Celsius to Fahrenheit Program in Python

Problem statement

Convert a temperature between Celsius and Fahrenheit using F = C × 9/5 + 32, and back using C = (F − 32) × 5/9.

Approach 1: simple loop

Apply the formula directly. The order matters exactly as written: multiply first, then add 32 — put brackets around anything you want done earlier.

c = 37

f = c * 9 / 5 + 32
print(f)
# Output: 98.6

Check the anchors as you go: 0°C must give 32°F (freezing point) and 100°C must give 212°F (boiling point). If those two pass, the formula is right.

Approach 2: reusable function

Wrap both directions in functions so the conversion is reusable and testable — and each formula lives in exactly one place.

def to_fahrenheit(c):
    return c * 9 / 5 + 32

def to_celsius(f):
    return (f - 32) * 5 / 9

print(to_fahrenheit(37))     # 98.6
print(to_celsius(98.6))      # 37.0
print(to_fahrenheit(100))    # 212.0

Notice the asymmetry in the brackets: the reverse formula needs (f - 32) grouped because subtraction happens before multiplication would not give the right order.

Approach 3: Pythonic alternative

Real programs convert ranges, not single values — and present them in a tidy table with f-string formatting.

def to_fahrenheit(c):
    return c * 9 / 5 + 32

print(f"{'C':>5} {'F':>7}")
for c in range(-2, 3):
    print(f"{c:>5} {to_fahrenheit(c):>7.1f}")
# Output:
#    -2    28.4
#    -1    30.2
#     0    32.0
#     1    33.8
#     2    35.6

{c:>5} right-aligns in 5 characters and :.1f shows exactly one decimal — a two-character change that turns output into a report.

Dry run

Trace C = 37: 37 × 9 = 333. 333 / 5 = 66.6. 66.6 + 32 = 98.6 — human body temperature. Now the anchors: C = 0 → 0 + 32 = 32 (freezing ✓). C = 100 → 900/5 + 32 = 180 + 32 = 212 (boiling ✓). And the reverse on your own answer: (98.6 − 32) × 5/9 = 66.6 × 5/9 = 333/9 = 37 — you land back where you started, which is the strongest check a conversion has.

Common errors

Using 9 // 5 — integer division makes it 1, so every result comes out as C + 32. Use true division /.
Forgetting the + 32 — the answer is off by exactly 32 degrees.
Multiplying by 5/9 in the forward direction — that is the reverse formula; F-to-C divides by 9/5 differently from C-to-F.
Omitting brackets in the reverse formula — f - 32 * 5 / 9 computes f - (32*5/9), not (f-32)*5/9.
Reading input() without float() — temperatures can be decimal, and string arithmetic raises TypeError.

Practice on PyDebug

Turn the idea into a debugging habit with these free browser exercises: