Temperature Converter

What Is Temperature, and What Do the Three Scales Measure?

Temperature measures how vigorously the particles in a substance are moving — not how much heat the object holds in total. A teaspoon of boiling water and a bathtub of boiling water sit at the same temperature; the bathtub simply contains far more energy. A thermometer reports the first quantity and says nothing about the second, and it is the first quantity that every scale on this page numbers.

The BIPM defines the kelvin as the SI unit of thermodynamic temperature, fixed by giving the Boltzmann constant the exact value 1.380 649 × 10⁻²³ joules per kelvin. Celsius and Fahrenheit are relabelings of that same quantity. NIST puts zero degrees Celsius at 273.15 K, makes one Celsius degree exactly one kelvin wide, and makes one Celsius degree equal to 1.8 Fahrenheit degrees.

Every number on this page follows from those two constants. 273.15 is the offset between Celsius and Kelvin; 1.8 — which is 9⁄5 written as a decimal — is the ratio between the sizes of a Celsius and a Fahrenheit degree.

Why the Three Scales Give Different Numbers

Each scale answers the same physical question and picks its own starting line.

Kelvin carries no degree sign — the BIPM writes 273.15 K, never 273.15 °K. The boiling row is deliberately approximate: NIST’s own wording is that water “boils at about 100 °C”, because the Celsius scale is now pinned to absolute zero and to the triple point of water at 273.16 K rather than to a kettle at sea level.

Where Kelvin Fits, and Where It Does Not

Kelvin exists so equations never have to handle a negative temperature. Gas laws, blackbody radiation and every thermodynamic relation are written in absolute temperature, where doubling the number really does double the thermal energy — a statement that is simply false in Celsius and Fahrenheit, whose zeros sit at arbitrary points on a thermometer.

This converter reports Kelvin on every run without being asked, because it costs one addition: a comfortable 20 °C room is 293.15 K, and 37 °C is 310.15 K. Forecasts and recipes are written in Celsius or Fahrenheit, so Kelvin sits in the breakdown rather than the headline — a design decision rather than an oversight.

How Do You Convert Temperature?

Every conversion here is one multiplication and one addition — performed in that order going forwards, and in the opposite order coming back.

The Formula, Written Out

Between freezing and boiling, Celsius counts 100 degrees while Fahrenheit counts 180, from 32 °F up to 212 °F. Divide 180 by 100 and you have 9⁄5, or 1.8: the number of Fahrenheit degrees inside one Celsius degree. The 32 is a separate fact doing a separate job — it records only that the two scales put their zeros in different places.

°F = °C × 9⁄5 + 32
°C = (°F − 32) × 5⁄9
K  = °C + 273.15

9⁄5 = 1.8   and   5⁄9 = 0.5555…
NIST writes the second line as (°F − 32) ÷ 1.8 — the same operation

The tool runs exactly those three lines. Whichever direction you pick it lands on Celsius first, then derives Fahrenheit and Kelvin from that value, which is why a single conversion always returns all three scales.

Converting Step by Step

Celsius to Fahrenheit, by hand:

• Multiply the Celsius figure by 9 and divide by 5 — or multiply by 1.8, which is the same thing.
• Add 32. Adding it before the multiplication instead of after puts the answer 25.6 °F too high, every time.
• Check the sign. Any Celsius value below −17.78 produces a negative Fahrenheit answer, because 0 °F sits at −17.78 °C.

Fahrenheit to Celsius reverses the order, and the order is the part that goes wrong:

• Subtract 32 first. Multiplying first and subtracting afterwards leaves the answer 14.22 °C low, whatever the temperature.
• Multiply what is left by 5 and divide by 9 — equivalently, divide by 1.8.
• Add 273.15 to the Celsius answer if you want Kelvin as well.

Worked Example: 100 °F in Celsius

Set Direction to Fahrenheit → Celsius and type 100. The headline reads 37.78 °C, and the three rows underneath it are:

• 37.78 °C = 100 °F = 310.93 K
• (100 − 32) × 5⁄9 = 37.78
• Handy anchors: −40° is the same in both · water freezes 0 °C / 32 °F · body temp 37 °C / 98.6 °F · water boils 100 °C / 212 °F

Fahrenheit → Celsius · Temperature 100

37.78 °C

By hand: 100 − 32 = 68, and 68 × 5 ÷ 9 = 37.777…, which the panel rounds to 37.78. A 100 °F day therefore sits a fraction above the 98.6 °F long quoted as body temperature.

