Convert °R to K degrees Rankine ↔ kelvins Free Online Secure
Type a value in degrees Rankine and read it in kelvins at once. The factor is exact by definition, the arithmetic happens in your browser, and nothing is sent anywhere.
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Common values — Convert °R to K (degrees Rankine ↔ kelvins)
| °R | K |
|---|---|
| 0 | 0 |
| 200 | 111.111111111 |
| 400 | 222.222222222 |
| 491.67 | 273.15 |
| 527.67 | 293.15 |
| 558.27 | 310.15 |
| 671.67 | 373.15 |
| 1000 | 555.555555556 |
Where these units come from
Fahrenheit set his zero in 1724 at the coldest thing he could make reliably — a brine of ice, water and salt — and put body heat near 96, so that his scale could be divided and divided again. Celsius published his in 1742 the other way up, with 0 for boiling and 100 for freezing; it was turned around after his death, which is why he is remembered for a scale he never used. Kelvin is the only one of the four that begins where there is nothing left to take away: absolute zero. Since 2019 it is defined by fixing the Boltzmann constant rather than by the triple point of water, so the kelvin no longer depends on a particular substance. Rankine is Fahrenheit's degree counted from absolute zero, and Réaumur, with 80 degrees between freezing and boiling, survives mostly in old recipes and in a few cheese dairies.
Where you meet them
Celsius runs weather, cooking and medicine almost everywhere; Fahrenheit does the same work in the United States and keeps a foothold in oven settings and in American recipes. Kelvin belongs to physics, to the colour temperature of lamps and screens, and to anything cold enough that negative numbers would be a nuisance. Rankine appears in American thermodynamics, and Réaumur mainly in historical texts — which is exactly why an old recipe can mislead someone reading it as Celsius.
Questions people ask
What is the Rankine scale?
An absolute scale with degrees the size of Fahrenheit degrees. Zero Rankine is absolute zero, and water freezes at 491.67 °R. To convert from kelvin you multiply by 9/5. It appears in some American engineering and thermodynamics work, where Fahrenheit-sized steps are convenient but a true zero is needed.
Why does the kelvin have no degree sign?
Because since 1967 it is a unit in its own right, written K and read as kelvin, not degrees Kelvin. Zero kelvin is absolute zero, which is −273.15 °C, and a step of one kelvin is exactly the same size as a step of one degree Celsius. That makes conversion a matter of adding or subtracting 273.15.
Is the answer exact?
Where both units are defined in terms of one another by whole numbers, yes — the page says so above the figure, and the only rounding is in how many digits are shown. Where the ratio has no finite decimal, the page shows twelve significant digits and says the relation is approximate rather than pretending otherwise.
Does anything I type leave my device?
No. The arithmetic runs in your browser; nothing is uploaded and there is no server to upload it to. Load the page once and it keeps working with the network switched off.
Flames, furnaces and engines: Rankine into kelvin
Combustion engineering deals in extreme heat, and American handbooks often express it in Rankine: methane burns in air near 4,000 °R, a hydrogen-oxygen flame reaches 5,544 °R, and gas inside an engine cylinder peaks around 4,860 °R. Papers published elsewhere give the same states in kelvin. To convert °R to K, divide by 1.8: 4,000 °R is 2,222 K, 5,544 °R is 3,080 K and 540 °R is 300 K. The calculator above does the division as you type, and the sections below connect the figures to flames, boilers, kilns, engines and emissions.
Expect the formula with worked examples, a short profile of each scale, a comparison, a tour of combustion practice and the slips to avoid. All arithmetic stays in your browser.
How to convert °R to K: divide by 1.8
The formula
The formula is K = °R ÷ 1.8, or K = °R × 5/9. So 540 °R gives 300 K, 1,000 °R gives 555.56 K and 3,000 °R gives 1,666.7 K. To go back, multiply by 1.8. Both scales start at absolute zero, so there is no offset to subtract.
Worked examples
Standard air at 518.67 °R is 288.15 K. A methane-air flame at 4,000 °R is 2,222 K. A diesel engine at the end of compression, about 1,620 °R, is 900 K, and an exhaust at 2,160 °R is 1,200 K.
Why only a division
Rankine and kelvin share the same zero. They differ only in the size of the degree, since a kelvin equals 1.8 Rankine degrees. That makes the conversion a single division.
