Celsius, Fahrenheit and Kelvin — and the one converter in this category where proportional reasoning does not work. Length, mass and area only ever multiply by a ratio; temperature does not, because the three scales put their zero in three different places, so the formula carries an offset as well as a factor. Twice 20 degrees is emphatically not twice whatever 40 degrees converts to. Celsius is the base scale here: enter a reading, say which scale it is on, and the other two appear together. It all happens in this tab and the temperature you measured is never sent anywhere, though the page around it still loads third-party resources such as advertising.
How to use it
- Enter the reading in the Value field, which starts at 1. A minus sign can be typed directly — a winter −5 needs no special setting.
- The From unit dropdown offers three entries — Celsius (°C), Fahrenheit (°F) and Kelvin (K) — and starts on Celsius. Pick the scale your reading is already on.
- With only three scales in play, the results area always shows exactly **two cards**: the other two scales, each with its own copy button.
- To check you have the direction right, try −40. Minus forty Celsius is still minus forty Fahrenheit; it is the single point where the two scales agree.
- Empty the field and both cards fall back to a dash, with the copy button returning an empty string.
When it comes in handy
Set an oven from an English recipe
The American 350 °F works out at 176.6666667 °C, which in practice means the 180 °C mark; a slow 220 °F is 104.4444444 °C. And 392 °F is exactly 200 °C — when a recipe prints an odd number like 392, it was almost certainly converted from a round metric one.
Read a forecast before travelling
US forecasts lead with Fahrenheit: 72 °F is 22.22222222 °C, comfortably short-sleeve weather, while a Taiwanese summer day of 26 °C reads 78.8 °F. The band that matters most is around freezing — 32 °F is 0 °C, so a forecast dipping below 32 means ice.
Interpret operating ranges on a datasheet
Commercial grade 0–70 °C and industrial grade −40–85 °C become 32–158 °F and −40–185 °F. When imported equipment quotes only Fahrenheit, converting back is how you find out whether it will survive an unair-conditioned server room.
Check medical thresholds
English-language sources put fever at 100.4 °F, which converts to exactly 38 °C, and normal body temperature at 98.6 °F, which is exactly 37 °C. Those numbers look suspiciously precise for a reason: they were converted from round Celsius values in the first place.
Bring absolute temperatures back to everyday scales
Laboratory and thermodynamic work runs in Kelvin: room temperature of 25 °C is 298.15 K, and absolute zero at 0 K is −273.15 °C. When a paper or a spec sheet talks in Kelvin, converting is what makes it intuitive.
Limits and behavior
- This is the only **non-linear** converter of the six. Fahrenheit to Celsius is
(v − 32) × 5 ÷ 9and Kelvin to Celsius isv − 273.15; the subtraction is the offset. Every proportional instinct fails as a result — doubling a temperature, taking 80% of it, or averaging a percentage rise all break down the moment you cross scales. - Celsius is the base, so there is no direct Fahrenheit-to-Kelvin formula: the value goes through Celsius and back out, two operations. That is why 100 °F displays as 37.77777778 °C — one ninth is a repeating fraction in binary, and the display stops at ten significant digits.
- **Nothing stops you going below absolute zero.** Enter −300 °C and the tool quietly returns −26.85 K; enter −1000 °C and it returns −726.85 K. Negative Kelvin does not exist physically, since 0 K is the floor, and no warning is shown. A negative Kelvin card simply means the input was out of physical range.
- What is converted is a **reading**, not a **difference**. Turning a thermostat up by 10 °C corresponds to a rise of 18 °F, but typing 10 here gives 50 °F, because 10 is treated as an absolute reading. Anything phrased as
±2 °Cor "a 5-degree rise" is a difference and cannot be converted with this tool. - In the dropdown, Celsius and Fahrenheit carry the degree sign as "Celsius (°C)" and "Fahrenheit (°F)", while Kelvin is written simply "Kelvin (K)". That is not an omission: SI treats the kelvin as a base unit rather than a degree, so the correct form is
273.15 K, never273.15 °K. - Only three scales are stocked. Rankine — Fahrenheit-sized degrees counted from absolute zero, common in US thermodynamics texts — and Réaumur, found in older European sources and a few cheese-making specifications, are both absent.
