9/5 is equal to 1.8, so we can also use this method: Celsius to Fahrenheit: °C × 1.8 + 32 = °F. Fahrenheit to Celsius: (°F − 32) / 1.8 = °C. To make ×1.8 easier we can multiply by 2 and subtract 10% , but it only works for °C to °F: Celsius to Fahrenheit: (°C × 2) less 10% + 32 = °F.