Convert Rankine to Kelvin (°R → K)

Temperature scales differ in both zero-point and degree size, so conversion uses a formula — not simple multiplication. Follow these steps to convert degrees Rankine to kelvins by hand.

1. 1
   Take your value in degrees Rankine

   Start with the number of degrees Rankine (°R) you want to convert.
2. 2
   Apply the formula

   Use the formula: K = °R × 5/9
3. 3
   Read the result in kelvins

   The result is your value in kelvins (K).

Temperature conversion uses an offset formula, not simple multiplication.

• ✓°R = °F + 459.67.
• ✓°R = K × 9/5. Identical absolute-zero anchor, different degree size.
• ✓Rankine is essentially obsolete outside specific US engineering specialisms.

• ✗Confusing Rankine with Réaumur (another historical scale).
• ✗Using Rankine outside US engineering contexts — nowhere else uses it.
• ✗Writing °R when Réaumur degree is meant — Réaumur is obsolete.

What is the Rankine?

The Rankine scale is the absolute version of Fahrenheit, named after Scottish engineer William John Macquorn Rankine (1820–1872). Like Kelvin, Rankine sets 0° at absolute zero, but uses Fahrenheit-sized degrees: 1°R = 1°F (in size, not in offset). Water freezes at 491.67°R and boils at 671.67°R at sea level. The scale is rarely used today except in some American engineering disciplines, particularly in older thermodynamics, refrigeration, and aerospace calculations where Fahrenheit is the working unit. The Rankine relates to Fahrenheit by °R = °F + 459.67 and to Kelvin by °R = K × 9/5. While Kelvin has largely replaced Rankine in modern science, Rankine retains a niche in certain US engineering textbooks and HVAC standards. Its main advantage is allowing absolute-temperature thermodynamic calculations within a Fahrenheit-based engineering context.

• US aerospace thermodynamics
• US steam-turbine and power-plant engineering
• Some US industrial combustion calculations

What is the Kelvin?

The Kelvin is the SI base unit of thermodynamic temperature, named after British physicist William Thomson, 1st Baron Kelvin (1824–1907), who proposed an absolute temperature scale in 1848. Kelvin starts at absolute zero — the theoretical lowest temperature where all classical molecular motion stops — at exactly 0 K. The Kelvin uses the same degree size as Celsius: 0°C = 273.15 K, water boils at 373.15 K. Note that Kelvin temperatures are not preceded by a degree sign (300 K, not 300°K). The Kelvin is essential in scientific work — gas laws, blackbody radiation, cryogenics, and astrophysics all use absolute temperature. Since 2019, the Kelvin has been defined by fixing the numerical value of the Boltzmann constant (k = 1.380649 × 10⁻²³ J/K). The Kelvin relates to Celsius by addition (K = °C + 273.15) and is universally used in physics and chemistry.

• Scientific and engineering thermodynamics
• Astronomy and astrophysics temperature reporting
• Colour temperature of light sources (e.g., 6500 K daylight)