Mental Math: Convert Celsius to Fahrenheit

As aviators, we are particularly concerned with temperature. We monitor our EGT or ITT, CHT, OAT, TAT, or SAT. Most of the time we deal with this alphabet soup of temperatures in terms of degrees Celsius. Most Americans however, still think in terms of degrees Fahrenheit. We need  a simple method to convert degrees Celsius from the ATIS report to degrees Fahrenheit.


Even pilots who deal with Metrics measurements on a regular basis are more “at home” with the Fahrenheit scale. We know that 90º F is hot, and that it might be wise to wear a coat if the temperature was forecast to be 40º F.


Now what if the ATIS reported 15º C at your destination? Do you bundle up? Is 25º C T-shirt weather? Pilots need a quick and simple way to convert degrees Celsius to Fahrenheit.

Recall the unintuitive formula for converting degrees Celsius into degrees Fahrenheit:

F = ( 1.8 * C ) + 32

In words, multiply the temperature Celsius by 1.8, then add 32.

Although this works, this formula poses two problems. First off, it is not easy to remember. Furthermore, the math is difficult to do in your head, especially while flying an airplane!

The Rule of Thumb:

To simplify the math, we can sacrifice a little accuracy for an easier formula:

F = ( 2 * C ) + 30

Simply double the temperature Celsius, then add 30.

So, do you still need your coat on at your destination? ATIS reports 15º C, lets figure it out.

2 * 15 = 30, We double 15 to get 30,

30 + 30 = 60, then add 30 to the result (30) to get a temperature of 60º F.

That’s pretty close to the actual temperature (from the first formula) of 59º F.


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About the author
Pat Flannigan is a professional pilot and aviation blogger. He has been flying for fifteen years and is currently working as an airline pilot in the United States.

11 Replies to Mental Math: Convert Celsius to Fahrenheit

  1. Nice trick, I’ll have to remember this one. Thanks, Patrick.

    • Yeah, I flew with a captain who was making those conversions on the fly for his PA announcements lightning fast. I couldn’t believe it so I asked him what the deal was.

      By the way, great community at PilotBuzz. I’m really enjoying interacting with all the people there.

  2. Jeffrey says:

    Patrick,

    Great tip. I use it all the time.

    Another useful conversion, but I always have to use a calculator is to convert knots to mph.

    mph = knots x 1.15

    I know there is a clever way for figuring out tips using the 15% but when I get into the 100s my brain shuts down!

    Nice blog!

    I’ll be back!

    Jeff

    • Funny you should mention the knots to mph conversion Jeffrey. My company has first flight certificates that we give out to first time fliers. The whole crew signs it, and the captain or I write down the cruising altitude and speed (in mph!).

      We had a 1st timer this morning and we were both struggling to figure that speed out.

  3. Lewis says:

    This “mental math” formula – which is similar to you “double” formula – will provide accuracy:

    1)Double it
    2)Take off a tenth
    3)Add 32.

    That’s it!

    For example:

    If it’s 50ºC in Paris, then in New York we’d have

    50 (double it) = 100
    (now, take off its tenth) = 100-10 = 90
    (and then add 32) = 90 + 32 = 122ºF.

    Wow. That’s hot! When you get more complicated Celsius numbers, it’s still the same:

    37 (double it) = 74
    (now, take off its tenth) = 74-7.4 = 66.6
    (and then add 32) = 66.6 + 32 = 98.6ºF.

    If you can’t do it in your head, the “double then add 30” is a good approximate.

  4. We don’t have Fahrenheit here in England any more but I guess the reverse is true – subtract 30 and divide by 2 to get back to Celsius – and this will be very helpful next time I’m in the US trying to work out if I need a coat or shorts!

  5. Sylvia says:

    I need the same for US gallons to litres, please!

    • Jay says:

      1 US Gallon = 3.78 litres. Easiest to assume that 1 US gallon = 4 litres. 12 US Gallons = 48 Litres.

      To make it more accurate for every 4 US gallons subtract 1 litre. So 12/4 = 3. So 12 US Gallons = 45 Litres.

      In reality it’s 45.42 litres

  6. pilot says:

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  7. Hey there, as a student pilot – this is a nice quick one … from a reputable source! Thanks!
    Terry Broadbent

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