Convert °F to K degrees Fahrenheit ↔ kelvins Free Online Secure
Type a temperature in degrees Fahrenheit and read it in kelvins at once. The relation is exact by definition, the arithmetic happens in your browser, and nothing is sent anywhere.
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Common values — Convert °F to K (degrees Fahrenheit ↔ kelvins)
| °F | K |
|---|---|
| -40 | 233.15 |
| 0 | 255.372222222 |
| 32 | 273.15 |
| 50 | 283.15 |
| 68 | 293.15 |
| 77 | 298.15 |
| 98.6 | 310.15 |
| 212 | 373.15 |
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
Is it true that −40 °C and −40 °F are the same?
Yes, and it is the only place the two scales meet. Fahrenheit degrees are smaller than Celsius ones — nine of them to every five — and the two scales start from different zeros; the one temperature where the difference in size and the difference in starting point cancel out exactly is minus forty. Below that, the Fahrenheit number is the lower of the two.
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.
Furnaces, jet engines and power plants: Fahrenheit into kelvin
American industry still describes a lot of hot equipment in degrees Fahrenheit: a steam turbine runs at 1,000 °F, a jet engine's turbine inlet sees 2,700 °F and a heat-treating furnace is set at 1,500 °F. Textbooks and international standards, though, speak in kelvin. To convert °F to K, add 459.67 and multiply by 5/9, or subtract 32, multiply by 5/9 and add 273.15: 212 °F is 373.15 K, 1,000 °F is 810.93 K and 2,700 °F is 1,755.37 K. The calculator above does all the steps as you type, and the sections below show what the figures mean for metals, engines, power generation, fire and flight.
You will find the formula with worked examples, a short profile of each scale, a comparison, a long set of cases from industry and aerospace, and the slips to avoid. The arithmetic runs in your browser, and what you type is not sent anywhere.
How to convert °F to K: add 459.67, then multiply by 5/9
The formula
The direct formula is K = (°F + 459.67) × 5/9. The equivalent form is K = (°F − 32) × 5/9 + 273.15. So 68 °F gives (68 + 459.67) × 5/9 = 293.15 K, 0 °F gives 255.37 K and 98.6 °F gives 310.15 K. To go back, multiply kelvin by 9/5 and subtract 459.67.
Worked examples
Boiling water at 212 °F is 373.15 K. Superheated steam at 1,000 °F is 810.93 K. A steel furnace at 1,500 °F is 1,088.71 K. Aluminium melts at 1,221 °F, which is 933.4 K, and tungsten at 6,192 °F, which is 3,695 K.
Why the offset is 459.67
Absolute zero is −459.67 °F. Adding that figure to a Fahrenheit reading gives degrees Rankine, which start at absolute zero but keep the Fahrenheit step. Dividing by 1.8 turns Rankine into kelvin. The two operations can be combined into one formula.
A quick sanity test
At −459.67 °F the result must be 0 K. If a method does not give absolute zero at that point, a step is missing.
The Fahrenheit scale
The scale of American industry
Fahrenheit puts 32 at the freezing point of water and 212 at its boiling point. In the United States it appears on furnace controllers, boiler gauges, engine data and safety data sheets, so any American specification for hot equipment is in these degrees.
Where it appears
Furnace controls, boiler gauges, aircraft manuals and safety data sheets.
The kelvin
The absolute scale
The kelvin is the SI unit of temperature, with zero at absolute zero. Thermodynamic and heat-transfer formulas need it, because they involve ratios of absolute temperatures. It is written K, with no degree sign.
Where it appears
Engineering calculations, material data, international standards and research papers.
Fahrenheit against kelvin
One kelvin is 1.8 Fahrenheit degrees. A 90 °F temperature rise across a heat exchanger is a 50 K rise. The absolute values differ more: 1,000 °F is 810.93 K, so the two numbers look unrelated at first glance. At very high temperatures the offset is small compared with the value, so 3,000 °F is about 1,922 K.
The formula is exact, so there is no rounding in the conversion itself. The traps are using 273 where 273.15 is needed and converting a difference as if it were a temperature.
Where the conversion shows up
Metals and melting
Lead melts at 621 °F, or 600.6 K, tin at 450 °F, or 505.1 K, aluminium at 1,221 °F, or 933.4 K, silver at 1,763 °F, or 1,235 K, gold at 1,948 °F, or 1,337 K and copper at 1,984 °F, or 1,358 K. Steels melt between about 2,500 and 2,800 °F, which is 1,644 to 1,811 K, depending on carbon content. A “convert °F to K” calculation for each puts a US handbook on the same footing as a European data sheet.
Jet engines and gas turbines
The gas entering a modern turbine can be 2,700 to 3,000 °F, which is 1,755 to 1,922 K, hotter than the melting point of the blade alloys, which survive thanks to internal cooling and ceramic coatings. Exhaust gas from a piston engine runs around 1,200 to 1,600 °F, or 922 to 1,144 K.
Power plants
Steam in a coal-fired plant is superheated to about 1,000 to 1,100 °F, which is 811 to 866 K. A pressurised-water reactor holds coolant near 600 °F, or 589 K, and a boiling-water reactor near 550 °F, or 561 K. Plant manuals in the US give the Fahrenheit figures, while international comparison needs kelvin or Celsius.
Heat treating steel
Steel is hardened by heating above about 1,500 °F, which is 1,088.7 K, then quenching, and tempered at 300 to 1,000 °F, or 422 to 811 K. Shops in the United States set controllers in Fahrenheit. Materials textbooks with phase diagrams are written in kelvin or Celsius, and a student matching a recipe with a diagram needs a “convert °F to K” calculation for each step.
Catalytic converters
A catalytic converter starts working at about 482 °F, or 523 K, known as light-off, and operates best between 750 and 1,500 °F, or 672 and 1,089 K. A cold engine pollutes more because the catalyst has not reached these temperatures. Automotive engineers in the US quote Fahrenheit and international standards use Celsius.
Engines and cooling
An engine coolant at 195 to 220 °F is at 364 to 378 K. The coolant boils higher under pressure: a 15 psi cap raises the boiling point of a 50 per cent glycol mix to about 265 °F, which is 402 K. Heat transfer calculations in the radiator use temperature differences, which are the same number of kelvin as of Celsius degrees.
Industrial boilers
A package boiler making saturated steam at 150 psig has a steam temperature near 366 °F, which is 459 K. Feedwater enters at 227 °F, or 381 K, from the deaerator. Every 10 to 11 degrees of feedwater preheat saves roughly one per cent of fuel. Boiler engineers work in Fahrenheit, and a “convert °F to K” calculation is the first step in comparing with a European design.
Glass and ceramics
Container glass is melted at about 2,700 °F, which is 1,755 K, and annealed near 1,050 °F, or 839 K. Fine porcelain is fired at roughly 2,400 °F, or 1,589 K. The kiln temperatures in a studio manual are in Fahrenheit, but a material data sheet lists the sintering point in kelvin, so potters and engineers often need to convert.
Semiconductor furnaces
Chip makers grow oxide layers at 1,500 to 2,000 °F, which is 1,089 to 1,366 K, and diffuse dopants at similar temperatures. Silicon melts at 2,577 °F, or 1,687 K. Process engineers in the US quote Fahrenheit for older furnaces and Celsius for newer tools, so a table with both and a “convert °F to K” calculation column is helpful.
Welding and soldering
Soft solder melts near 361 °F, or 456 K, for tin-lead, and silver brazing alloys melt at 1,100 to 1,600 °F, or 866 to 1,144 K. A steel weld pool exceeds 2,800 °F, or 1,811 K. American welding standards give preheat temperatures in Fahrenheit, such as 200 °F for thick sections, which is 366.5 K.
