Temperature Converter

A temperature converter changes a value between Celsius, Fahrenheit, and Kelvin using the correct scale-specific formulas. It helps when reading weather forecasts from another country, following a recipe written in Fahrenheit, converting lab notes to Kelvin, checking device readouts, or comparing scientific and everyday temperature references. Temperature is different from length or weight because the scales do not share the same zero point: Fahrenheit and Celsius require both multiplication and an offset, while Kelvin starts at absolute zero. This tool gives a numerical unit conversion, not a judgment about comfort, fever, food safety, or equipment calibration. For medicine, engineering, laboratory thresholds, and safety-critical decisions, use the required professional standard and rounding rules.

Temperature scales are linear and share two anchor points but with different zeros and step sizes, so each conversion is one multiplication and one addition. The Celsius scale is defined by the relation C = K - 273.15 in the SI, where the kelvin itself is defined since the 2019 SI revision in terms of the fixed numerical value of the Boltzmann constant k = 1.380649e-23 J/K. Fahrenheit is defined by F = C * 9/5 + 32, and Rankine by R = F + 459.67 = K * 9/5. The Reaumur scale, occasionally seen in older European texts, follows Re = C * 4/5. The converter applies these formulas directly: C = (F - 32) * 5/9, K = C + 273.15, F = K * 9/5 - 459.67, and chained conversions (for example Fahrenheit to Kelvin) pass through Celsius rather than composing two floating-point rounded coefficients. Round-trip precision matters because IEEE 754 doubles cannot represent 1/9 or 5/9 exactly, so a value converted F -> C -> F may differ from the original by a few units in the last place; the tool keeps full double precision internally and only rounds for display. Physical floors are enforced: absolute zero is exactly -273.15 C, 0 K, or -459.67 F, and Kelvin or Rankine readings cannot be negative in any physically meaningful context. ISO 80000-5 defines the symbols and abbreviations used (degree Celsius is a single symbol, the K for kelvin carries no degree sign), which the UI follows so a copy-pasted value stays standards-compliant when dropped into a lab notebook or engineering report.

• Reference: kelvin defined via Boltzmann constant k = 1.380649e-23 J/K since the 2019 SI revision; Celsius = K - 273.15
• Core formulas: F = C * 9/5 + 32; C = (F - 32) * 5/9; K = C + 273.15; R = F + 459.67 = K * 9/5; Re = C * 4/5
• Chained conversions route through Celsius to avoid composing two IEEE 754 rounding errors into the displayed result
• Absolute zero floor: -273.15 C = 0 K = -459.67 F = 0 R; negative Kelvin or Rankine is rejected
• Floating-point note: 1/9 and 5/9 are not exact in double precision, so F -> C -> F may differ by a few units in the last place
• Display vs storage: full double precision is kept internally, rounding is only applied to the rendered string
• Notation: ISO 80000-5 — degree Celsius as a single symbol, kelvin without the degree sign, Fahrenheit and Rankine with the degree sign