This distinction matters more than people expect. A handful of imperial units were redefined in terms of metric ones by international agreement in 1959, so those conversions are exact by definition and carry no rounding error at all:
Others are measured constants, not definitions, and any digit you quote beyond the source precision is invented. A nautical mile is exactly 1852 m by convention, but the metric horsepower and the mechanical horsepower differ by about 1.4% and neither is exact.
The most common real-world conversion error in the English-speaking world. There are two gallons still in use and they are not close:
| Unit | In litres | Used in |
|---|---|---|
| US liquid gallon | 3.785412 | United States |
| Imperial gallon | 4.546092 | UK, Canada, Caribbean |
The imperial gallon is 20% larger. A car rated at 30 miles per US gallon is 36 miles per imperial gallon for exactly the same fuel consumption — the car did not change, only the container. The same split runs through pints (473 ml US, 568 ml imperial) and fluid ounces (29.57 ml US, 28.41 ml imperial), where the ounce is confusingly larger in the US while the gallon is smaller. Always check which system a recipe, spec sheet or fuel figure came from.
A kilogram is mass. A pound-force is weight — a force. Converting kg to lb works on Earth because we quietly assume standard gravity of 9.80665 m per second squared. On the Moon your mass is unchanged and your weight is about one sixth. For everyday shopping and shipping this never matters; in engineering it does, which is why the kilogram-force and the newton exist as separate units. If a spec sheet mixes kgf, lbf and N, read it carefully before converting.
Converting 5 feet to 1524.0 mm implies you measured to a tenth of a millimetre. You did not. A conversion cannot create precision that was not in the original measurement, so the sensible rule is to keep the number of significant figures you started with:
This tool shows a generous number of decimals so you can round to whatever your task deserves. It will not decide for you how precise your original measurement was.
A frequent mistake: since 1 m = 3.2808 ft, people assume 1 square metre is 3.2808 square feet. It is 3.2808 squared, or 10.764 sq ft. For volume the factor cubes: 1 cubic metre is 35.315 cubic feet, not 3.28. Doubling a length multiplies area by four and volume by eight, which is also why a pizza twice the diameter is four times the pizza.
Almost always a rounding choice, occasionally a different definition of the same unit name. Compare the factor each tool uses, not the final digit. If the factor differs, check whether one of you is using the US and the other the imperial version.
Temperature needs its own treatment because the scales have different zero points, so a simple multiplication does not work. Use the dedicated temperature converter instead.
Metric, in almost every case. It is the legal standard nearly everywhere, its prefixes scale cleanly by powers of ten, and it avoids the gallon and ounce ambiguity entirely. Convert to imperial only at the point of presenting to an audience that needs it.
No. The factors are built into the page and the arithmetic happens in your browser. Nothing is sent to a server.
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