To convert a temperature reading in Fahrenheit to Celsius, subtract 32 from the reading, and then multiply the result by 5/9. For 98.6°F, for example, first subtract 32, which is equal to 66.6, then multiply by five, which is 333. Finally, divide 333 by nine, which equals 37°C.
For a rough Fahrenheit-to-Celsius calculation, subtract 30 from the Fahrenheit figure, and divide the result by two. As this method is inaccurate, it should only be used for rough temperature conversions.
Instead of using the 5/9 fraction in the Fahrenheit-to-Celsius conversion, you can use 1.8. Subtract 32 from the Fahrenheit figure, as before, but for the next step simply divide the result by 1.8. Using the same Fahrenheit figure as an example, 66.6 divided by 1.8 also gives 37 as the quotient.
Another way to convert a Fahrenheit figure to a Celsius reading is adding 40 to the figure, multiplying by 5/9, and then subtracting 40 from the result. For 98.6°F, 98.6 + 40 is 138.6, which when multiplied by 5/9 gives 77. 77 minus 40 then gives 37, which is also an accurate Celsius reading.
For a Celsius-to-Fahrenheit conversion, multiply the Celsius figure by nine, divide by five, and then multiply the result by 32.