Celsius to Fahrenheit (°C to °F)

The degree Celsius (°C) is an SI-derived unit of temperature defined by the equation t/°C = T/K − 273.15, where T is the thermodynamic temperature in kelvin (the SI base unit). The Celsius and kelvin scales differ only by a fixed offset of exactly 273.15: a temperature change of 1 °C is identical to a temperature change of 1 K, but the absolute reference point of 0 °C is the freezing point of water at standard atmospheric pressure (273.15 K) rather than absolute zero. The two scales are interconvertible by addition or subtraction without any multiplicative factor, distinguishing the Celsius/kelvin pair from the Fahrenheit/Rankine pair (where the same fixed-offset relationship holds with a different offset) and from any pair across the two systems (where the conversion is affine: scale by 9/5 or 5/9 plus an offset). Since the 2019 SI redefinition, the kelvin — and so the Celsius scale — is defined by fixing the Boltzmann constant k at exactly 1.380649 × 10⁻²³ J/K. The earlier definitional anchor at the triple point of water (exactly 273.16 K, or 0.01 °C, by the 1954 CGPM) survives as a useful realisation rather than as a definition. The Celsius scale is part of the International System of Units in the sense that it is fully derivable from kelvin, even though kelvin is the SI base unit for temperature. The unit is recognised by every national metrology institute and is the legal scale for trade, weather reporting, medicine and engineering across nearly every country in the world.

The Celsius scale is the only major temperature scale whose original direction was the opposite of its modern form. Anders Celsius, professor of astronomy at Uppsala University, presented his thermometric scale to the Royal Swedish Academy of Sciences in 1742 with 0° marked at the boiling point of water and 100° at its freezing point — an inversion that placed warm temperatures at low numbers and cold at high. Celsius's choice was deliberate: an inverted scale avoided negative readings during the Swedish winter, when temperatures regularly dropped below the freezing point of water but rarely above its boiling point. The scale was used in this inverted form during Celsius's lifetime and on the thermometers built at Uppsala under his direction. Celsius died of tuberculosis in 1744, two years after publishing the scale, and within a year of his death the inversion had been reversed by his colleagues at Uppsala. The reversal is conventionally credited to the botanist Carl Linnaeus in correspondence dated December 1745, although the Swedish instrument maker Daniel Ekström and the astronomer Mårten Strömer have both been put forward as alternative or co-authors of the change. The reversed scale, with 0° at the freezing point of water and 100° at its boiling point, was the form that spread through European science across the second half of the eighteenth century. The name "centigrade" — from the Latin centum gradus, "a hundred steps" — was the dominant English term for the scale into the mid-twentieth century. The 9th General Conference on Weights and Measures (CGPM) renamed the scale "Celsius" in 1948, partly to honour the original author and partly to avoid confusion with the centesimal grade (gon, gradian — 1/100 of a right angle) used in French surveying. The 10th CGPM in 1954 anchored the scale to the triple point of water at exactly 273.16 K (0.01 °C), and the 2019 SI redefinition tied the kelvin — and transitively the Celsius scale — to the Boltzmann constant fixed at exactly 1.380649 × 10⁻²³ J/K.

