Kelvin equals Fahrenheit minus 32, times 5/9, plus 273.15. It is the only temperature conversion on this site that needs three operations, because it crosses both a different degree size and a different zero point at once.
About the units
The kelvin is the SI base unit, absolute and defined through the Boltzmann constant since 2019. Fahrenheit is a relative scale with an arbitrary zero — originally a brine freezing mixture — and a degree that is 5/9 the size of a kelvin. The two have almost nothing in common, which is why the conversion is awkward. The scale that bridges them is Rankine, which uses Fahrenheit-sized degrees measured from absolute zero: converting Fahrenheit to Rankine is a single addition of 459.67, and Rankine to kelvin is a single multiplication by 5/9.
The exact factor
The formula is K = (°F − 32) × 5/9 + 273.15, or equivalently K = (°F + 459.67) × 5/9, which is the shorter route through Rankine. The order matters: subtracting 32 must come before the multiplication, and adding 273.15 must come after. Reference points: absolute zero is −459.67 °F, water freezes at 32 °F or 273.15 K, and body temperature at 98.6 °F is 310.15 K. As with Celsius, temperature differences convert without offsets — a change of 9 °F is a change of 5 K.
Where you meet this conversion
American engineering thermodynamics, where gas properties and combustion calculations require absolute temperature while the surrounding documentation is in Fahrenheit. Cryogenic equipment sold into the United States carries both scales. Scientific instruments manufactured for American markets sometimes display Fahrenheit while their underlying calculations run in kelvin, and reconciling a displayed value with a computed one requires the full three-step conversion.