Which formula converts Celsius to Fahrenheit?

• A. C = 5/9(F - 32)
• B. F = 9/5(C) + 32
• C. F = 5/9(C) + 32
• D. C = 9/5(F) + 32

Answer: (B)

Converting Celsius to Fahrenheit uses a linear formula: Fahrenheit equals nine-fifths of the Celsius temperature, plus 32. This comes from two known points: 0°C is 32°F and 100°C is 212°F. If you write Fahrenheit as F = mC + b, then 0°C gives F = 32, so b = 32. Using 100°C, 212 = 100m + 32, so m = 180/100 = 9/5. Put together, F = (9/5)C + 32.

This form matches everyday temperature conversions. For example, 20°C is 68°F since (9/5)×20 = 36, and 36 + 32 = 68.

The other expressions mix up the direction or use the reciprocal slope, so they don’t give Fahrenheit from Celsius. For instance, using 5/9 instead of 9/5 scales Celsius to a smaller Fahrenheit value, and forms that place Celsius on the left or swap variables describe converting Fahrenheit to Celsius, not Celsius to Fahrenheit.