Instantly convert between Celsius, Fahrenheit, Kelvin, Rankine, Délisle, Newton, Réaumur and Rømer. Real-time results for all 8 temperature scales.
Temperature is a fundamental physical quantity that measures the thermal energy of matter. Over the centuries, scientists and engineers devised multiple scales, each anchored to different reference points. Today, Celsius (°C) is used worldwide for everyday weather and cooking, Fahrenheit (°F) remains dominant in the United States, and Kelvin (K) is the SI base unit used in science and engineering. Three historical scales — Délisle, Newton, and Réaumur — are no longer in official use, but appear in historical texts and are interesting for their mathematical elegance. Rankine, an absolute scale analogous to Kelvin but based on Fahrenheit-sized degrees, is used in some US thermodynamic engineering contexts.
Unlike all other unit conversions, temperature is an affine (non-linear) transformation — you cannot simply multiply or divide by a single factor. Each scale has its own zero point and degree size. This converter routes every conversion through Celsius as an intermediate step, applying the correct two-step affine formula automatically. Enter a value, choose the input scale, and all eight outputs update in real time.
Because temperature scales have different zero points (offsets), every conversion is a two-step affine transformation: first convert the source value to Celsius, then convert from Celsius to the target scale. The general form is:
Where f_toC converts the source scale to Celsius, and f_fromC converts Celsius to the target scale. The formulas for each scale are listed in the panels below.
→ Celsius (input)
Celsius → (output)
Key Landmarks
Normal human body temperature is 37 °C (98.6 °F, 310.15 K). A fever is generally considered to start at 38 °C (100.4 °F). These values are critical reference points used daily by healthcare professionals across the UK and Commonwealth countries.
A cold British winter day might be −5 °C (23 °F, 268.15 K), while a hot summer in London can reach 38 °C (100.4 °F). In Réaumur, the comfortable spring temperature of 20 °C equals 16 °Ré — a unit that was once standard in French and Russian scientific writing.
British and Irish recipes sometimes use gas marks alongside Celsius. Gas Mark 6 corresponds to 200 °C (392 °F, 473 K). Gas Mark 7 is 220 °C (428 °F). For baking bread at 230 °C that is 446 °F, 503.15 K, or in Newton's scale: approximately 75.9 °N.
Absolute zero, the theoretical minimum temperature, is 0 K = −273.15 °C = −459.67 °F = 0 °R. At this temperature all classical atomic motion ceases. It was first defined using the Kelvin scale, established by Lord Kelvin (William Thomson) at the University of Glasgow in 1848.
Enter the temperature value you wish to convert in the input field. Positive and negative numbers, as well as decimal values, are all supported.
Select the source scale from the dropdown. All 8 scales are listed with their full name and symbol.
The converter first calls the toC function for your source scale to obtain the Celsius equivalent, then calls the fromC function for each target scale.
Results are grouped into two sections: Common Scales (Celsius, Fahrenheit, Kelvin, Rankine) and Historical & Scientific Scales (Délisle, Newton, Réaumur, Rømer).
All conversions update instantly as you type — no button press required. Very large or very small values are displayed in scientific notation.
Quick mental shortcut for Celsius ↔ Fahrenheit: °F ≈ °C × 2 + 30 (not exact but handy in everyday life). The exact formula is °F = °C × 9/5 + 32.
Kelvin has no degree symbol — it is simply written 'K', never '°K'. One kelvin is the same size as one degree Celsius; only the zero point differs (by 273.15).
Rankine is the absolute Fahrenheit scale: 0 °R = absolute zero, and one Rankine degree equals one Fahrenheit degree. It appears in some US aerospace and chemical engineering calculations.
Délisle scale runs in reverse: higher Délisle values correspond to lower temperatures. Water boils at 0 °De and freezes at 150 °De — the opposite of Celsius!
The Rømer scale, devised by Danish astronomer Ole Rømer in 1701, is the oldest surviving temperature scale. It inspired Fahrenheit, who visited Rømer and later adapted his instrument.
Most unit conversions (length, mass, area) are linear: one unit is simply a fixed multiple of another. Temperature scales, however, have different zero points — 0 °C is not the same physical state as 0 °F (which is 32 degrees below the freezing point of water). Because the zeros don't align, you must apply an offset (addition or subtraction) in addition to scaling the degree size. This makes temperature an affine transformation rather than a purely linear one.
Kelvin and Celsius use the same degree size (1 K = 1 °C in terms of interval), but have different zero points. 0 °C = 273.15 K (the freezing point of water). 0 K = −273.15 °C (absolute zero, the coldest possible temperature). Kelvin is the SI base unit for thermodynamic temperature, used in physics, chemistry, and engineering calculations where absolute values matter.
Celsius and Fahrenheit are equal at −40°. That is, −40 °C = −40 °F. This is the only point where the two scales intersect, because at that value the 9/5 scaling factor and the +32 offset happen to cancel each other exactly.
The United States adopted the Fahrenheit scale in the late 18th century, before the Metric System was established. It remained deeply embedded in American customary units, everyday language and infrastructure. While Celsius is used in US science, medicine and engineering, Fahrenheit persists in public weather forecasts, home thermostats and colloquial temperature references.
These four scales are no longer used in practice. They appear primarily in the study of the history of science and metrology. Réaumur was widely used in 18th and 19th century European chemistry and was the standard in parts of France and Russia. Newton's scale was one of the first attempts at a scientific temperature measurement. Rømer's scale inspired Fahrenheit. Délisle was used at the Paris Observatory for decades.
Both Rankine and Kelvin are absolute temperature scales (their zero is absolute zero, −273.15 °C / −459.67 °F). The difference is the degree size: one Rankine degree equals one Fahrenheit degree (5/9 of a kelvin), while one kelvin equals 9/5 Rankine degrees. To convert: K = °R × 5/9, or °R = K × 9/5. Rankine is used in some US thermodynamic engineering, particularly in older HVAC and aerospace standards.
All conversions are calculated using exact affine formulas. Values are rounded to 8 significant figures for display. For precision scientific or engineering work, always verify results with calibrated instrumentation.