Global weather reporting is the Celsius scale's single largest public-facing domain. Every national meteorological service except the US National Weather Service reports temperature in degrees Celsius: the UK Met Office, Météo-France, the Deutscher Wetterdienst, the Japan Meteorological Agency, the China Meteorological Administration, the Australian Bureau of Meteorology and the World Meteorological Organization (WMO) all denominate surface-air-temperature observations and forecasts in °C. The WMO's Manual on Codes (WMO-No. 306) specifies Celsius as the international meteorological standard for SYNOP, METAR, TAF and upper-air radiosonde reports, and even US-domestic aviation weather reports use Celsius for terminal aerodrome forecasts (TAFs) and METARs while the same airport's public-facing weather page reports in Fahrenheit. Medical practice worldwide uses Celsius for body temperature. The WHO's 2008 Pocket Book of Hospital Care for Children, the British National Formulary, the European Medicines Agency clinical guidelines and clinical-laboratory norms across the world denominate normal core body temperature at 37.0 °C, with thresholds at 38.0 °C (low-grade fever), 39.0 °C (significant fever) and 40.0 °C (medical emergency in adults, often life-threatening in infants). Even US clinical practice has been moving toward Celsius for inpatient charting since the early 2000s, although outpatient consumer thermometers continue to display Fahrenheit on the US-domestic retail market. WHO Cold Chain handling of vaccines specifies storage in degrees Celsius (2 to 8 °C standard refrigeration; −25 to −15 °C for frozen mRNA vaccines). European, Asian and Australian cooking: domestic ovens, commercial bakeries and food-safety regulators denominate temperatures in °C. EU Regulation (EC) No 178/2002 on food safety, and the UK Food Standards Agency's Cooking Temperatures guidance, both denominate cooking and holding temperatures in Celsius (cook poultry to 75 °C, hold hot food above 63 °C, cold-chain food below 8 °C). UK cookery shows occasional historical "gas mark" references — a parallel scale running gas mark 1/4 ≈ 110 °C through gas mark 9 ≈ 240 °C, inherited from pre-decimal British gas-cooker dial markings — but modern UK recipes give Celsius as primary. HVAC and building services: ASHRAE Standard 55 (Thermal Environmental Conditions for Human Occupancy) specifies the comfortable indoor air-temperature range as 20–25 °C in winter and 23–27 °C in summer, the values that underlie the BREEAM (UK), LEED (international metric specs) and Passivhaus building-certification schedules. Domestic thermostats in metric countries (Honeywell, Nest, Tado, Hive in EU/UK markets) display in °C by default, with 18–22 °C the residential heating setpoint range. Industrial process control: the IEC 60584 thermocouple standard, the ISO 17025 calibration-laboratory accreditation framework and most national-standard reference thermometers calibrate in Celsius. Pharmaceutical cold-chain logistics under WHO Pre-Qualification (PQ) standards specify transport temperatures in °C, with deviations triggering regulatory reporting. Power-station condenser water, refining process streams and food-processing pasteurisation cycles are all denominated in degrees Celsius outside US-domestic facilities.

The degree Fahrenheit (°F) is a unit of temperature defined relative to the Celsius scale by the equation t/°F = (t/°C × 9/5) + 32, equivalent to t/°F = (T/K × 9/5) − 459.67, where T is the thermodynamic temperature in kelvin. It is one of two scales using a fixed-offset relationship to a thermodynamic-temperature scale (the other being Rankine, which is to Fahrenheit what kelvin is to Celsius), and conversion between Fahrenheit and Celsius is a true affine transformation: a multiplicative factor of 9/5 and an offset of 32, both required, distinguishing this conversion from the addition-only Celsius/kelvin pair. The two scales intersect at exactly −40°: −40 °F = −40 °C is the only point of equality between them, and the value is the standard mental check for translation between the systems. The Fahrenheit scale is not part of the SI; it is recognised by the US National Institute of Standards and Technology as a customary unit under the same Federal Register notice 24 FR 5445 that recognises the pound and the inch. Modern Fahrenheit retains the original 1724 reference points (32 °F freezing, 212 °F boiling at 1 atm) but has been recalibrated against the SI-anchored Celsius scale rather than against any independent Fahrenheit-specific reference; the unit's anchor since the 2019 SI redefinition is therefore the Boltzmann constant, transitively through Celsius and kelvin.

Daniel Gabriel Fahrenheit (1686-1736) was a German-born instrument maker who spent his working life in the Dutch Republic. Born in Danzig (now Gdańsk, Poland) into a Hanseatic merchant family, he was apprenticed in Amsterdam after his parents' deaths in 1701 and spent the next two decades developing the mercury thermometer — a critical advance over the alcohol thermometers of the period, since mercury's uniform thermal expansion gave linear, readable scales over a much wider temperature range than alcohol could deliver. Fahrenheit presented his temperature scale to the Royal Society of London in 1724, and the scale's three-reference-point calibration is its distinctive structural feature. He set 0 °F at the lowest temperature he could reproducibly achieve in his Danzig workshop — a brine-ice-salt freezing mixture that bottoms out at about −17.8 °C in modern terms — chosen because it was the coldest temperature any thermometer of the era could be safely calibrated against. He set 32 °F at the equilibrium of ice and pure water, and 96 °F at the temperature of a healthy human body. The three-point calibration produced a scale on which water boiled at 212 °F, and the freezing-to-boiling interval came out to 180 degrees — a number Fahrenheit did not deliberately choose but that subsequent users have noted is highly composite, with eighteen whole-number divisors that made subdividing temperature ranges easy before digital instruments. The scale was the dominant temperature reference across the British Empire, the United States and much of northern Europe through the nineteenth century. The geographic footprint shrank rapidly across the twentieth: most of continental Europe switched to Celsius by the 1960s, the United Kingdom completed its weather-forecasting switch through the 1970s and 1980s, and by 2026 only the United States and a handful of Caribbean and Pacific dependencies — Belize, the Bahamas, the Cayman Islands, Palau, the Marshall Islands and the Federated States of Micronesia — retain Fahrenheit as the primary public-facing temperature scale.