A quick sanity test
The kelvin value is 5/9 of the Rankine value, about 56 per cent. If a “convert °R to K” calculation result is larger than the input, you multiplied instead of dividing.
The Rankine scale
The absolute scale of English-unit combustion data
Rankine is absolute temperature in Fahrenheit-sized degrees. American combustion and gas-dynamics texts use it alongside Btu and pounds, especially for flame temperatures and gas-law calculations. The symbol is °R.
Where it appears
Combustion handbooks, boiler manuals and engine thermodynamics in English units.
The kelvin
The SI unit of temperature
The kelvin starts at absolute zero. Combustion science, kinetics and international standards use it. A temperature in kelvin enters the Arrhenius equation, the gas law and radiation formulas directly. It is written K, with no degree sign.
Where it appears
Research papers, international standards and kinetics tables.
Rankine against kelvin
One kelvin equals 1.8 degrees Rankine. A flame that is 900 °R hotter than another is 500 K hotter. Both scales are absolute, so temperature ratios are the same: a flame at 4,000 °R and air at 540 °R give a ratio of 7.4, just as 2,222 K and 300 K do.
The division is exact, so no rounding enters the conversion. The only common slip is confusing Rankine with Fahrenheit.
Where the conversion shows up
Adiabatic flame temperatures
The temperature a flame would reach with no heat loss depends on fuel and oxidiser. Methane in air gives about 4,000 °R, which is 2,222 K. Propane in air gives 4,081 °R, or 2,267 K, hydrogen in air 4,469 °R, or 2,483 K, acetylene in oxygen 6,138 °R, or 3,410 K, and hydrogen in oxygen 5,544 °R, or 3,080 K. A “convert °R to K” calculation reconciles a US handbook with a European paper.
Engines
Peak gas temperature in a spark-ignition engine is near 2,700 K, which is 4,860 °R. Diesel compression ends near 900 K, or 1,620 °R, enough to ignite the fuel. Exhaust gas leaves the cylinder at about 1,200 K, or 2,160 °R, and cools as it moves down the pipe.
Furnaces and kilns
A glass furnace runs near 1,900 K, which is 3,420 °R. A cement kiln burns raw meal near 1,723 K, or 3,101 °R. A lime kiln runs near 1,370 K, or 2,466 °R, and a waste incinerator at about 1,373 K, or 2,471 °R. Operators use Fahrenheit or Celsius for control, and Rankine only in textbooks.
Thermal NOx
Nitrogen oxides form in flames when the temperature exceeds roughly 1,800 K, which is 3,240 °R. The rate rises steeply with temperature, so a reduction of 100 K, or 180 °R, can cut emissions sharply. Low-NOx burners lower the peak temperature by mixing flue gas or staging the air. Environmental papers quote kelvin, and US engineering notes give °F or °R.
Heat recovery
Flue gas leaving a boiler at 450 K, which is 810 °R, carries energy that a heat exchanger can recover. Cooling it to 380 K, or 684 °R, recovers heat but risks acid condensation below the dew point. The designer works with temperature differences and with absolute temperatures for the entropy balance of the exchanger.
Spreadsheet formulas
Place Rankine values in column A and use =A2/1.8 in column B. For the reverse, use =B2*1.8. For differences, the same factors apply. Format with one decimal and write the unit in each header, so the sheet can be passed to a colleague without a note.
Rounding and precision
The division is exact but often non-terminating: 1,000 °R is 555.555… K. Match the digits to the source. Flame temperatures are known to a few tens of kelvin at best, so 2,222 K is better written as about 2,220 K. Do not report more precision than the underlying calculation justifies.
Estimating without a calculator
Halve the Rankine figure, then add a tenth of that half. For 4,000 °R the half is 2,000 and the tenth is 200, giving 2,200 K against the exact 2,222. For 540 °R the result is 297 K. At flame temperatures the estimate is within one per cent, which is finer than the data deserve.
Landmarks to memorise
491.67 °R is 273.15 K, 518.67 °R is 288.15 K, 540 °R is 300 K, 671.67 °R is 373.15 K and 1,800 °R is 1,000 K. For flames, 4,000 °R is about 2,220 K. These landmarks help you spot an error in a table at a glance.
Reference ladder
0 °R is 0 K. 36.5 °R is 20.3 K. 139.25 °R is 77.4 K. 540 °R is 300 K. 1,000 °R is 555.6 K. 1,800 °R is 1,000 K. 3,240 °R is 1,800 K. 4,000 °R is 2,222 K. 5,544 °R is 3,080 K. A ladder like this speeds up checking.