- This is pure scale conversion and touches none of the derived weather figures. Feels-like temperature, heat index, wind chill, dew point and wet-bulb temperature all need humidity or wind speed as extra inputs, which makes them a different problem entirely.
Frequently asked questions
If a room goes from 20 °C to 40 °C, is it "twice as hot"? Does that hold in Fahrenheit?
No on both counts, for the same underlying reason. 20 °C is 68 °F and 40 °C is 104 °F — twice 68 is 136, not 104. Ratios do not survive a change of scale when the zeros sit in different places. The only scale where doubling means anything is Kelvin, which counts from absolute zero: 20 °C is 293.15 K, and twice that is 586.3 K, or 313.15 °C. That is doubling in the thermodynamic sense. The everyday phrase "twice as hot" has essentially no physical content.
A spec says "±2 °C". Is that ±35.6 °F?
No — the right answer is ±3.6 °F, and this tool cannot produce it. Typing 2 gives you 35.6 °F, which is the Fahrenheit reading corresponding to a Celsius reading of two. But ±2 describes the width of an interval, and widths convert by multiplying by 9/5 with no 32 added. The test is linguistic: if the sentence says difference, rise, drop, range or tolerance, it is an interval, so multiply by 1.8; if it says something *is*, *reaches* or *falls below* a temperature, it is a reading and belongs here. For the same reason, a difference in kelvin and a difference in Celsius are identical — a change of 1 K is a change of 1 °C.
I typed −300 °C and still got an answer. Why?
Because there is no physical range check; the formula is simply applied to whatever you type. −300 °C converts to −26.85 K, and Kelvin is an absolute scale on which negatives cannot exist. Absolute zero is −273.15 °C, −459.67 °F and 0 K, and any input below those three numbers is outside physical reality. You can turn this into a sanity check: glance at the Kelvin card after every conversion, and a negative value means you mistyped.
Can I convert a 6500K bulb's colour temperature to Celsius?
Arithmetically yes — it comes to 6226.85 °C — but the result means nothing useful. Colour temperature describes the colour of light an ideal black body would emit if heated to that temperature; it borrows the kelvin to label a hue, not to describe the lamp, and a 6500K LED bulb is merely warm to the touch. Camera white balance and monitor colour settings use the kelvin the same way, so converting them to Celsius only produces a misleading number.
Why does −40 convert to −40? Is something broken?
Nothing is broken; it is the one point where Celsius and Fahrenheit coincide, and it has to exist. The two scales have different slopes and an offset, so the lines cross exactly once — solve v = v × 9/5 + 32 and you get −40. Above it the Fahrenheit number is the larger one (0 °C = 32 °F); below it the Fahrenheit number is smaller (−50 °C = −58 °F). Industry rather likes the coincidence: a rating of −40 needs no scale marked next to it.
Is Rankine (°R) available? I need it for thermodynamics homework.
No — only Celsius, Fahrenheit and Kelvin. Rankine is an absolute scale zeroed at absolute zero with Fahrenheit-sized degrees, and it turns up in US engineering and thermodynamics texts. To get there, convert to Fahrenheit here and add 459.67 yourself; subtract the same to come back. The other omission is Réaumur, on which water freezes at 0 and boils at 80, now confined to historical European sources and a handful of cheese-making standards.
What should I actually set the oven to for a 350 °F recipe?
The exact figure is 176.6666667 °C, but domestic oven dials move in steps of ten, so in practice you set 180 °C. That rounding is fine for baking: a home oven typically drifts more than ten degrees on its own, and opening the door costs more than that again. Where precision genuinely matters — sous vide, sugar work — trust a probe thermometer rather than the dial. The other common settings, 325, 375 and 425 °F, land near 163, 191 and 218 °C, which most cookbooks simply print as 160, 190 and 220.