Worked Example: 180 °C in Fahrenheit

Switch Direction to Celsius → Fahrenheit and type 180 — the setting a European recipe means by “moderate oven”. The headline reads 356 °F, with:

• 180 °C = 356 °F = 453.15 K
• 180 × 9⁄5 + 32 = 356

Celsius → Fahrenheit · Temperature 180

356 °F

356 is not a number any US oven dial offers, which is why recipes print 350 °F. NIST’s table of legacy oven descriptions pairs 350 °F with 180 °C directly, so the 6 °F you give up by rounding down is a convention both kitchens already agreed on.

The Mental Shortcut, and Exactly Where It Breaks

Double the Celsius and add 30. It is fast, it is wrong by a predictable amount, and the amount is worth knowing: the error is 0.2 × (°C − 10) degrees Fahrenheit. That makes the shortcut exact at 10 °C and one degree worse for every 5 °C you move away from it, in either direction.

Reversed — subtract 30, then halve — the shortcut is exact at 50 °F and drifts roughly 1 °C for every 18 °F away from it: 68 °F becomes 19 °C instead of 20 °C, and 90 °F becomes 30 °C instead of 32.22 °C. Across a normal weather range either version is close enough to dress by. Neither belongs near an oven, a fever or a datasheet.

Temperature Conversion Chart

Both ladders below were produced by the converter itself and are rounded the way it rounds — two decimal places, trailing zeros dropped.

Celsius to Fahrenheit

Two rows carry more weight than the others. −40 is the crossing point, the single temperature that reads the same on both scales. And 37 °C → 98.6 °F is the only row whose Fahrenheit figure is not a whole number — the Kelvin column carries .15 all the way down — which is the whole reason American thermometers display that odd figure: 98.6 was never chosen by anyone, it is 37 converted.

Fahrenheit to Celsius

Round Fahrenheit numbers rarely produce round Celsius ones, and that asymmetry is the practical difference between the two directions. Coming up from Celsius you land on whole numbers nearly every time; coming down from Fahrenheit you land on 26.67 and 32.22, and every printed chart has quietly rounded them on your behalf.

Temperature Examples: The Anchors Worth Memorizing

A handful of fixed points turns most conversions into estimation rather than arithmetic. These eleven keep recurring, each figure run through the converter.

NIST publishes a version of this list too, and rounds it. Its “Touchstone Temperatures (Approximate)” table gives body temperature as 37 °C and 99 °F, where the exact conversion is 98.6 °F. The word “approximate” in that heading is doing real work — and so is the same word on any wall chart.

Weather: Reading a Forecast in the Other Scale

NIST teaches the Celsius weather range as a four-line rhyme: 30 °C is hot, 20 °C is nice, 10 °C is cold, 0 °C is ice. In Fahrenheit that is 86, 68, 50 and 32 — four pairs covering most of what a traveler actually needs to decide.

Outside the rhyme the pairs stop being tidy. 0 °F is −17.78 °C and 100 °F is 37.78 °C: two round Fahrenheit numbers bracketing almost every inhabited climate, neither converting to anything memorable. Learn the Celsius side instead, because that is the side holding the whole numbers.

Body Temperature and Fever

The 37 °C / 98.6 °F pairing traces to one physician. From 1851 Carl Reinhold August Wunderlich took millions of underarm readings from 25,000 patients in Leipzig, establishing 37 °C — 98.6 °F — as the standard, with a normal range of 36.2 to 37.5 °C. The measurements came first and the book came later: the English edition, Medical Thermometry and Human Temperature, appeared in 1871, twenty years after the readings began.

Modern figures run lower. Obermeyer, Samra and Mullainathan analyzed 35,488 outpatients at a US academic hospital for the BMJ in 2017 and put mean oral temperature at 36.6 °C, which this converter renders as 97.88 °F. A separate 2020 eLife study traced the drift back through time: across Union Army veterans, NHANES I and a Stanford cohort, mean body temperature fell steadily by 0.03 °C per birth decade. MedlinePlus still gives normal as “usually around 98.6 °F (37 °C)” while stressing that it varies from person to person.

For the threshold that changes what you do, the NHS puts a high temperature in adults at 38 °C or above. Convert it once and it stays converted: 38 °C is exactly 100.4 °F, which is why 100.4 is the number printed on American fever guidance.

