The formula
One inch is defined as exactly 2.54 centimeters. That means the conversion runs in one direction only: divide the number of centimeters by 2.54 to get inches.
inches = cm / 2.54. If you prefer multiplying, multiply centimeters by 0.393701 - the result is the same.
Worked examples
30 cm / 2.54 = 11.81 inches. A standard A4 sheet is 21 cm wide, which is 8.27 inches.
180 cm / 2.54 = 70.87 inches, or about 5 feet 10.9 inches. To get feet and inches, divide the inch result by 12 - the whole number is feet, the remainder is inches.
Common reference points: 1 cm is roughly the width of a small paperclip, 2.54 cm is exactly one inch, and 30 cm is very close to one foot.
Quick mental shortcut
For a rough estimate without a calculator, remember that 1 inch is about 2.5 cm. Halve the centimeter value twice and you will be within a couple of percent - close enough for shopping or packing.
When accuracy matters - cutting material, ordering parts, medical measurements - always use 2.54 rather than the shortcut.