US weather forecasting is the Fahrenheit scale's largest public-facing domain. The US National Weather Service (NWS), the National Oceanic and Atmospheric Administration (NOAA), the Weather Channel, AccuWeather and Weather Underground all denominate public-facing temperature reports in degrees Fahrenheit, and US local-television weather presentation is uniformly in °F. The internal aviation-weather products generated by the same agencies — METAR and TAF reports for airports — use Celsius per WMO international convention, so US air-traffic control and pilot weather briefings work in Celsius even while the same agency's public-facing forecast on weather.gov for the same airport is in Fahrenheit. The hybrid is unique to the US among major economies. US-domestic cooking and food preparation: domestic and commercial ovens sold in the United States are calibrated in Fahrenheit, with standard baking ranges 350 °F (177 °C, the all-purpose default for cookies, casseroles and roasted vegetables), 375 °F (190 °C, for cakes and quick breads), 400 °F (204 °C, for roasting and crisping) and 450 °F (232 °C, for pizza and bread crusts). The FDA's Food Code, the USDA Food Safety and Inspection Service guidance and the ServSafe foodservice-handler certification all denominate cooking-and-holding temperatures in °F (cook poultry to 165 °F internal, hold hot food above 135 °F, cold-chain food below 41 °F). US cooking publications give temperatures in Fahrenheit by default, with Celsius as a parenthetical for international readers. US HVAC and thermostats: the standard residential heating-and-cooling setpoint on US thermostats is 68–72 °F in winter and 76–78 °F in summer, the values that map to ASHRAE Standard 55's 20–25 °C and 23–27 °C metric ranges. Honeywell, Nest, ecobee and Trane domestic thermostats sold in the US default to °F display; the same units sold in EU/UK markets default to °C. US medical and consumer thermometers: the conventional "normal" adult body-temperature reference of 98.6 °F is the figure printed on US drugstore consumer-grade ear, oral and forehead thermometers. The reference traces to the German physician Carl Reinhold August Wunderlich's 1868 study of about 25,000 Leipzig patients, in which he measured an average axillary temperature of 37.0 °C (98.6 °F); modern studies — including Mackowiak et al. (1992, JAMA) and Protsiv et al. (2020, eLife) — have found the population mean has drifted downward by about 0.5 °F (0.3 °C) since the nineteenth century, possibly because of declining chronic-infection rates with antibiotics and vaccines, putting the modern healthy-adult average closer to 97.5–97.9 °F. Caribbean and Pacific Fahrenheit holdouts: Belize, the Bahamas, the Cayman Islands, Palau, the Marshall Islands and the Federated States of Micronesia retain Fahrenheit as the primary public-facing temperature scale, in most cases through inherited US-territory or US-association status. Tourism-industry weather reports and consumer-grade thermometer retail in these jurisdictions parallel US conventions, with Celsius offered as a secondary unit on weather apps for international visitors. UK partial holdout: UK weather forecasting fully transitioned to Celsius through the 1970s and 1980s, but Fahrenheit retains a parallel cultural existence in tabloid summer-weather headlines ("it's going to be 80 degrees!") and in older-generation conversational temperature reporting. UK consumer thermometers and digital body-temperature monitors offer Fahrenheit as a settable display alternative, and the UK Met Office's public-forecast app provides a °F toggle for users who prefer it.