Oven Temperatures

US recipes step in 25 °F; European recipes step in 10 or 20 °C. The two ladders were never designed to line up, and the exact conversions show how far apart they sit.

Two common US dials are absent because NIST does not list them: 425 °F is 218.33 °C and 475 °F is 246.11 °C, whose nearest ten-degree marks are 220 °C and 250 °C. Where a name spans two rows it is because NIST’s bands overlap — 350 °F closes “moderately slow” and opens “moderate”.

UK recipes add a third system this converter does not handle. A gas mark is a dial index rather than a reading on a temperature scale, so neither 9⁄5 nor 5⁄9 applies to it — read the equivalent off the chart in the oven’s own manual.

Scaling the quantities as well as converting the oven setting? Send the ingredient list through the Recipe Converter.

Doneness temperatures cross the same border. The USDA’s safe minimum internal temperature for poultry, 165 °F, reads as 73.89 °C on a probe thermometer sold in Celsius.

How to Read Your Result

A successful conversion always returns four lines — a headline plus three rows — and the same four whichever direction you picked.

The Four Lines the Panel Returns

The page loads with Direction on Celsius → Fahrenheit and Temperature pre-filled with 100. Both fields already hold a value, so the converter runs once on arrival and the panel is showing a finished conversion before you touch anything:

• 212 °F — the headline: your reading, in the scale you asked for.
• 100 °C = 212 °F = 373.15 K — all three scales for the same point, so Kelvin arrives whether or not you wanted it.
• 100 × 9⁄5 + 32 = 212 — the arithmetic actually performed, with your number substituted in.
• Handy anchors: −40° is the same in both · water freezes 0 °C / 32 °F · body temp 37 °C / 98.6 °F · water boils 100 °C / 212 °F — a fixed reference line, identical on every run.

Change a value and nothing moves until you press Calculate; typing alone does not recalculate. Reset puts the form back to Celsius → Fahrenheit and 100 rather than emptying it, and returns the panel to its empty state. Figures are rounded to two decimal places with trailing zeros dropped, which is why 100 °C prints as 212 °F and not 212.00 °F. Negative answers carry a leading minus sign, and −40 is the one input that hands back its own value in either direction.

When the Panel Turns Amber

Three separate conditions produce the amber “Check your inputs” notice, and each names its own cause.

• Leave Temperature empty and the form refuses before the formula even runs: “Enter a value for Temperature.”
• Type something that is not a number — a stray unit, a word — and the formula answers: “Enter a temperature (negatives are fine)”.
• Ask for a point below absolute zero and you get: “Below absolute zero (−273.15 °C / −459.67 °F) — no such temperature”.

That third one is a physical floor, not a display limit. −273.15 °C is accepted and returns 0 K; −273.16 °C is refused. On the Fahrenheit side the boundary sits at −459.67 °F, which goes through, while −460 °F does not. Direction can never be blank — it is a two-button toggle with Celsius → Fahrenheit already selected.

Limits: Where a Temperature Conversion Is the Wrong Tool

This is exact arithmetic on a single reading. Several things people bring to it are not that.

• A difference is not a reading. NIST’s rule is that an interval of one Celsius degree corresponds to an interval of 1.8 Fahrenheit degrees, with no +32 anywhere — so a 10 °C rise is an 18 °F rise, and a thermometer accurate to ±0.5 °C is accurate to ±0.9 °F.
• Round trips can move the last digit. 101 °F converts to 38.33 °C; type 38.33 back in and you get 100.99 °F, because two decimal places is where the display stops. Same story at 103 °F → 39.44 °C → 102.99 °F.
• Water does not boil at exactly 100 °C. NIST’s own wording is “about 100 °C”: boiling temperature moves with air pressure, and the scale is defined against absolute zero and the triple point of water at 273.16 K rather than against a boiling pot.
• “Feels like” numbers are not conversions. Wind chill, heat index and humidex are models of human perception — convert the raw air temperature, then look the derived figure up in the scale it was published in.
• An oven dial is a request, not a measurement. Converting 180 °C to 356 °F tells you what the recipe meant; it says nothing about what the cavity reaches.
• Reference charts round to whole degrees; this converter stops at two decimal places. When a chart and a conversion disagree by a fraction of a degree — 99 °F against 98.6 °F for 37 °C, say — the chart has been rounded for reading.

None of that is a reason to distrust the figure in the panel. It is a reason to be clear about the question you asked: this tool restates one reading on another scale, to two decimal places, and deliberately does nothing else.