Temperature Conversion List: Celsius, Fahrenheit and Kelvin

A temperature conversion list shows how to change a reading among Celsius (°C), Fahrenheit (°F), and kelvin (K). Enter a temperature and choose its starting and target scales to get the equivalent reading; the formulas and reference points below also let you convert values by hand.

Formula

Let T be the input temperature, in the scale selected under “Convert from.” Let C, F, and K denote the corresponding numerical readings in Celsius, Fahrenheit, and kelvin. The standard conversion equations are:

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

The calculator uses the selected scale numbers to apply the matching relationship. Its first output is the converted reading, rounded to two decimal places. The displayed output unit is a generic scale label because the unit depends on the selection; read it together with the “Convert to” choice.

Worked Example

Using the default input, 20 °C, with “Convert from” set to Celsius and “Convert to” set to Celsius:

1. Identify the starting reading: T = 20 °C.
2. Because the target and starting scales match, no conversion is needed.
3. The calculator returns 20.00 °C.

To see a useful cross-scale example, convert the same 20 °C to Fahrenheit: (20 × 9/5) + 32 = 36 + 32 = 68 °F. This illustrates why matching the exact source and target scale matters; the default settings intentionally demonstrate the identity conversion.

Temperature Conversion List

These familiar reference values can help check a conversion or estimate a temperature mentally. Values are rounded where appropriate.

• −40 °C = −40 °F = 233.15 K
• 0 °C = 32 °F = 273.15 K: water’s freezing point at standard atmospheric pressure
• 20 °C = 68 °F = 293.15 K: a mild indoor-temperature reference
• 25 °C = 77 °F = 298.15 K
• 37 °C = 98.6 °F = 310.15 K: a commonly cited approximate human body temperature, not a universal value
• 100 °C = 212 °F = 373.15 K: water’s boiling point at standard atmospheric pressure

Water’s freezing and boiling points depend on pressure and purity, so they are useful reference points rather than rules for every setting. The −40-degree point is a particularly handy check: Celsius and Fahrenheit have the same numerical value there.

How the Scales Work

Celsius and Fahrenheit use different-sized degrees and different zero points. A change of 1 °C equals a change of 1.8 °F, but 0 °C does not equal 0 °F. That offset is why converting between them requires both multiplication and addition or subtraction.

Kelvin is an absolute temperature scale used widely in science. Its increments are the same size as Celsius degrees, but its zero is absolute zero, the theoretical lower limit of thermodynamic temperature. Kelvin values are written without a degree sign: 300 K, not 300 °K. A temperature difference of 10 K has the same size as a difference of 10 °C, although absolute readings on the two scales differ by 273.15.

Choosing a Scale and Using the Result

Use the scale shown by the source reading, not the scale you expect or prefer. Celsius is common in most countries and in everyday scientific contexts; Fahrenheit remains common for everyday weather and household temperatures in the United States; kelvin is useful for absolute temperatures in science and engineering. A converted number is meaningful only when its unit is included.

Keep enough decimal places for the task. Two decimal places are convenient for general conversion, but they do not make an imprecise thermometer more accurate. For everyday comparisons, rounding to the nearest whole degree may be adequate; technical work should follow the precision and conventions required for that measurement.

Common Conversion Mistakes

• Forgetting the offset: Celsius-to-Fahrenheit conversion is not simply multiplication by 1.8; add 32 afterward.
• Reversing the order: For Fahrenheit-to-Celsius, subtract 32 first, then multiply by 5/9.
• Using the wrong kelvin notation: Write K without a degree symbol.
• Confusing temperature with temperature change: For a difference, 10 °C equals 18 °F; do not add 32 to a difference.
• Rounding too early: Keep the full value through the calculation and round the final result.

Also check for a scale label on the original instrument or data. A number such as 30 by itself cannot be converted reliably: 30 °C, 30 °F, and 30 K describe very different temperatures.

Frequently Asked Questions

What is the formula to convert Celsius to Fahrenheit?
Multiply the Celsius value by 9/5, then add 32: °F = (°C × 9/5) + 32. For example, 20 °C equals 68 °F.

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

How do you convert Celsius to kelvin?
Add 273.15 to the Celsius value. For example, 20 °C is 293.15 K. Do not add a degree sign to kelvin.

At what temperature are Celsius and Fahrenheit equal?
They have the same numerical value at −40: −40 °C equals −40 °F.

Is a change of 1 °C the same as a change of 1 °F?
No. A 1 °C change is equal to a 1.8 °F change. The conversion offset of 32 applies to temperature readings, not to temperature differences.

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in temp | Temperature | °C | 20
sel from | Convert from | Celsius=1, Fahrenheit=2, Kelvin=3
sel to | Convert to | Celsius=1, Fahrenheit=2, Kelvin=3
out converted | Converted temperature | selected scale | (from - 1) * (to - 1) + temp * (from - 1) + (temp - 32) * (from - 2) * (to - 1) / 1.8 + (temp - 273.15) * (from - 3) * (to - 1) + (temp * 1.8 + 32) * (from - 1) * (to - 2) + (temp - 273.15) * (from - 3) * (to - 2) * 1.8 + (temp + 273.15) * (from - 1) * (to - 3) + (temp - 32) * (from - 2) * (to - 3) / 1.8 + temp * (from - 3) * (to - 3) | 2