Temperature Conversion: Celsius, Fahrenheit and Kelvin

This temperature conversion calculator converts a value between Celsius, Fahrenheit, and Kelvin. Enter a temperature in any one scale, then use the formulas below to convert it to the other two; temperature differences are handled separately because they do not use the same offsets as absolute readings.

Formula

The calculator accepts Celsius, C, and gives Fahrenheit, F, and Kelvin, K:

• Fahrenheit: F = (C × 9/5) + 32
• Kelvin: K = C + 273.15

C is the temperature entered in degrees Celsius; F is the result in degrees Fahrenheit; and K is the result in kelvins. Celsius and Fahrenheit values are written with degree symbols (°C and °F). Kelvin is written K, without a degree symbol. The formulas use the exact scale relationships; the calculator displays results rounded to two decimal places.

Worked Example

Using the default input, C = 20°C:

1. Convert to Fahrenheit: 20 × 9/5 + 32 = 36 + 32 = 68°F.
2. Convert to Kelvin: 20 + 273.15 = 293.15 K.

The calculator therefore returns 68.00°F and 293.15 K. The extra zero in the Fahrenheit result reflects the display setting, not greater measurement precision.

How the Temperature Scales Work

Celsius and Fahrenheit use different-sized degrees and different zero points. At standard atmospheric pressure, water freezes at 0°C (32°F) and boils at 100°C (212°F). The interval between these points is 100 Celsius degrees but 180 Fahrenheit degrees, so one Celsius degree equals 1.8 Fahrenheit degrees. That difference in scale size explains the factor 9/5 in the conversion formula; the added 32 accounts for where the scales place zero.

Kelvin uses degree-sized increments equal to Celsius but starts at absolute zero, the theoretical lower limit of thermodynamic temperature. Thus 0 K equals −273.15°C, and a change of 1 K has the same size as a change of 1°C. Kelvin readings are not written with a degree sign.

The freezing and boiling points above are useful reference points, not universal descriptions of water under every condition. Pressure and dissolved substances can affect phase-change temperatures. For ordinary household comparisons, however, the familiar values provide a practical check on arithmetic.

Converting in Either Direction

To convert a Fahrenheit reading back to Celsius, subtract 32 first, then multiply by 5/9: C = (F − 32) × 5/9. For example, 68°F becomes (68 − 32) × 5/9 = 20°C. Reversing the order or forgetting to subtract 32 is a common source of error.

For Kelvin conversions, use K = C + 273.15 and C = K − 273.15. To convert directly between Fahrenheit and Kelvin, either convert through Celsius or use the combined formulas:

• K = (F − 32) × 5/9 + 273.15
• F = (K − 273.15) × 9/5 + 32

Converting through Celsius is often easiest to check because it makes the offset and scale change visible.

Absolute Temperatures and Temperature Changes

A key distinction is whether a number describes an absolute temperature or a temperature difference. For an absolute reading, the conversion between Celsius and Fahrenheit includes both a multiplier and an offset. For a difference, there is no offset: multiply a Celsius difference by 9/5 to get a Fahrenheit difference, or multiply a Fahrenheit difference by 5/9 to get a Celsius difference.

For example, if a room warms from 18°C to 23°C, its temperature increase is 5°C, equivalent to a 9°F increase. Applying the absolute-temperature formula to the 5-degree change would incorrectly add 32. Similarly, a change of 5 K equals a change of 5°C, not 278.15°C. This distinction matters in weather comparisons, laboratory notes, and calculations involving heating or cooling.

Choosing the Right Scale and Avoiding Mistakes

Celsius is widely used for everyday weather and many scientific measurements; Fahrenheit remains common in everyday use in the United States. Kelvin is used in scientific contexts where an absolute scale is important, including many physics and thermodynamics calculations. Pick the scale required by the instrument, instructions, or calculation rather than assuming one is more precise: precision depends on the measurement and its uncertainty, not the unit name.

• Check the input unit before calculating. A number without °C, °F, or K is ambiguous.
• For Fahrenheit-to-Celsius, subtract 32 before multiplying by 5/9.
• Do not add 32 when converting a temperature difference.
• Keep extra digits during intermediate calculations and round only the displayed result when practical.
• Use K, not °K, for kelvin; use a degree symbol for Celsius and Fahrenheit.

Rounding can make a converted value appear slightly different from a source that gives more decimal places. For example, 20°C is exactly 68°F, while 21°C is 69.8°F. A displayed result such as 69.80°F does not mean the original measurement was accurate to a hundredth of a degree.

Frequently Asked Questions

How do I convert Celsius to Fahrenheit?
Multiply the Celsius value by 9/5, then add 32. For example, 25°C is (25 × 9/5) + 32 = 77°F.

How do I convert Fahrenheit to Celsius?
Subtract 32 from the Fahrenheit value and multiply the remainder by 5/9. For example, 50°F is 10°C.

What is 20°C in Fahrenheit and Kelvin?
20°C equals 68°F and 293.15 K.

Is a degree Celsius the same size as a kelvin?
Yes. A temperature change of 1°C has the same magnitude as a change of 1 K, though the scales have different zero points.

Why is there no 32 in a temperature-difference conversion?
The 32 adjusts the zero point when converting absolute Celsius and Fahrenheit readings. Differences compare two temperatures, so the shared offset cancels out.

Can Kelvin be negative?
In the ordinary thermodynamic temperature scale, Kelvin readings are not below 0 K, which is absolute zero. Celsius and Fahrenheit can show negative values.

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in c | Celsius temperature | °C | 20
out f | Fahrenheit temperature | °F | c * 9 / 5 + 32 | 2
out k | Kelvin temperature | K | c + 273.15 | 2