40 °C to °F, 40 degrees Celsius in Fahrenheit.

40 °C is 104 °F. To do 40 C to F, multiply 40 by 1.8 to get 72, then add 32. That gives 104, and the answer is exact, with nothing rounded. This page works out 40 °C in Fahrenheit, shows the sum, and has a box where you can try any other temperature.

How to use the converter

1. The box Degrees Celsius (°C) starts at 40. Type another number to change it. A minus sign works.
2. The big card shows the answer in °F. Under it the sum is written out in two steps.
3. Decimals sets how many digits show. The first choice shows up to 2 and drops zeros at the end.
4. Press Turn it around: °F to °C to go the other way. The answer moves into the box, so 104 °F shows 40 °C.
5. Copy result copies "40 °C = 104 °F". Copy link copies an address that opens this page with your number. A row in the table loads when you click it, and Back to 40 °C returns to the start.

The sum for 40 °C

NIST Handbook 44 (2026), Appendix C gives the equation: degree Fahrenheit = (degree Celsius × 1.8) + 32, and kelvin = degree Celsius + 273.15 (page read 24 Oct 2026). Both 1.8 and 32 are exact. So 40 × 1.8 = 72, and 72 + 32 = 104. In kelvin, 40 °C is 313.15 K, and in degrees Rankine it is 563.67 °R.

Why the sum looks like this

A Celsius degree is bigger than a Fahrenheit degree. The formula fits 100 Celsius degrees into the same gap as 180 Fahrenheit degrees, so one Celsius degree is 180 ÷ 100 = 1.8 Fahrenheit degrees. The zero marks are in different places too, because 0 °C sits at 32 °F. So 40 °C is 40 Celsius degrees above that mark, which is 40 × 1.8 = 72 Fahrenheit degrees above 32 °F, and 72 + 32 = 104 °F.

Check it in your head

The quick habit is to double the Celsius number and add 30. For 40 °C that gives 110 °F, which is 6 °F too high. The gap is 0.2 °F for each degree Celsius away from 10 °C, so it grows as the number grows. A closer way: double it (80), take away a tenth of the double (8) to get 72, then add 32 for 104. That works because 2 − 0.2 is 1.8.

40 °C and the UK record

The Met Office says the UK's highest July temperature on record is 40.3 °C, at Coningsby in Lincolnshire on 19 July 2022, and that six more stations reached 40 °C or higher (Met Office, July weather records, read 24 Oct 2026). 40.3 °C is 104.54 °F, so that record sits 0.54 °F above the 104 °F on this page.

More worked examples

• 39 °C. 39 × 1.8 = 70.2, plus 32 is 102.2 °F.
• 40.5 °C. 40.5 × 1.8 = 72.9, plus 32 is 104.9 °F.
• Twice the temperature. 80 °C is 176 °F, not 208 °F, because the 32 is added once and not doubled.

Worked examples near 40 °C

On the Celsius scale 40 °C is 40 per cent of the way from 0 °C to 100 °C. The formula puts those two marks at 32 °F and 212 °F, and 104 °F is 72 of the 180 Fahrenheit degrees between them.

Each row is the same sum as the one above. The middle column is the step before the last one, so you can follow the arithmetic.

Rounding

The answer is exact, so there is nothing to round. It is already a whole number of degrees, 104 °F.

Going back

Going back, 104 − 32 = 72, and 72 ÷ 1.8 = 40, so 104 °F is 40 °C. A one degree step the other way is smaller: 105 °F is 40.56 °C and 103 °F is 39.44 °C, because one Fahrenheit degree is 5/9 of a Celsius degree.

Mistakes that matter at this number

• Forgetting the 32. 40 × 1.8 = 72, and stopping there gives 72 °F, which is 32 °F too low.
• Adding the 32 first. (40 + 32) × 1.8 = 129.6 °F, which is wrong. Multiply first, then add.
• Reading the answer on the wrong scale. A thermometer that says 40 °F is 4.44 °C, not 40 °C.

How accurate is this page

The sum uses exact fractions, not the floating point numbers most calculators use, so no tiny error hides in the last digits. The answer is rounded once, at the end, to the number of decimals you pick. The factors come from NIST Handbook 44 (2026), Appendix C, which gives 1.8 and 32 as exact. Everything runs in your browser.

However you type it

40 C to F, 40 degrees C to F, 40c in f and 40 Celsius to Fahrenheit are the same sum. Check the unit letters before you trust any answer, because the right number on the wrong scale is the commonest slip. The arrow keys step the number in the box up and down, which helps when you have a short list to run through, and only your decimals choice is remembered. Other values close by: 37 °C is 98.6 °F; 38 °C is 100.4 °F; 45 °C is 113 °F; 180 °C is 356 °F.

More conversions

For any other temperature use the Celsius to Fahrenheit converter, or go the other way with the Fahrenheit to Celsius converter. For the kelvin scale see Celsius to kelvin.