Temperature Converter

Why temperature conversion is different from every other unit

Converting metres to feet is a single multiplication, because both measure the same thing from the same zero. Temperature is not like that. The Celsius and Fahrenheit scales place their zeros in different physical spots and step in differently sized degrees, so converting between them requires both a scale factor and an offset — a line with a slope and an intercept, not a simple proportion. This converter handles five scales at once (Celsius, Fahrenheit, Kelvin, Rankine, and Réaumur) by routing every value through one common reference, the Kelvin scale, so any input can be expressed in any of the others instantly as you type.

How the five scales relate

Notice the two patterns. Celsius, Kelvin, and Réaumur share the same size step between freezing and boiling-just spread over 100, 100, and 80 degrees respectively-while Fahrenheit and Rankine use a smaller degree (180 between freezing and boiling). Kelvin and Rankine are absolute scales: their zero is absolute zero, the coldest possible temperature, so they never go negative for any real substance.

The math, through Kelvin

Internally the tool converts your input to Kelvin, then out to each target. The two conversions you will use most are:

• Fahrenheit to Celsius: C = (F − 32) × 5/9
• Celsius to Fahrenheit: F = C × 9/5 + 32

The 5/9 factor accounts for the different degree size (100 Celsius degrees span the same range as 180 Fahrenheit degrees, and 100/180 = 5/9), and the 32 handles the offset between the two zeros. The tool also displays the live formula set for whichever scale you last edited, so you can see exactly how each result was produced rather than trusting a black box.

A worked example: normal body temperature

Type 98.6 into the Fahrenheit box. The tool computes C = (98.6 − 32) × 5/9 = 66.6 × 5/9 = 37 °C, then K = 37 + 273.15 = 310.15 K, °R = 310.15 × 9/5 = 558.27, and °Ré = 37 × 4/5 = 29.6. Every box fills in at once. The single most common temperature-conversion mistake is multiplying 98.6 by 5/9 and forgetting to subtract 32 first — that gives 54.8, a meaningless number. The offset is not optional.

Built-in reference points

The tool includes a reference table showing six landmark temperatures across all five scales at a glance: absolute zero (−273.15 °C / 0 K), water freezing (0 °C), water boiling (100 °C), body temperature (37 °C), room temperature (22 °C), and a moderate oven (180 °C). This is handy for a quick sanity check — if your conversion result lands far from a nearby landmark, you have likely entered the wrong scale or sign. The table is also a small reference card in its own right: at a glance you can confirm that a 180 °C oven is 356 °F, or that comfortable room temperature is about 72 °F, without typing anything.

Why everything routes through Kelvin

Rather than store a separate rule for every one of the twenty possible scale-to-scale conversions, the tool defines each scale only by how to reach Kelvin and how to leave it. To convert Fahrenheit to Réaumur, for instance, it goes Fahrenheit → Kelvin → Réaumur — two steps it already knows — instead of needing a bespoke Fahrenheit-to-Réaumur formula. This "hub" design is why all five boxes can update from any one of them the moment you type, and why adding another scale would only require two more small functions rather than rewriting the whole table.

Practical tips and common mistakes

• Kelvin has no degree sign and no negatives. Write "300 K," not "300 °K," and remember that any Kelvin or Rankine result below zero signals a data-entry error — nothing can be colder than absolute zero.
• Watch the sign on cold values. −40° is the one temperature where Celsius and Fahrenheit coincide (−40 °C = −40 °F); it makes a great quick test that a converter is wired correctly.
• For mental Fahrenheit-to-Celsius estimates, subtract 30 and halve: 70 °F → (70−30)/2 = 20 °C (actual 21.1 °C). Close enough for weather, but use the tool when precision matters.
• Cooking is unforgiving. A 180 °C oven is 356 °F, not 360 — small differences matter for baking, so convert rather than approximate.
• Rankine and Réaumur are niche (some US engineering, some traditional cheese-making) but included for completeness; you will mostly live in Celsius, Fahrenheit, and Kelvin.

The conversion runs entirely in your browser with fixed formulas, so nothing you type is uploaded and the same input always returns the same exact answer, even offline.