Checking by reversing
To check any convert °R to K result, multiply by 1.8 and see whether you return to the start. If 540 °R became 300 K, then 300 times 1.8 is 540. A second test is the ratio: the Rankine value must be 1.8 times the kelvin value.
Relation to Fahrenheit and Celsius
Rankine equals Fahrenheit plus 459.67, and kelvin equals Celsius plus 273.15. Starting from a Fahrenheit flame temperature of 3,540 °F you add 459.67 to reach 4,000 °R, divide by 1.8 to reach 2,222 K and subtract 273.15 to reach 1,949 °C. Each step is easy, and the chain provides a check on the final value.
Differences versus absolute values
A gap of 180 °R equals a gap of 100 K, and a 900 °R rise across a boiler equals 500 K. Since the two scales share a zero, gaps convert exactly like absolute readings. This contrasts with Fahrenheit and Celsius, where an offset applies only to absolute values and never to gaps.
Writing the unit properly
Write Rankine as °R and kelvin as K, with a space after the number. In reports for an international audience, give both: 4,000 °R (2,222 K). A bare 4,000 is ambiguous in a combustion paper, because it could be Rankine, Fahrenheit or kelvin, and the three describe very different flames.
Gas turbines
A modern gas turbine inlet reaches 1,700 K, which is 3,060 °R. Higher inlet temperature means higher efficiency, but blades must survive it with internal cooling and thermal barrier coatings. The efficiency of the ideal Brayton cycle depends on pressure ratio only, while the specific work increases with the temperature ratio between 3,060 °R and 518.67 °R, which is 5.9.
Fuel properties in English units
American fuel tables list heating values in Btu per pound or per cubic foot. Methane has about 21,500 Btu/lb, which is 50 MJ/kg. The conversion factor is 2.326 kJ/kg per Btu/lb. Temperature is just one of several quantities to convert, and a “convert °R to K” calculation is the easiest of them.
Computer codes for combustion
Chemical kinetics software accepts temperature in kelvin. Legacy data from English-unit sources may carry temperatures in Rankine. A user who feeds a Rankine value to a code that expects kelvin gets a result that is wrong by a factor of 1.8, which a “convert °R to K” calculation at the input stage prevents.
Teaching combustion
An instructor can demonstrate the quantities in a candle flame with a thermocouple, which gives around 1,600 K, or 2,900 °R, in the luminous zone. Students then calculate the radiant power of the flame in both unit systems. The exercise teaches why absolute temperature matters and how the “convert °R to K” calculation step precedes every calculation.
Soot and flame colour
The yellow glow of a candle comes from hot soot particles that radiate like tiny black bodies at around 1,600 K, which is 2,880 °R. A blue gas flame has little soot and glows from excited molecules instead. A Bunsen burner with the air hole open burns hotter and cleaner than one with it closed. Chemists write the temperatures in kelvin, and a classroom sheet from the US may have them in °R.
Fire investigation
Investigators estimate fire temperatures from the effects on materials. Aluminium melts at 933 K, which is 1,680 °R, glass softens near 1,000 K, or 1,800 °R, and steel loses half its strength near 870 K, or 1,566 °R. A report that cites melted aluminium tells the reader the fire exceeded 933 K, a value that appears in US documents as 1,220 °F or 1,680 °R.
Heat released by burning fuel
A kilogram of methane gives about 50 MJ and needs roughly 17 kg of air to burn. Spread over about 18 kg of products with a specific heat near 1.2 kJ/(kg·K), that heat would raise the temperature by about 2,300 K, which matches real flames, once dissociation of products is allowed for. In an English-unit calculation the same heat is 21,500 Btu per pound and the rise is quoted in °R. Converting the final temperature with a “convert °R to K” calculation lets the two calculations be compared.
Oxygen enrichment
Burning in oxygen-enriched air raises the flame temperature and cuts the volume of flue gas. Methane in pure oxygen reaches about 3,050 K, which is 5,490 °R, against 2,223 K, or 4,001 °R, in air. Glass and metal furnaces use oxy-fuel firing for this reason. Papers on the subject, written for a global audience, give kelvin, while American trade notes use °F or °R, so a “convert °R to K” calculation appears on nearly every comparison.
