaller than a Celsius degree: a change of 1 °C equals a change of 1.8 °F. The scales also have different zero points, which is why converting a temperature requires both multiplication and addition or subtraction.

How to Choose the Right Conversion

Choose the scale that matches the purpose of the measurement. Celsius is usually the simplest choice for everyday metric temperatures. Kelvin is useful for scientific calculations involving absolute temperature, such as many thermodynamics and gas-law equations. Fahrenheit conversion is helpful when comparing a value with a source or instrument that reports that scale.

• For a temperature reading, use the full formula, including the offset. For example, 25 °C equals 77 °F, not 45 °F.
• For a temperature difference, do not add the scale offsets. A rise of 10 °C is a rise of 10 K or 18 °F.
• For calculations that require absolute temperature, convert to kelvins before using the equation, and follow that equation’s assumptions.
• Keep extra digits through intermediate steps, then round the final displayed result to the precision that the measurement supports.

Common Conversion Mistakes

The most frequent error is using the ratio between degree sizes but forgetting the scales’ offset. Multiplying Celsius by 9/5 gives the Fahrenheit-sized change, not a complete Fahrenheit temperature: the conversion is °F = (°C × 9/5) + 32. For the reverse direction, subtract 32 before multiplying by 5/9.

Another common mix-up is confusing a temperature with a temperature difference. Since Celsius and kelvin have equal-sized increments, a difference of 15 °C is 15 K. But 15 °C as an absolute temperature is 288.15 K. The same distinction matters with Fahrenheit: a difference of 15 °C equals 27 °F, while an absolute 15 °C reading equals 59 °F.

Finally, remember that temperatures below 0 °C are valid, but a kelvin temperature cannot be below 0 K. A negative Celsius value can still correspond to a positive kelvin value; for example, −10 °C is 263.15 K.

Frequently Asked Questions

How do you convert Celsius to Fahrenheit?
Multiply the Celsius temperature by 9/5, then add 32. For instance, 20 °C × 9/5 + 32 = 68 °F.

How do you convert Fahrenheit to Celsius?
Subtract 32 from the Fahrenheit reading, then multiply by 5/9. Thus, 68 °F converts to (68 − 32) × 5/9 = 20 °C.

How do you convert Celsius to kelvin?
Add 273.15. For example, 20 °C + 273.15 = 293.15 K. Do not add a degree symbol to kelvin.

Is a degree Celsius the same size as a kelvin?
Yes. A change of 1 °C has the same magnitude as a change of 1 K. The difference between the scales is their zero point, not the size of each increment.

Why is 0 °C not 0 K?
Zero Celsius is the freezing point of water under a commonly used reference condition, while zero kelvin is absolute zero. The conversion is K = °C + 273.15, so 0 °C equals 273.15 K.

Can a temperature in Celsius be negative?
Yes. Celsius readings below zero are common in cold conditions. Convert them to kelvins by adding 273.15; for example, −20 °C equals 253.15 K.

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in c | Celsius | °C | 20
in k | Kelvin | K | 293.15 | 0
in f | Fahrenheit | °F | 68
out c | Celsius | °C | (k - 273.15 + (f - 32) * 5 / 9 + c) / 3 | 2
out k | Kelvin | K | c + 273.15 | 2
out f | Fahrenheit | °F | c * 9 / 5 + 32 | 2