Fahrenheit to Celsius Converter

C = (F − 32) × 5⁄9 is the algebraic inverse of F = (C × 9⁄5) + 32: subtract 32 to shift the Fahrenheit origin (water freezing) down to the Celsius origin, then multiply by 5⁄9 to scale the smaller Fahrenheit degree up to the larger Celsius degree. The 5⁄9 ratio is the inverse of 9⁄5 = 1.8: each Celsius degree is 9⁄5 of a Fahrenheit degree, so each Fahrenheit degree is 5⁄9 = 0.555… of a Celsius degree. The fractional answer is exact in rational arithmetic; this page rounds to four decimal places for display.

Daniel Gabriel Fahrenheit's 1724 paper (Phil. Trans. Roy. Soc. London 33, no. 381, pp. 1–3) calibrated the upper end of his scale at 96 °F at human body temperature — a number chosen partly for arithmetic convenience (96 = 32 + 64, with 32 °F as the freezing point and 64 = 2⁶ as the span, allowing easy halving on a thermometer ruler). Later 19th-century thermometry, particularly Carl Reinhold August Wunderlich's 1868 axillary-temperature survey of around 25,000 patients ('Das Verhalten der Eigenwärme in Krankheiten'), refined the average to 37 °C / 98.6 °F. Modern medical research (Mackowiak et al. 1992, JAMA 268(12):1578–1580) actually finds the population mean closer to 36.8 °C ≈ 98.2 °F, with substantial individual and time-of-day variation; the 98.6 figure is conventional, not strictly accurate.

Absolute zero is the lower limit of the thermodynamic temperature scale — the point at which classical thermal motion ceases — and equals exactly −459.67 °F = −273.15 °C = 0 K by definition. From 1954 to 2019, the kelvin was defined by setting the triple point of water to exactly 273.16 K (a property of a specific material under specific conditions). The 26th CGPM (November 2018) approved a redefinition that took effect on 20 May 2019: the kelvin is now defined by fixing the Boltzmann constant kB to exactly 1.380649×10⁻²³ J·K⁻¹, anchoring temperature to a fundamental physical constant rather than a material property — making realisations of the kelvin reproducible without reference to a particular sample of water.

Kelvin and Rankine are linear shifts away from Celsius and Fahrenheit respectively, so deriving them is straightforward arithmetic without a separate tool: K = °C + 273.15, and R = °F + 459.67. This page is scoped to the everyday Celsius–Fahrenheit conversion that everyone needs (US weather, oven settings, body temperature). For scientific work that requires the absolute scales, the conversion is one extra add: F → C produces a Celsius value, and adding 273.15 yields kelvin (K = °C + 273.15). The Rankine equivalent is T(°R) = T(°F) + 459.67 — Rankine and Fahrenheit share degree size, so only the origin shifts. The 4-decimal display precision is enough for almost every practical use; ultra-precise work calls for the original formula directly with full IEEE-754 double precision.

The Celsius result and the absolute-zero warning region are inside an aria-live="polite" region — the W3C WCAG Success Criterion 4.1.3 (Status Messages, introduced in WCAG 2.1, Recommendation 5 June 2018; carried unchanged into WCAG 2.2, Recommendation 5 October 2023) pattern. Polite live regions queue announcements after any speech in progress, so updating the Fahrenheit input announces the new Celsius value without interrupting the user mid-sentence. Screen readers (NVDA, JAWS, VoiceOver) consume the live region automatically; the user does not need to do anything else.