Flame stability and blow-off
A flame stays attached to a burner when the unburned gas flows at a speed close to the burning velocity. The burning velocity rises with the unburned gas temperature, roughly with the square of absolute temperature. Preheating a mixture from 300 K, which is 540 °R, to 600 K, or 1,080 °R, quadruples the speed in this rough rule. The ratio of 2 in absolute temperature is the same in either scale.
Boilers
The furnace of a large boiler runs near 1,500 K, which is 2,700 °R, while the steam it makes is at about 811 K, or 1,460 °R. The gap between the two is a source of irreversibility, which is why combined cycles use the gas first in a turbine and then in the boiler. A “convert °R to K” calculation for each point turns a US schematic into a European one.
Wood fires and stoves
A wood stove flame is around 1,300 K, or 2,340 °R, while a forest fire flame averages near 1,100 K, or 1,980 °R. A candle flame is at about 1,670 K, or 3,006 °R, in its hottest region. Wood begins to ignite near 570 K, which is 1,026 °R. Fire safety standards in different countries quote these in their own scales.
Welding gases
An oxyacetylene flame reaches about 3,410 K, which is 6,138 °R, hot enough to melt steel at around 1,811 K, or 3,260 °R. Arc welding plasmas exceed 6,000 K, or 10,800 °R. Welding manuals list gas flow and flame types, and the temperatures are given by the manufacturer in whichever scale suits the market.
Rocket combustion
Chamber temperature in a kerosene-oxygen engine is about 3,500 K, which is 6,300 °R, and for hydrogen-oxygen about 3,300 K, or 5,940 °R. The exhaust velocity depends on the square root of the chamber temperature, so a 10 per cent increase gives a 5 per cent higher velocity. Rocket texts from the US often use °R in the equations.
Compression ignition
Diesel fuel ignites because air compressed 16 to 1 reaches about 900 K. Starting from 300 K, or 540 °R, the end temperature is 300 × 16^0.4 = 909 K, which is 1,636 °R. Cold starting is hard because a cold engine loses heat to the walls, and glow plugs help. Students compute the end temperature in kelvin or Rankine, never in Celsius or Fahrenheit.
Autoignition
Hydrogen autoignites near 773 K, which is 1,391 °R, propane near 723 K, or 1,301 °R, and gasoline near 530 K, or 954 °R. Paper ignites near 506 K, or 911 °R. These figures determine safe surface temperatures for equipment in hazardous areas, and regulations give them in Celsius, with US documents adding Fahrenheit.
Thermal radiation from flames
A flame radiates according to the fourth power of its absolute temperature. A flame at 2,000 K radiates 16 times as much per unit area as one at 1,000 K. In English units this is 3,600 °R against 1,800 °R. Furnace designers use whichever scale their formula needs, and a “convert °R to K” calculation reconciles the results.
More values for convert °R to K
A longer list of conversions
If the value you need is not in the first list, these further conversions may be close enough to read off directly:
- -273.15 °R = -151.75 K
- -100 °R = -55.5556 K
- -20 °R = -11.1111 K
- 10 °R = 5.55556 K
- 15 °R = 8.33333 K
- 25 °R = 13.8889 K
- 30 °R = 16.6667 K
- 50 °R = 27.7778 K
Typical errors in a “convert °R to K” calculation
Applying an offset
The two scales share a zero, so neither 459.67 nor 273.15 plays a role. 540 °R is 300 K.
Treating Rankine as Fahrenheit
Same degree, different zero. 540 °R equals 80.33 °F, not 540 °F.
Multiplying where division is needed
Kelvin is the larger unit, so the figure shrinks. If 540 °R became 972, the factor was applied backwards.
Quick questions
What is the method for a “convert °R to K” calculation?
Divide the Rankine value by 1.8.
What is 540 °R in kelvin?
300 K.
What is 491.67 °R in kelvin?
273.15 K.
What is 0 °R in kelvin?
0 K.
Is my figure stored or sent?
No, it stays on your device.
Where to go next
Other temperature tools are listed here: Convert °R to °F, Convert °R to °C, Convert °C to K, Convert °F to K, Convert K to °C, Convert km to mi, Convert mi to km, Convert L to gal, Convert gal to L. For the opposite direction, use Convert K to °R.
The takeaway: a “convert °R to K” calculation divides by 1.8 and labels the result K with no degree sign. The figure you enter stays on your device.