Fahrenheit To Celsius - How To Convert

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Calculating Fahrenheit and Celsius

But how would you be able to convert those temperatures for yourself without access to one of those widgets on your phone or laptop?

It is made more complicated by that difference between freezing points, 32°F versus 0°C. From there on up to boiling, 180 degrees in Fahrenheit are equivalent to 100 degrees Celsius.

Official conversion formula

Therefore the conversion formula to convert Fahrenheit to Celsius is:

Fahrenheit to Celsius Formula:

[°C] = ([°F] - 32) × 5/9

In other words, take your Fahrenheit reading, subtract 32, multiply the answer by five and then divide by nine. So 66 degrees Fahrenheit minus 32 would be 34, times five makes 170 divided by nine equals 18.88 recurring, or 18.9°C.

To go the other way, from Celsius to Fahrenheit:

Celsius to Fahrenheit Formula:

[°F] = [°C] × 9/5 + 32

Here, you multiply your Celsius temperature by nine, divide by five and then add 32 to find the Fahrenheit reading.

So, 13 degrees Celsius times nine is 117, divided by five is 23.4, plus 32 makes 55.4°F.

You can see where the problem is in both these sums; itís that nine.

Easier methods of conversion

Thereís a rough and ready way round this, based on 9/5 being close to doubling and 5/9 being near half. Under this system, double your Centigrade and add 32 to find Fahrenheit.

Alternatively, take Fahrenheit, subtract 32 and halve the answer to get to Celsius.

Under this system, that 66°F would convert to 17°C, nearly two degrees lower than the correct answer while the 13°C would convert to 58°F, which is close to three degrees higher than the true figure.

Weíre left with an uncomfortable choice between cumbersome sums and distinctly dodgy accuracy: is there any way to work around this dilemma?

The Rule of 40

For a start, we can use 40 instead of 32. This is because it so happens that -40°F is also -40°C. So under this system to go from Fahrenheit to Celsius, you add 40, multiply by 5, divide by 9, then subtract 40. Going from Celsius to Fahrenheit you also add 40 to the centigrade reading, multiply by nine, divide by 5, then subtract 40.

It may sound strange but it does indeed work, like this:

Our original 66 degrees Fahrenheit plus 40 makes 106, times five equals 530, divided by nine gives us 58.88 recurring, rounding up to 58.9, minus 40 takes us once again to 18.9°C.

And working the other way, from Celsius to Fahrenheit:

That 13 degrees Celsius times plus 40 gives 53, which multiplied by nine makes 477, divided by five to make 95.4, minus 40 gives us, once again, 55.4°F.

Using decimals

Thereís one last thing we can do to make life a little simpler, using decimals instead of fractions. That means using 1.8 instead of 9/5.

This gives modified formulae of:

  • Celsius to Fahrenheit: (°C + 40) × 1.8 - 40 = °F
  • Fahrenheit to Celsius: (°F + 40) / 1.8 - 40 = °C

Trying this out in action on a hot day (and remembering that multiplying by 1.8 is the same as doubling and then subtracting 10 per cent) we could have 36°C plus 40 is 76, times 1.8 is 136.8 minus 40 is 96.8°F, which is firmly in 'Scorcha' territory.

And the other way, on a rather chillier day, 48°F plus 40 is 88, divided by 1.8 is 48.88 recurring (48.9) minus 40 equals 8.9°C, which actually does sound quite cold.

Thermometer stuck in snow
It always looks chillier in Celsius...

Mental reference points

In actual life, the best way might be to fix some reference points in your mind:

  1. A genuine heat wave day is 95°F and thatís 35°C.
  2. Temperatures over 40°C (105°F) are dangerous, with real risk of heat stroke or dehydration. Stay in the shade and drink plenty of water.
  3. A really chilly take-your-coat-and-gloves-and-donít-forget-your-woolly-hat-and-scarf day is around 2°C; a little under 36°F.
  4. The temperature that most of us like to lounge around in at home is 70 degrees Fahrenheit, so set the central heating thermostat to 20°C. If youíre in the air conditioning zone, then the numbers are 75°F or 24°C.
  5. On a health note, anything over 103°F (39.4°C) constitutes a dangerous fever. If the patientís temperature reaches this point, you need urgent medical assistance.

Reference Chart 1: Fahrenheit to Celsius

Celsius readings rounded up or down to single decimal points.

A chart of direct conversions from Fahrenheit to Celsius

Reference Chart 2: Celsius to Fahrenheit

Fahrenheit readings rounded up or down to nearest whole degree

A chart of direct conversions from Celsius to Fahrenheit

Of course, if you're looking for a pain-free method of doing the calculation right here and now then you can use our handy Fahrenheit and Celsius converter, below.

Fahrenheit and Celsius Converter

Our website provides a comprehensive list of online conversion tools. So, should you wish to convert between fahrenheit, celsius and any other units of temperature, give the temperature converter a try.

Written by Nick Valentine

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Last update: 18 July 2016

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