Celsius to Fahrenheit: Formula, Chart and Code (Ruby, JavaScript, Python)

To convert Celsius to Fahrenheit, multiply by 9/5 (or 1.8) and add 32: °F = °C × 9/5 + 32. So 25 °C is 77 °F. To go back, subtract 32 and divide by 1.8. Below is a quick chart, a mental shortcut, and the code in Ruby, JavaScript and Python, including the two bugs that make the results come out wrong.

Key takeaways

• Celsius to Fahrenheit: °F = °C × 9/5 + 32.
• Fahrenheit to Celsius: °C = (°F − 32) × 5/9.
• -40 is the same in both scales.
• In Ruby, 9 / 5 is 1. Use 9.0 or the result is wrong.
• Round only when you display the number, never while you calculate.

What is the formula to convert Celsius to Fahrenheit?

Multiply the Celsius temperature by 9/5 and add 32. The U.S. National Institute of Standards and Technology (NIST) writes the same formula as (°C × 1.8) + 32.

°F = °C × 9/5 + 32

Example: 25 × 9/5 = 45, and 45 + 32 = 77 °F.

How do you convert Fahrenheit to Celsius?

Subtract 32 and multiply by 5/9 (or divide by 1.8).

°C = (°F − 32) × 5/9

Example: 350 °F − 32 = 318, and 318 × 5/9 ≈ 176.7 °C.

Celsius to Fahrenheit chart

Is there a quick way to convert Celsius to Fahrenheit in your head?

Double the Celsius number and add 30. 20 °C becomes 40 + 30 = 70 °F (the exact answer is 68 °F). It’s close for everyday weather and drifts as temperatures climb, so don’t use it for cooking or a fever.

To go from Fahrenheit to Celsius, subtract 30 and halve it. 80 °F becomes 50 / 2 = 25 °C (exact: 26.7 °C).

How do you convert Celsius to Fahrenheit in Ruby?

Use a float in the formula: celsius * 9.0 / 5 + 32.

def celsius_to_fahrenheit(celsius)
  celsius * 9.0 / 5 + 32
end

def fahrenheit_to_celsius(fahrenheit)
  (fahrenheit - 32) * 5.0 / 9
end

celsius_to_fahrenheit(37)   # => 98.6
fahrenheit_to_celsius(350)  # => 176.66666666666666

Watch out for integer division. In Ruby, dividing two integers returns an integer, so 37 * 9 / 5 + 32 gives 98 instead of 98.6. Make one side a float (9.0), or use a rational (9r / 5) and call .to_f at the end.

How do you convert Celsius to Fahrenheit in JavaScript?

Use the formula directly: celsius * 9 / 5 + 32.

const celsiusToFahrenheit = (celsius) => celsius * 9 / 5 + 32;
const fahrenheitToCelsius = (fahrenheit) => (fahrenheit - 32) * 5 / 9;

celsiusToFahrenheit(37); // 98.6

JavaScript numbers are floating point by default, so integer division isn’t a problem here.

How do you convert Celsius to Fahrenheit in Python?

Use the formula directly: celsius * 9 / 5 + 32.

def celsius_to_fahrenheit(celsius):
    return celsius * 9 / 5 + 32

def fahrenheit_to_celsius(fahrenheit):
    return (fahrenheit - 32) * 5 / 9

celsius_to_fahrenheit(37)  # 98.6

In Python 3, / always returns a float. Python 2 had the same integer division trap as Ruby, which PEP 238 changed.

Why does my conversion return 97.88000000000001?

Because computers store most decimals as approximations. In all three languages, 36.6 * 9 / 5 + 32 returns 97.88000000000001, not 97.88. The Python docs explain why. Round when you display the value, not while you calculate:

celsius_to_fahrenheit(36.6).round(1)  # => 97.9

Math.round(celsiusToFahrenheit(36.6) * 10) / 10; // 97.9

round(celsius_to_fahrenheit(36.6), 1)  # 97.9

Frequently asked questions

At what temperature are Celsius and Fahrenheit equal?

At -40. Plug -40 into either formula and you get -40 back.

What is 37 °C in Fahrenheit?

98.6 °F, the textbook normal body temperature.

What is 350 °F in Celsius?

About 176.7 °C. Recipes usually round it to 175 or 180 °C.

Is the formula 9/5 or 1.8?

Both. 9/5 equals 1.8 exactly, so °C × 1.8 + 32 gives the same result.

The bottom line

Multiply by 9/5 and add 32. In code, the formula is the easy part: use a float, and round only when you show the result.

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