Understanding Significant Figures and Rounding in Unit Conversion
Every converter on this site shows results to six decimal places by default, and every formula page shows the full, unrounded conversion factor rather than a tidy approximation. That's a deliberate choice, and understanding why it's structured that way helps you decide when to use the full precision and when it's safe to round.
Exact Constants vs Measured Values
Almost every conversion factor on this site is an exact defined constant, not a measured approximation. 1 inch equals exactly 2.54 cm, 1 pound equals exactly 0.45359237 kg, 1 mechanical horsepower equals exactly 745.69987158227022 watts. These aren't rounded to a certain number of significant figures because they're not measurements with inherent uncertainty, they're definitions, fixed by international agreement, that carry as many digits as you choose to write. That's why this site's formula sections state factors like 0.3937007874015748 in full rather than rounding to "0.3937," the extra digits are real and exact, not noise.
Why the Result Still Needs Rounding
Even with an exact conversion factor, the practical result usually needs rounding, because your input value itself has limited precision. If you measured a board at 24 inches using a tape measure that reads to the nearest 1/16 inch, converting that to millimeters with fifteen decimal places of "precision" is false precision, your actual measurement uncertainty is much larger than that. The right amount of rounding depends on how precisely you actually know your starting number, not on how many digits a calculator can display.
A Practical Rule of Thumb
A reasonable working rule: round your final answer to roughly the same number of significant figures as your least-precise input. If you're converting a body weight given to the nearest pound, the kilogram result shouldn't be reported to six decimal places, one or two decimal places is more honest about what you actually know. For engineering, cooking, or construction, matching your result's precision to what your measuring tool can actually resolve, a ruler, a kitchen scale, a tape measure, keeps the number meaningful rather than misleadingly precise.
When Full Precision Actually Matters
There are real exceptions where the extra digits matter: scientific calculations, financial or engineering specifications with tight tolerances, or any case where a result feeds into a further calculation rather than being a final human-readable answer. In those cases, rounding early and reusing the rounded value in subsequent steps is itself a common source of compounding error, better to carry full precision through every intermediate step and round only the final displayed number.