Temperature Converter

About the Temperature Converter

Temperature is the one common measurement where converting is not just a matter of multiplying. Celsius and Fahrenheit start from different zero points and use different-sized degrees, and Kelvin shares the Celsius degree but starts at absolute zero. This converter takes a reading in any of the three scales and restates it in another, using the exact formulas, so an oven setting from an American recipe, a thermostat from a European apartment or a lab value in kelvins can all be compared on the same footing.

Under the hood the value is first converted to Celsius and then out to the scale you want. From Fahrenheit that means subtracting 32 and multiplying by 5/9, from Kelvin it means subtracting 273.15, and in the other direction the steps are reversed. Because the page works from exact formulas, the only rounding is in the display, which keeps up to six decimal places. A built-in check also rejects any reading below absolute zero, which is 0 K, -273.15 degrees Celsius or -459.67 degrees Fahrenheit.

People reach for it when cooking from a foreign recipe, setting a thermostat while traveling, reading a weather forecast in an unfamiliar unit, checking a fever reading, following a science assignment, or handling brewing and aquarium instructions. The input field accepts values from -1000 to one million, so everyday numbers and extreme laboratory values are both fine. The result is a single converted reading, not a difference between two temperatures, a distinction worth keeping in mind.

When to Use It

How to Use It, Step by Step

1. Find the reading you want to convert and note the unit it is in. A degree sign alone is ambiguous, so look for the letter C, F or K on the thermometer, label or recipe.
2. Type the number into the Value field. It accepts values between -1000 and 1,000,000 and allows decimals, so you can enter negative readings such as -12 or fractional ones such as 98.6.
3. Choose the starting scale in the From list: Celsius (°C), Fahrenheit (°F) or Kelvin (K). The page opens on Celsius.
4. Choose the scale you want in the To list. It opens on Fahrenheit, so for a conversion into Celsius or Kelvin, change it before you press the button.
5. Press the Convert Temperature button. The summary line restates the reading, for example 425°F = 218.333333°C, and the highlighted In box shows the converted value with its scale label.
6. If you see the message that the temperature cannot be below absolute zero, re-check the unit and the sign. A value like -300 is impossible on the Celsius scale but it is a normal reading in Fahrenheit.
7. Round the result to suit the use: the nearest whole degree for weather and ovens, a tenth for body temperature, and two decimals for lab work. Then change the To scale if you want to see the third unit as well.

Understanding Your Results

The result is the same physical temperature written on a different scale. A reading of 218.333333°C and one of 425°F are identical heat levels, so you should never add or subtract anything to compensate for the change of unit. The number of decimals shown is a by-product of the exact formulas, not a sign of extra accuracy; a thermometer rarely resolves better than one degree, and a home oven can be off by a noticeable margin.

Use landmarks to check that the result is plausible. Water freezes at 0°C or 32°F and boils at 100°C or 212°F at normal sea-level pressure, and the two scales read the same at -40. Room temperature is roughly 20 to 22°C, or 68 to 72°F, and a usual body temperature is around 37°C, or 98.6°F. Kelvin readings are always the Celsius figure plus 273.15, so 0°C is 273.15 K.

When the result looks wrong, test the formula by hand with a friendly number. Converting 100°C should give 212°F and 373.15 K. Remember that the tool converts a reading, not a change in temperature. A rise of 10 degrees Celsius is a rise of 18 degrees Fahrenheit, with no 32 added, so use multiplication alone for differences.

More Worked Examples

Tips for Better Results

• For a fast Celsius to Fahrenheit estimate, double the Celsius figure and add 30. It gives 70 for 20°C against the true 68, accurate enough for weather but not for baking or lab work.
• For a fast Fahrenheit to Celsius estimate, subtract 30 and halve. It is close in the range of everyday weather but drifts at very high or low temperatures, so use the tool for anything that matters.
• Convert oven temperatures to the nearest value your dial supports and verify with an inexpensive oven thermometer, since the temperature inside many ovens differs noticeably from the set point.
• If an oven is fan-assisted, recipes written for conventional ovens are commonly adjusted downward, often by around 20°C. Check your oven's manual before relying on a rule of thumb.
• Keep -40 in mind as a landmark: the two scales read the same there, so a forecast of -40 is an extreme cold in either unit and the conversion needs no arithmetic.
• For temperature differences, such as a 15 degree rise in a greenhouse, multiply or divide by 1.8 only. Do not add or subtract 32, which belongs to readings alone and would give a nonsense result.

Common Mistakes to Avoid

• Using the reading formula on a temperature change. A 10 degree Celsius increase is 18 degrees Fahrenheit, not 50, so leave out the 32 when you convert a difference.
• Confusing the scales when a number looks plausible in both. 30 is a hot summer day in Celsius and a cold winter day in Fahrenheit, so double-check the letter before you enter it.
• Writing kelvin with a degree sign. The unit is simply K or kelvin, and negative kelvin readings do not exist, so a value below zero in that scale means an input error.
• Rounding before converting. Round the result at the end, since rounding 98.6 to 99 first and then converting makes the Celsius answer 37.2 instead of 37.0.
• Treating the result as precise because it has many decimals. A body thermometer reads to about one tenth of a degree and a kitchen oven is much looser, so the extra digits are an artifact of the formula.

Assumptions and Limits

• The conversions use the standard definitions: Fahrenheit equals Celsius times 9/5 plus 32, and Kelvin equals Celsius plus 273.15. These are exact for the scales as defined and not a rule-of-thumb approximation.
• Any input that works out below absolute zero, 0 K or -273.15°C or -459.67°F, produces an error message instead of a result, since no temperature can be that low. The rejection is checked after converting to Celsius.
• The tool does not cover Rankine, Réaumur, gas marks or other scales, and it does not account for any measurement error in the thermometer you read.
• For fever thresholds, food safety limits, medical storage or industrial process control, follow the written guidance from a clinician, health authority or manufacturer, which may use specific rounding rules.

Key Terms

Celsius

A scale that sets the freezing point of water at 0 degrees and its boiling point at 100 degrees at standard sea-level pressure. It is used in most countries.

Fahrenheit

A scale on which water freezes at 32 degrees and boils at 212 degrees. It is the everyday scale in the United States and a few other places.

Kelvin

The scientific temperature scale that begins at absolute zero. One kelvin step is the same size as one Celsius degree, and the unit is written K without a degree sign.

Absolute zero

The lowest temperature that is physically possible, equal to 0 K, -273.15°C and -459.67°F. At this point particles have their minimum possible energy.

Offset

The fixed amount added or subtracted when converting between scales whose zero points differ, such as the 32 in the Fahrenheit formula or the 273.15 in the Kelvin formula.

Temperature interval

A difference between two temperatures. It converts with the scale factor alone, so 1 Celsius degree equals 1.8 Fahrenheit degrees and 1 kelvin.