About this tool
The Temperature Converter changes one reading into all five temperature scales at once: Celsius, Fahrenheit, Kelvin, Rankine and Réaumur. Enter a value, choose the scale you measured in, and every other scale updates live in the results grid.
Unlike length or weight, temperature scales are affine — they have different zero points, so a simple ratio is wrong. The tool first converts your input to Celsius using the exact linear relations: from Fahrenheit C = (F − 32) × 5/9, from Kelvin C = K − 273.15, from Rankine C = (R − 491.67) × 5/9, and from Réaumur C = Ré × 5/4. It then converts Celsius to each target: F = C × 9/5 + 32, K = C + 273.15, R = (C + 273.15) × 9/5, and Ré = C × 4/5.
Because Kelvin and Rankine are absolute scales, the tool rejects any input that falls below absolute zero (0 K, 0 °R, or −273.15 °C) rather than showing a physically impossible result. Everything runs locally in your browser with no network calls, and values are rounded to two decimals for readability.
Frequently asked questions
Why can't I just multiply to convert temperature?
Temperature scales have different zero points, so they are affine, not proportional. You must add or subtract an offset (like 32 or 273.15) as well as scale, which is why the tool routes every conversion through Celsius using the exact linear formulas.
What is Rankine and Réaumur?
Rankine (°R) is an absolute scale using Fahrenheit-sized degrees, where 0 °R is absolute zero (491.67 °R = 32 °F). Réaumur (°Ré) sets water's freezing point at 0 and boiling at 80 degrees; it appears in some older European and culinary references.
What happens if I enter a temperature below absolute zero?
The tool blocks it. Any Kelvin or Rankine value below 0, or any input that computes to below −273.15 °C, is flagged as invalid because no matter can be colder than absolute zero.
How precise are the results?
Conversions use the exact defining constants (5/9, 9/5, 273.15, 491.67) at full double precision, then round the displayed value to two decimal places so the grid stays easy to read.
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