Celsius, Kelvin and Réaumur converter

What is a temperature converter?

A Celsius, Kelvin and Réaumur converter is a tool that allows you to quickly and accurately switch between three different temperature scales used in science and daily life. Temperature is a measure of the average kinetic energy of particles in a substance, and depending on the context—scientific, industrial, or geographical—different scales might be preferred.

This converter is particularly useful for students, researchers, and professionals working in thermodynamics, chemistry, and meteorology, as well as for anyone needing to interpret older texts or international data where less common scales like Réaumur might appear. The special advantage of this converter is its instant operation, with no need to press a “calculate” button — the result updates automatically.

Historical background

The three scales—Celsius, Kelvin, and Réaumur—each have unique origins:

• The Celsius scale was developed in 1742 by Anders Celsius. Originally, it was inverted (0 °C represented the boiling point of water and 100 °C the freezing point), but later reversed to the format we use today.
• The Kelvin scale was created in 1848 by Lord Kelvin (William Thomson) and is an absolute scale based on thermodynamic principles, starting from absolute zero—where all molecular motion theoretically stops.
• The Réaumur scale, less known today, was proposed by René Antoine Ferchault de Réaumur in 1731. It was once common in parts of Europe, particularly France, Germany, and Russia, but gradually lost popularity to Celsius.

Knowing how to convert among these scales allows for a richer understanding of temperature data across different scientific and historical contexts.

Formula

From Celsius to Kelvin

$$T(K) = T(°C) + 273.15$$

From Kelvin to Celsius

$$T(°C) = T(K) - 273.15$$

From Celsius to Réaumur

$$T(°Ré) = T(°C) \times \frac{4}{5}$$

From Réaumur to Celsius

$$T(°C) = T(°Ré) \times \frac{5}{4}$$

From Kelvin to Réaumur

$$T(°Ré) = (T(K) - 273.15) \times \frac{4}{5}$$

From Réaumur to Kelvin

$$T(K) = T(°Ré) \times \frac{5}{4} + 273.15$$

These formulas maintain precision across scientific, laboratory, and educational applications.

Relationship between degree scales

Each temperature scale uses a different fixed point and step size. The key relationships among Celsius, Kelvin, and Réaumur are determined by two physical reference points: the freezing point and boiling point of water.

The comparison shows the relationship between the increments of one degree in each system:

$$1\ \text{°Ré} = 1.25\ \text{°C}$$ $$1\ \text{°C} = 1\ \text{K} \quad (\text{difference only in zero point})$$

One degree Celsius and one Kelvin are the same size. The only difference is at the starting point: 0 K = -273.15 °C. Therefore, for temperature differences, the following holds: Δ1 °C = Δ1 K.

These ratios help form the conversion formulas.

Examples

Example 1: Converting Celsius to Kelvin

Temperature: 25 °C

$$T(K) = 25 + 273.15 = 298.15\ K$$

A room temperature of 25 °C corresponds to 298.15 K.

Example 2: Converting Celsius to Réaumur

$$T(°Ré) = 25 \times \frac{4}{5} = 20°Ré$$

Example 3: Converting Kelvin to Réaumur

If a temperature is 310 K:

$$T(°Ré) = (310 - 273.15) \times \frac{4}{5} = 36.85 \times 0.8 = 29.48°Ré$$

Example 4: Converting Réaumur to Kelvin

If we have 60°Ré:

$$T(K) = 60 \times \frac{5}{4} + 273.15 = 75 + 273.15 = 348.15\ K$$

This is equivalent to 75 °C.

Example 5: Comparing divisions

To check the relationship step by step: An increase of 1 °Ré corresponds to an increase of 1.25 °C or 1.25 K.
Thus, a temperature difference of 80 °Ré equals 100 °C.

Interesting facts

1. Absolute zero (0 K or −273.15 °C) is the lowest possible temperature where atomic motion stops. It’s physically unreachable but approached in laboratory conditions (in the nanokelvin range).
2. The Réaumur scale was once used in early thermometers that contained spirit or ethanol rather than mercury, suited to colder regions.
3. The Kelvin scale does not use the degree symbol (°). Hence: 300 K, not 300 °K.
4. The Celsius scale is directly related to the metric system, allowing simple integration in scientific formulas involving energy, entropy, and pressure.

Common conversion table

This table helps visualize how each scale progresses relative to the others.

Notes

When using the converter, remember that only the Kelvin scale has no negative numbers because it uses an absolute thermodynamic reference. Meanwhile, Celsius and Réaumur can show negative values since they measure relative to water’s freezing point.

The converter is an essential tool for education, laboratories, and industries that deal with thermal processes, ensuring precise data comparison among systems. Its real-time conversion feature enhances user experience and avoids manual errors in calculation.

Frequently Asked Questions

How to convert 37 °C (normal body temperature) into Kelvin and Réaumur?

$$T(K) = 37 + 273.15 = 310.15\ K$$ $$T(°Ré) = 37 \times \frac{4}{5} = 29.6°Ré$$

How many degrees Réaumur correspond to absolute zero?

$$T(°Ré) = (-273.15) \times \frac{4}{5} = -218.52°Ré$$

Which scale increases faster, Réaumur or Celsius?

Celsius degrees are larger; a 1 °Ré rise equals a 1.25 °C rise. Thus, when a Réaumur thermometer increases by one degree, a Celsius thermometer under the same conditions increases by 1.25 °C.

Why is Kelvin preferred in scientific formulas?

Because Kelvin starts from absolute zero, it reflects true thermal energy and ensures proportionality in thermodynamic equations, such as $PV = nRT$ for ideal gases.