Convert between Celsius, Fahrenheit, Kelvin, and Rankine.
Formulas: F = C × 9/5 + 32. K = C + 273.15. °R = (C + 273.15) × 9/5. Absolute zero = −273.15 °C = 0 K.
Temperature is the single most universally measured physical quantity in everyday life — and the world cannot agree on a unit. The metric world uses Celsius (boiling water at 100 °C, freezing at 0 °C); the United States and a handful of countries (Bahamas, Belize, Cayman Islands, Liberia, Palau, Marshall Islands) use Fahrenheit (boiling 212 °F, freezing 32 °F); science globally uses Kelvin (absolute zero at 0 K, water freezing at 273.15 K); engineering thermodynamics in the English-speaking world historically used Rankine (Fahrenheit's absolute equivalent, 0 °R = absolute zero, water freezing at 491.67 °R). Anyone reading a recipe, a US weather report, a Russian engineering paper, or a thermodynamics textbook eventually needs to convert between two of these. The conversions are simple linear transformations, but the offsets and scale factors are easy to mis-remember (was it C = (F − 32) × 5/9 or × 9/5?), and the boundary case of absolute zero in the wrong sign trips up novices. This calculator converts any of the four common units to all the others in one shot, and adds a context note that classifies the value into a familiar regime — refrigerator, comfort, fever, sauna, oven, plasma — so the abstract number lands in a recognisable scale.
The four scales are linearly related by two reference points each. Celsius–Fahrenheit: F = C × 9/5 + 32 (or equivalently F = C × 1.8 + 32). The 32 offset reflects the freezing point of water in F (32 °F), and the 9/5 ratio reflects the difference in scale-step sizes (the 100 °C between freezing and boiling becomes 180 °F over the same range). Celsius–Kelvin: K = C + 273.15. Same scale-step (1 °C step = 1 K step), different zero (Kelvin starts at absolute zero, Celsius at the freezing point of water). Celsius–Rankine: °R = (C + 273.15) × 9/5 = K × 9/5. Rankine is the absolute version of Fahrenheit, so it shares F's scale-step (1 °R step = 1 °F step) and Kelvin's zero (absolute). The calculator picks the input unit, converts internally to Celsius (the metric reference), then computes the other three from C. The classification note is keyed off Celsius: below absolute zero impossible; below −40 extreme cold; below 0 sub-freezing; up to 4 refrigerator; up to 35 comfort; up to 100 hot/boiling; up to 1000 industrial; above plasma/stellar.
Two inputs: a numeric value and a unit dropdown (Celsius, Fahrenheit, Kelvin, Rankine). The defaults — 100 in Celsius — represent the boiling point of water at 1 atm, and the result panel shows 212 °F, 373.15 K, 671.67 °R alongside the regime note. Change the input to 32 °F and watch all four panels update: 0 °C, 273.15 K, 491.67 °R. Try absolute zero: enter 0 K and the result is −273.15 °C, −459.67 °F, 0 °R — and the note flags the impossibility of going lower. The four KPI tiles are deliberately given equal visual weight; there is no privileged "target" unit, because the right unit depends on your audience.
A user in Boston reads a French recipe that calls for an oven at 180 °C. Enter 180, Celsius: results — 356 °F, 453.15 K, 815.67 °R. The note: "Industrial / cooking / metalwork." The 356 °F result matches the standard moderate-oven setting on US ranges, so the recipe transfers cleanly. Now consider a thermodynamics problem: a thermal cycle has a hot reservoir at 1500 R and a cold one at 500 R; what is the Carnot efficiency? Enter 1500 °R: 833.33 K, 560.18 °C, 1040.33 °F — the cold side at 500 °R: 277.78 K, 4.63 °C, 40.33 °F. Carnot efficiency = 1 − T_cold/T_hot in absolute units = 1 − 500/1500 = 67 %. The calculator does not compute the efficiency directly, but it produces the absolute-temperature inputs the formula needs. Or a weather example: −40 °C is the famous point where Celsius and Fahrenheit converge — enter it and observe both readouts say −40, the unique fixed point of the affine transformation between the two scales.
First, mixing additive and multiplicative steps. The conversion C → F has both an offset (32) and a scale (9/5); applying them in the wrong order gives a value off by the magnitude of the offset times the scale, which can be tens of degrees. Always do scale-then-offset: multiply by 9/5 first, add 32 second. Second, treating Kelvin as a temperature with units of "degrees". Kelvin is written without the degree sign (273.15 K, not 273.15 °K), reflecting its status as an SI base unit; the older "degrees Kelvin" notation was abolished in 1968. Third, applying the Celsius–Fahrenheit conversion to temperature differences rather than absolute readings. A 10 °C rise is a 10 × 9/5 = 18 °F rise, not 10 × 9/5 + 32 = 50 °F — for differentials, the offset is irrelevant. Fourth, comparing absolute and Celsius zeroes. A 5 K change and a 5 °C change describe the same temperature step (the Kelvin and Celsius scales share their step size), but a 5 K reading and a 5 °C reading differ by 273.15 — the former is far below human comfort, the latter is a cool spring morning. Fifth, going below absolute zero. Negative Kelvin values are mathematically possible in the formulas but physically impossible (and indeed the calculator flags this in the note). Negative absolute Rankine values have the same problem.
Several lesser-known scales exist. Réaumur (used in 18th–19th century Europe) puts freezing at 0 °Ré and boiling at 80 °Ré. Delisle (used in 18th century Russia) inverts: 150 °De at freezing, 0 °De at boiling. Both are now historical curiosities. Newton's degree had 33 between freezing and boiling. Romer had 7.5 at freezing, 60 at boiling. None of these survived into modern engineering use; the calculator covers the four still in active use. Astronomical and physical extremes stretch the regime note: the temperature of the cosmic microwave background is 2.725 K (−270.4 °C); the sun's core reaches 1.5 × 10⁷ K (15 million °C); the Planck temperature, the theoretical upper limit of physical temperature, is 1.4 × 10³² K. The note classifies values up to 1 000 °C; above that the regime is solidly industrial and the user already knows the context. Wind chill and heat index are derived metrics that combine temperature with wind speed or humidity respectively to express "feels like" — they are different from temperature itself and require a separate calculator. The temperature converter implements only the linear conversions between the four standard scales, which is what most users actually need most of the time.