Spreadsheet formulas
Put Fahrenheit values in column A and use =(A2+459.67)*5/9 in column B. For the reverse, use =B2*9/5-459.67. For differences, use =A2*5/9 with no offset. Format with two decimals, label each unit, and keep the original values for traceability.
Rounding and precision
The formula is exact, but process instruments are not. A furnace thermocouple good to ±1% at 1,500 °F is uncertain by 15 °F, or 8 K. Report 1,089 K, not 1,088.71 K. Keep full precision inside the calculation and round only the final figure.
A mental shortcut
Subtract 32, multiply by 0.556 and add 273. For 1,000 °F: 968 times 0.556 is 538, plus 273 gives 811 K, close to the exact 810.93. The method is good to a fraction of a kelvin if you keep three decimals in 0.556.
Landmarks to memorise
32 °F is 273.15 K, 212 °F is 373.15 K, 68 °F is 293.15 K, 1,000 °F is 810.93 K and −459.67 °F is 0 K. Memorising these helps in spotting typing errors in data tables.
Cement and lime kilns
A cement kiln burns raw meal at about 2,640 °F, or 1,723 K, equal to 1,450 °C. Lime kilns run lower, near 1,650 to 2,000 °F, or 1,172 to 1,366 K. The flame inside can exceed 3,500 °F, or 2,200 K. Operators use Fahrenheit and Celsius in different countries, and the same kiln data may need a “convert °F to K” calculation for a global company.
Reference ladder
−459.67 °F is 0 K. −320 °F is 77.6 K. −40 °F is 233.15 K. 0 °F is 255.37 K. 32 °F is 273.15 K. 100 °F is 310.93 K. 500 °F is 533.15 K. 1,000 °F is 810.93 K. 2,000 °F is 1,366.5 K. A ladder like this helps in checking a table quickly.
Checking by reversing
To check any convert °F to K result, multiply it by 9/5 and subtract 459.67. If 68 °F became 293.15 K, then 293.15 times 1.8 is 527.67, and subtracting 459.67 gives 68. A second test is the sign: kelvin values are never negative.
The Rankine scale
Rankine measures from absolute zero with Fahrenheit-sized degrees, so °R = °F + 459.67. 68 °F is 527.67 °R, and dividing by 1.8 gives 293.15 K. Older American thermodynamics texts use Rankine in the gas law. Converting through Rankine is a reliable way to avoid slips.
Temperature differences
When a heat exchanger raises a stream by 90 °F, the rise is 50 K. A drop of 36 °F is a drop of 20 K. Never add 459.67 to a difference. Specific heat in the US appears in Btu per pound per °F, while in SI it is joules per kilogram per kelvin, and the two are related by a factor of 4186.8.
Reading an American data sheet
Safety data sheets list melting point, boiling point, flash point and autoignition temperature in Fahrenheit. A “convert °F to K” calculation for each makes international comparison easier. Check whether the sheet says "°F" or "°C", because some give both, and a mix-up between them can turn a cool storage limit into a hot one.
Heat exchangers and approach temperatures
A shell-and-tube exchanger cooling oil from 300 °F to 150 °F with water heated from 80 °F to 120 °F has a hot-side drop of 150 °F, or 83.3 K, and a cold-side rise of 40 °F, or 22.2 K. The log-mean temperature difference, the figure used for design, is computed from differences, so it is the same in Fahrenheit-based and kelvin-based units after a factor of 5/9.
Thermal expansion of steel pipe
A steel pipe expands about 0.0000065 per °F, or 0.0000117 per K. A 100 ft pipe heated from 70 to 400 °F, a rise of 330 °F or 183 K, grows by about 2.6 inches. The coefficient is given per °F in US codes and per K in SI codes, and the factor between them is 1.8, which is why a “convert °F to K” calculation on the range is a common check.
