Temperature Converter Online Free
Type a temperature in Celsius, Fahrenheit or Kelvin and get all three conversions instantly.
Why temperature conversion needs offsets, not just multipliers, and how this tool handles eight scales through one hub unit
Every other converter on this site works by multiplying a value by a fixed factor. Temperature cannot work that way, and the reason is structural, not a technicality. A length unit’s zero point is always the same physical nothing, zero metres is zero feet is zero of anything. Temperature scales do not share a zero point. Zero on the Celsius scale is the freezing point of water, zero on the Fahrenheit scale is a brine mixture Daniel Fahrenheit used as a lab reference, and zero on the Kelvin scale is absolute zero itself. Converting between them needs both a multiplication for the different sized degree steps and an addition or subtraction for the different starting points.
All of this runs locally in your browser. No temperature value you type is sent anywhere.
Celsius as the hub, and why offsets are unavoidable
The tool converts any input to Celsius first, then from Celsius to whatever output scale you want. Every formula in both directions carries an addition or subtraction term alongside its multiplication, which is the tell that distinguishes an affine conversion from a pure ratio.
(F - 32) * 5 / 9. The subtraction of 32 shifts Fahrenheit’s zero point to align with Celsius’s before the ratio 5/9 rescales the degree size.
K - 273.15, a pure offset with no multiplication at all, since a kelvin and a Celsius degree are the exact same size.
C * 9/5 + 32, rescale first, then shift the zero point.
Notice every single branch pairs a multiply with an add or subtract, except Kelvin to Celsius, which is offset only because the two scales share the same degree size. That asymmetry is the whole reason this converter cannot reuse the simple factor table structure that the length, weight or pressure converters use.
All eight scales and their exact formulas from Celsius
| Scale | Symbol | Formula from °C |
|---|---|---|
| Celsius | °C | c (reference) |
| Fahrenheit | °F | c × 9/5 + 32 |
| Kelvin | K | c + 273.15 |
| Rankine | °R | (c + 273.15) × 9/5 |
| Delisle | °De | (100 − c) × 3/2 |
| Réaumur | °Ré | c × 4/5 |
| Rømer | °Rø | c × 21/40 + 7.5 |
| Newton | °N | c × 33/100 |
The lesser known scales in the list
Rankine
An absolute scale like Kelvin, but sized in Fahrenheit degree increments rather than Celsius ones. Its zero point is also absolute zero, which is why the formula from Celsius adds 273.15 before rescaling by 9/5, mirroring Fahrenheit’s degree size.
Delisle
An inverted historical French scale where 0 degrees represents boiling water and larger numbers mean colder temperatures, which is why its formula subtracts from 100 rather than adding an offset.
Réaumur
An 18th century European scale that set water’s freezing and boiling points 80 degrees apart instead of 100, giving it the 4/5 ratio relative to Celsius. It saw common use in parts of Europe into the 20th century.
Rømer
One of the earliest practical thermometer scales, predating both Celsius and Fahrenheit, with water freezing at 7.5 degrees, which is why every formula involving Rømer carries that specific additive offset.
Temperature scale references
- BIPM SI Brochure defines the kelvin as the SI base unit of thermodynamic temperature and fixes the exact Celsius offset of 273.15.
- NIST kelvin reference explains the 2019 redefinition of the kelvin via the Boltzmann constant.
- Historical thermometry references document the origins of the Rømer, Delisle, and Réaumur scales predating standardized SI temperature.
Temperature across three scales
Converting a US oven temperature in Fahrenheit into Celsius for a recipe written abroad, checking a scientific reading given in Kelvin against a familiar Celsius figure, translating an engine coolant spec listed in Rankine for aerospace documentation, working out a fever reading between Celsius and Fahrenheit for a household thermometer, and cross checking historical weather or lab records that used Réaumur or Rømer before Celsius became standard.
Questions About Temperature Converter Online Free
Results use the exact standard formulas (C to F: multiply by 9/5 then add 32). Accurate to 4 decimal places.
Absolute zero is 0 Kelvin, which equals minus 273.15 Celsius and minus 459.67 Fahrenheit.
Yes. Enter a negative number and the converter handles it correctly, including sub-zero Fahrenheit and Celsius values.
Normal body temperature is 37 Celsius, 98.6 Fahrenheit and 310.15 Kelvin.
Kelvin starts at absolute zero so there are no negative values. This makes it practical for thermodynamic calculations.
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