Temperature Converter
A temperature converter turns one reading into the other scales. Enter a number, pick its scale, and all four appear at once to two decimals: 20 °C is 68.00 °F, 293.15 K and 527.67 °R. The Fahrenheit formula is °F = °C × 9/5 + 32, so 100 °C gives 212.00 °F and −40 °C gives −40.00 °F, the one point where the two scales agree.
Below absolute zero (-273.15 °C / -459.67 °F / 0 K)
Conversion Formulas
About Temperature Converter
Type a figure into Temperature, choose Celsius, Fahrenheit, Kelvin or Rankine in Scale, and the four cards update together. The card for the scale you typed in is outlined, so you can see which number is your input and which three are conversions. Results are rounded to two decimals for display only.
Internally each value is converted to Celsius and back out again, which is why the four cards always agree with each other. The Formulas panel prints the three expressions that apply to your current scale — from Celsius it shows °F = °C × 9/5 + 32, K = °C + 273.15 and °R = (°C + 273.15) × 9/5, and it swaps to the reverse forms when you change the dropdown. Kelvin is the SI base unit of thermodynamic temperature and starts at absolute zero, so 0 °C is 273.15 K by definition; Rankine is the same idea applied to Fahrenheit degrees, with absolute zero at 0 °R and the freezing point of water at 491.67 °R.
Five preset buttons load the reference points and label them in Celsius: absolute zero (−273.15 °C), freezing point (0 °C), room temperature (20 °C), body temperature (37 °C) and boiling point (100 °C). The thermometer beside them is a bar covering −40 °C to 100 °C, filling proportionally and changing colour at 0, 20 and 37 °C, so anything colder than −40 °C or hotter than 100 °C simply pins the bar at one end. Copy puts all four scales on the clipboard as four labelled lines.
Limits: it converts single values, not ranges or lists, there is no bulk paste and no history, and temperature differences are not handled — a one-degree interval is 1.8 Fahrenheit degrees, but this tool converts points on the scale. Below absolute zero a red warning appears and the cards dim, because the value is not physically meaningful. Everything is calculated in the page.
Use Cases
How to use
Type the reading into the Temperature field; the arrows step it by 0.1 at a time.
Choose the scale you typed in from the Scale dropdown: Celsius, Fahrenheit, Kelvin or Rankine.
Read the four cards below — the outlined one is your input, the other three are the conversions to two decimals.
Check the Formulas panel underneath for the exact expression used for each conversion from the current scale.
Use the preset buttons for absolute zero, freezing, room, body or boiling point, and press Copy to take all four scales as text.
Pro Tips
- The card with the coloured outline is the scale you typed in, so if a result looks wrong, check that the Scale dropdown matches what you meant to enter.
- The preset buttons always set Celsius, whatever the dropdown currently says — pressing body temperature loads 37 with the scale switched to Celsius, not 37 °F.
- The input has a step of 0.1, so the up and down arrows in the number field move by a tenth of a degree rather than a whole one.
- The thermometer bar only spans −40 °C to 100 °C: below or above that it sits flat at the bottom or top, which is expected rather than a bug.
- Copy takes all four scales at once as four labelled lines, so you can paste a complete conversion into a note instead of retyping three numbers.
Troubleshooting
Every card shows a conversion of zero even though the field looks empty.
An empty or unreadable input is treated as 0, so the cards fall back to 0 °C, 32.00 °F, 273.15 K and 491.67 °R. That is the tool's default, not a stuck value — type a figure into Temperature and all four update immediately.
The results are dimmed and there is a red line under the input.
That is the below-absolute-zero warning: your value converts to colder than −273.15 °C. Most often the Scale dropdown is on the wrong scale — 200 entered as Celsius is fine, but −300 is not, and a Kelvin figure entered as Rankine can fall below the limit too.
A preset button changed my scale as well as the number.
The presets are defined in Celsius, so loading one sets both the value and the Scale dropdown to Celsius. The label on each button shows this — body temperature reads 37 °C. Change the dropdown afterwards if you wanted the same figure read as another scale.
The thermometer bar does not move when I change the temperature.
The bar covers −40 °C to 100 °C only. Anything colder sits empty at the bottom and anything hotter fills it completely, so a jump from 150 °C to 400 °C leaves it looking identical. The numbers in the cards are the accurate readout.
Copy did not put anything on my clipboard.
Copying uses the browser clipboard API, which needs a secure page and a real click and can be blocked by an extension. If the button does not switch to its tick, select the four numbers in the cards and press Ctrl+C, or Cmd+C on a Mac.
Frequently Asked Questions
Multiply by 9/5 and add 32: °F = °C × 9/5 + 32. The tool prints that formula in the Formulas panel whenever Celsius is the selected scale. Worked through, 20 °C gives 68.00 °F, 37 °C gives 98.60 °F and 100 °C gives 212.00 °F. Going the other way, subtract 32 first and then multiply by 5/9.
The usual shortcut is to subtract 30 and halve. It is exact only at 50 °F: the error grows by 0.2 °F for every degree Celsius away from 10 °C. At 20 °C the shortcut predicts 70 °F where the true value is 68.00, and at 30 °C it predicts 90 °F against a true 86.00. Use it for a rough feel and the converter when the number matters.
It depends on whether you mean a temperature or a difference. As a temperature, 1 °C is 33.80 °F. As a difference, one Celsius degree equals 1.8 Fahrenheit degrees, because only the 9/5 factor applies and the 32 offset does not. This tool converts temperatures, so entering 1 gives you 33.80 °F, not 1.8.
Nearly, but not exactly. 100 °F converts to 37.78 °C, which is close to body temperature rather than to 40 °C. The other way round, 40 °C is 104.00 °F. Type either figure into the converter with the matching scale selected and the difference is visible in the cards.
40 °F is much colder. Converted, 40 °F is 4.44 °C — a cold spring day — while 40 °C is 104.00 °F, a heatwave. The two scales only give the same number at −40, where −40 °C and −40 °F are the same temperature, and that is the single crossing point of the two lines.
30 °C, by a long way. 30 °C is 86.00 °F, while 30 °F is −1.11 °C, just below freezing. Because Celsius and Fahrenheit only agree at −40, every reading above that has a higher number in Fahrenheit than in Celsius for the same physical temperature.
Kelvin is the SI base unit of thermodynamic temperature, counted from absolute zero rather than from a property of water, which is why it is written as 293.15 K and not 293.15 °K. A kelvin is the same size as a Celsius degree, so the conversion is a pure offset: K = °C + 273.15, and 0 °C is exactly 273.15 K.
Rankine is to Fahrenheit what Kelvin is to Celsius: an absolute scale that starts at absolute zero but uses Fahrenheit-sized degrees. Absolute zero is 0 °R, water freezes at 491.67 °R and boils at 671.67 °R. The conversions are °R = °F + 459.67 and °R = K × 9/5, and it still appears in US thermodynamics and HVAC work.
158.00 °F. Applying °F = °C × 9/5 + 32 gives 70 × 1.8 = 126, plus 32 = 158. The same input shows 343.15 K and 617.67 °R in the other two cards. If you are checking a hot-water setting, note that many domestic systems are set well below this.
Because nothing can be colder than −273.15 °C, 0 K or 0 °R. If your input converts to less than that, a red line appears under the field and the four result cards dim to show the numbers are not physically meaningful. Check the Scale dropdown first — entering a Kelvin figure with Celsius selected is the usual cause.