Waste incineration
Regulations for hazardous waste incineration often require at least 1,800 °F, which is 1,255 K, or about 980 °C, with a residence time of two seconds. The high temperature breaks down organic compounds. Environmental permits in the US quote Fahrenheit, while European directives use 850 °C or 1,100 °C, so a conversion helps compare rules.
Flash points of fuels
Gasoline has a flash point near −45 °F, or 230.4 K, so it can ignite even on a cold day. Jet A fuel flashes at 100 °F or higher, or 310.9 K. Diesel flashes at 125 °F or higher, or 324.8 K. Safety data sheets in the US list these numbers in Fahrenheit, and fire engineers convert them for international comparisons.
Autoignition temperatures
Gasoline autoignites near 495 °F, or 530.4 K. Propane needs about 842 °F, or 723.2 K, and hydrogen about 932 °F, or 773.2 K. Paper ignites near 451 °F, or 506.5 K, the figure that gave a famous novel its title. A “convert °F to K” calculation on these numbers places them on a scale used in thermodynamic safety calculations.
Wildfires and flames
Flames in a forest fire reach 1,500 to 2,000 °F, which is 1,089 to 1,366 K, and the temperature at the ground can be hundreds of degrees lower. Wood begins to char from about 540 °F, or 555 K. Fire scientists model radiation using kelvin, while public bulletins use Fahrenheit, so a “convert °F to K” calculation links the two.
Supersonic flight
At Mach 3, the skin of the SR-71 Blackbird reached about 600 °F, or 589 K, requiring titanium construction. The Concorde's nose reached 260 °F, or 400 K, at Mach 2. At such speeds aerodynamic heating dominates, and the temperature is calculated from the stagnation formula, which needs absolute temperature.
The standard atmosphere
The International Standard Atmosphere sets sea-level temperature at 59 °F, which is 288.15 K, falling to −69.7 °F, or 216.65 K, at 11 km, where it stays through the lower stratosphere. Pilots in the US read outside air temperature in Celsius on most instruments, but flight manuals may use Fahrenheit for performance charts.
Re-entry and heat shields
The Space Shuttle's tiles protected the airframe from surface temperatures near 2,300 °F, or 1,533 K, while the reinforced carbon-carbon on the nose and wing edges saw up to 3,000 °F, or 1,922 K. The aluminium structure behind had to stay below about 350 °F, or 450 K. Heat shield engineers work from NASA reports in Fahrenheit and kelvin.
More values for convert °F 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 °F = 103.622 K
- -100 °F = 199.817 K
- -20 °F = 244.261 K
- 10 °F = 260.928 K
- 15 °F = 263.706 K
- 25 °F = 269.261 K
- 30 °F = 272.039 K
- 50 °F = 283.15 K
Common slips when you convert °F to K
Forgetting the 459.67
Multiplying by 5/9 alone gives a result that is far too small. 212 °F is not 117.8 K but 373.15 K.
Converting a difference as a temperature
A rise of 18 °F is a rise of 10 K. Do not add 459.67.
Rounding the constants
Use 459.67 and 273.15. Rounding them to 460 and 273 introduces errors of a third of a kelvin in precise work.
Short answers
How do I convert °F to K?
Add 459.67 and multiply by 5/9.
What is 68 °F in kelvin?
293.15 K.
What is 32 °F in kelvin?
273.15 K.
What is absolute zero in Fahrenheit?
−459.67 °F.
Does my number leave my device?
No, the calculation runs in your browser.
More pages and a wrap-up
For other temperature conversions see Convert °F to °Ré, Convert °F to °R, Convert °R to K, Convert °C to K, Convert K to °C, Convert km to mi, Convert mi to km, Convert L to gal, Convert gal to L. The reverse direction is Convert K to °F.
In short, a “convert °F to K” calculation means adding 459.67 and multiplying by 5/9, then writing the unit beside the answer. Your number is computed locally, with no account and no upload.