Most unit conversions people need aren't exotic, they're the same handful that come up constantly across cooking, travel, fitness, and work. Here's what actually matters about each, and where rounding gets people into trouble.
Length: the inch-to-centimeter trap
1 inch is exactly 2.54 centimeters, by international agreement since 1959, not an approximation. Where people go wrong is compounding rounded intermediate values: converting feet to inches to centimeters and rounding at each step introduces error that a single direct conversion wouldn't have. For anything precision-sensitive (a manufacturing tolerance, a doorway measurement), convert once, directly, at full precision, then round only the final answer.
Weight: kilograms vs. pounds, and the stone wildcard
1 kilogram equals approximately 2.20462 pounds. The number people forget: 1 stone (used for body weight in the UK and Ireland) equals 14 pounds, not a round metric-friendly number at all, it's a historical unit that predates the metric system entirely and just never got fully replaced for that specific use case.
Temperature: the formula, not just the two fixed points
Everyone remembers that 0°C is 32°F and 100°C is 212°F, but the actual formula for everything in between is °F = (°C × 9/5) + 32. The common mistake is assuming a linear +32 shift works everywhere; it doesn't, because Celsius and Fahrenheit also have different-sized degrees (a Fahrenheit degree is 5/9 the size of a Celsius one), which is exactly what the 9/5 multiplier corrects for.
Data storage: why your "1TB" drive shows less than 1000GB
This one causes more confused support tickets than almost any other conversion. Drive manufacturers use decimal units, where 1TB = 1000GB = 1,000,000,000,000 bytes. Operating systems typically report binaryunits, where what they call a gigabyte is actually a gibibyte, 1,073,741,824 bytes. The physical drive has exactly the storage advertised; the two measurement systems just disagree on what "giga" means, and the gap compounds as the numbers get bigger. A data storage converter that clearly separates decimal and binary avoids the confusion entirely instead of picking one silently.
Time zones: why "just add the offset" breaks twice a year
A fixed UTC offset works fine most of the year, until daylight saving time shifts in one region but not another (or shifts on a different date), and suddenly your fixed-offset math is off by an hour. Real timezone conversion has to account for the actual daylight saving rules of both locations on the specific date in question, not just today's offset. A timezone converterthat uses your browser or system's real timezone database, rather than a hardcoded offset table, gets this right automatically, including for dates in the past or future.
Currency: the one conversion you should never hardcode
Unlike every conversion above, exchange rates aren't a fixed mathematical relationship, they move constantly based on markets. Any tool or spreadsheet with a hardcoded exchange rate is wrong the moment rates move, which is why we deliberately don't offer a currency converter with a static rate baked in: it would quietly become inaccurate over time in a way that looks authoritative. For real transactions, always pull a live rate at the moment you need it.
The pattern underneath all of this
Length, weight, and temperature are fixed mathematical relationships: convert once at full precision and you're done forever. Data storage is a definitional disagreement between two industries, know which one you're looking at. Time zones and currency are the two that actually change over time or by date, and are the two most likely to silently go wrong if you treat them like the fixed ones above.
ToolVyne's Unit Converter covers length, weight, volume, area, and speed with exact, non-compounding conversion factors in one place, alongside the dedicated data storage and timezone converters for the two cases that need special handling.