Most conversion mistakes come from three sources: getting a prefix wrong, keeping too many or too few figures, and not checking whether the answer is plausible. The converter above handles the arithmetic exactly, but the inputs and the interpretation are still up to you. This guide covers the habits that make conversions reliable.
Prefixes do most of the work
SI units use the same prefixes for every quantity. Kilo means a thousand, mega a million and giga a billion. Milli means a thousandth, micro a millionth and nano a billionth. So a kilowatt is 1,000 watts, a megajoule a million joules, and a millimetre a thousandth of a metre. Case matters in the symbols: mW is a milliwatt, MW a megawatt, a difference of a billion times. When copying a figure from a datasheet, check the case of the prefix as carefully as the number.
How many figures to keep
A conversion can't be more precise than the original measurement. If a room is measured as "about 4 metres", converting it to 13.1234 feet gives a false impression of accuracy; "about 13 feet" is honest. As a rule of thumb, keep the same number of significant figures as the least precise input. Keep full precision during the calculation, though, and only round at the end. Rounding at intermediate steps is how small errors become large ones.
Quick sanity checks
A few rough facts catch most errors. A metre is a little over three feet. A kilogram is a little over two pounds. A litre is a little under two UK pints. A mile is about 1.6 kilometres. A kilowatt-hour is enough to run a 100-watt light bulb for ten hours. If your converted answer is wildly different from what these suggest, check the units and the direction of the conversion. Multiplying when you should have divided is the single most common slip.
Worked example
A specification lists a cable as rated for 2.5 kilowatts. Converting to watts gives 2,500 W. A 3 kW kettle draws 3,000 W, more than the rating, so the cable isn't suitable. The conversion is trivial; the value lies in noticing the prefix and comparing like with like.
Units that look alike
Several units share names but differ in size. Tonnes, long tons and short tons differ by up to 12%: a metric tonne is 1,000 kg, a UK long ton about 1,016 kg, and a US short ton about 907 kg. Ounces come in avoirdupois and troy versions, and fluid ounces in UK and US sizes. Gallons, pints and cups differ between the UK and US. Calories on food labels are kilocalories. Whenever a unit could come from either side of the Atlantic, or from a specialist field, check which version is meant before converting.
Converting compound units
Compound units such as speeds, densities and fuel economy need each part converted. To convert kilograms per cubic metre to pounds per cubic foot, convert the kilograms to pounds and the cubic metres to cubic feet separately, then divide. The converter does this automatically, but if you're working by hand, writing the units out at each step, and cancelling them like algebra, is the most reliable way to avoid errors.
When precision matters most
For everyday tasks, three significant figures are usually enough. For engineering, medicine and anything with safety margins, use the full conversion factor and round only at the end, then check the result against the specification it's meant to meet.
One more worked example
A product listing describes a garden water butt as holding 55 gallons. If the listing is American, that's about 208 litres; if it's British, about 250 litres. The difference, more than 40 litres, could matter for both the price comparison and the space it needs. Checking where the listing comes from before converting is the quickest way to get the right answer.