Temperature Conversion: Formulas, Scales, and Examples

Temperature conversion in plain terms

What is temperature conversion? It is the process of changing a temperature from one scale to another. The heat level stays the same. Only the number and unit change.

This skill matters in science, weather, cooking, medicine, and software. A forecast may use Celsius, while a recipe uses Fahrenheit. Engineers often use Kelvin for science work. The right formula keeps these readings clear.

Temperature scales do not share the same zero point or step size. So, you cannot convert by changing the unit label alone. You must use a formula that matches both scales.

Why do different temperature scales exist?

People built temperature scales for different needs. Daniel Fahrenheit made his scale in the early 1700s. Anders Celsius later proposed a scale based on water's key phase changes.

These scales spread through different regions and fields. Fahrenheit remains common in the United States. Celsius is the main scale in most countries. Kelvin is the standard scale for much scientific work.

Each scale sets fixed points and divides the gap between them. Water freezes at 0°C and boils at 100°C under standard pressure. The same points are 32°F and 212°F on the Fahrenheit scale.

• Zero point: the starting value chosen by a scale
• Degree size: the temperature change represented by one degree
• Reference points: fixed events used to set the scale

The three temperature scales you will use most

The Celsius scale uses water's freezing point as 0°C. Its boiling point is 100°C at standard sea-level pressure. The gap between those points has 100 equal steps.

The Fahrenheit scale places water's freezing point at 32°F. Its boiling point is 212°F under the same pressure. That gap contains 180 equal steps.

The Kelvin scale begins at absolute zero. Absolute zero is the lowest possible temperature. It equals 0 K, or −273.15°C. Kelvin uses the same step size as Celsius, but it does not use a degree sign.

Event	Celsius	Fahrenheit	Kelvin
Water freezes	0°C	32°F	273.15 K
Water boils	100°C	212°F	373.15 K
Absolute zero	−273.15°C	−459.67°F	0 K

These water values assume standard pressure. Boiling changes at high altitudes. The NIST guide to temperature units explains the Kelvin scale and its link to Celsius.

Temperature conversion formulas and how to use them

A temperature conversion formula has two parts. First, it may change the scale's zero point. Second, it may change the size of each step.

Celsius and Fahrenheit

To convert Celsius to Fahrenheit, multiply by 9/5. Then add 32.

°F = (°C × 9/5) + 32

To convert Fahrenheit to Celsius, subtract 32 first. Then multiply by 5/9.

°C = (°F − 32) × 5/9

Celsius and Kelvin

To convert Celsius to Kelvin, add 273.15.

K = °C + 273.15

To convert Kelvin to Celsius, subtract 273.15.

°C = K − 273.15

Fahrenheit and Kelvin

For Fahrenheit to Kelvin, subtract 32. Then multiply by 5/9. Finally, add 273.15.

K = (°F − 32) × 5/9 + 273.15

For Kelvin to Fahrenheit, subtract 273.15. Multiply by 9/5. Then add 32.

°F = (K − 273.15) × 9/5 + 32

Keep the order clear. Parentheses matter in formulas with subtraction. Also, Kelvin values use K rather than °K.

Worked temperature conversion examples

Examples make each formula easier to apply. Write the known value first. Then choose the formula that ends with your target unit.

Example 1: Convert 25°C to Fahrenheit

Use the Celsius to Fahrenheit formula.

°F = (25 × 9/5) + 32

First, 25 × 9/5 equals 45. Add 32 to get 77°F. Thus, 25°C equals 77°F.

Example 2: Convert 68°F to Celsius

Use the Fahrenheit to Celsius formula.

°C = (68 − 32) × 5/9

Subtract 32 to get 36. Then multiply 36 by 5/9. The result is 20°C.

Example 3: Convert 20°C to Kelvin

Add 273.15 to the Celsius value.

K = 20 + 273.15 = 293.15 K

So, room temperature near 20°C equals 293.15 K. Kelvin values never fall below zero.

• Write the starting unit beside the number.
• Use parentheses when the formula calls for them.
• Round only after completing the main calculation.
• Check that your answer has the target unit.

How to remember temperature conversion formulas

How to remember temperature conversion formula steps? Start with the two Celsius and Fahrenheit anchor points. Remember that 0°C equals 32°F. Also remember that 100°C equals 212°F.

The scale gap gives the key ratio. A change of 100 Celsius degrees equals a change of 180 Fahrenheit degrees. Reduce 100/180 to 5/9. The reverse ratio is 9/5.

Use this short memory chain: “multiply, then add” for Celsius to Fahrenheit. Use “subtract, then multiply” for Fahrenheit to Celsius. The 32 moves with the direction.

1. Celsius to Fahrenheit: multiply by 9/5, then add 32.
2. Fahrenheit to Celsius: subtract 32, then multiply by 5/9.
3. Celsius to Kelvin: add 273.15.
4. Kelvin to Celsius: subtract 273.15.

Test your memory with easy values. Zero Celsius should become 32 Fahrenheit. A Kelvin answer should sit about 273 points above Celsius. These checks catch most mistakes.

Other temperature scales worth knowing

Celsius, Fahrenheit, and Kelvin cover most daily and science tasks. Still, other scales appear in older books and specialist fields. Knowing their names helps you read older data.

The Rankine scale starts at absolute zero. It uses Fahrenheit-sized degrees. The Delisle scale runs in the reverse direction. The Réaumur scale sets water's freezing and boiling points at 0 and 80 degrees.

These scales are now uncommon. Most modern tools convert them through Celsius, Kelvin, or Fahrenheit. Their main value is historical context.

Scale	Main feature	Typical use
Rankine	Absolute scale with Fahrenheit steps	Older engineering work
Réaumur	Water gap split into 80 steps	Historical records
Delisle	Values rise as temperature falls	Historical science

Use the right scale with confidence

Temperature conversion changes a reading from one scale into another. The three main scales are Celsius, Fahrenheit, and Kelvin. Their formulas account for both zero points and degree sizes.

For quick work, keep the Celsius and Fahrenheit formulas close at hand. Remember 0°C equals 32°F. Remember 100°C equals 212°F. Kelvin is simpler because it shares Celsius step sizes.

When a result seems wrong, test a known point. Freezing water is a useful check. So is room temperature near 20°C, 68°F, or 293.15 K.

With these anchors, you can solve most conversion tasks by hand. A calculator can then confirm your result.