Round at the End
Do intermediate calculations with full precision and round only the final result. Rounding at each step compounds small errors. For example, converting 12.5 inches to cm: 12.5 × 2.54 = 31.75 cm exactly — if you rounded 2.54 to 2.5 first, you'd get 31.25, which is wrong.
Match the Input's Precision
A result should not appear more precise than the measurement it came from. A length measured to the nearest foot, converted to meters, should be reported to the nearest 0.3 m (one decimal), not to six decimal places.
Common Rounding Rules
Round half up is the everyday rule: 2.5 rounds to 3. Round half to even (banker's rounding) reduces bias in large datasets: 2.5 rounds to 2, 3.5 rounds to 4. Choose the method that fits your context.