Match the quantity before converting
Length, area, volume, temperature, speed, pressure, energy, power, mass, and digital storage may all appear as one number followed by a short unit. They are not interchangeable quantities. A factor that correctly changes metres to feet does not change square metres to square feet, and a temperature scale needs an offset rather than a single multiplier.
Read the convention on the source
US and Imperial gallons are different. Decimal gigabytes and binary gibibytes are different. Gauge pressure and absolute pressure have different reference points. Before entering a figure, copy the unit exactly as it is written on the label, instrument, plan, or bill; the right arithmetic cannot repair a mismatched convention.
Keep the original precision
Convert the measured value before rounding for a packing list, layout, recipe, or calculation that will use the result again. The final display may be rounded to a practical increment, but early rounding can accumulate when several converted measurements are added together.
Conversion is only the first operation
A converted area still needs a waste allowance, a converted speed still needs route conditions, and a converted pressure still needs an equipment specification. These tools state the numerical relationship between units; the document or physical system supplies the decision rule.
Check direction and magnitude
Before relying on an answer, estimate whether it should be larger or smaller. Metres become more feet, litres become fewer US gallons, and decimal gigabytes become more megabytes. Temperature is the exception that reveals why a rule of thumb is not enough: the Fahrenheit reading changes because its zero point differs. A quick direction check catches reversed factors and misplaced decimal points without needing another calculator.
Preserve the source record
Write down the original reading, its unit, the conversion convention, and any rounding used in the final document. This creates an audit trail for a contractor, traveller, student, or equipment operator who later needs to reproduce the result. It also makes clear whether a disagreement comes from arithmetic, a different gallon or storage prefix, or from a measurement that was never suitable for the purpose.