Every conversion goes through one base unit
Each category has a base unit - the meter, the kilogram, the liter, the square meter, the meter per second, the second, the byte, the kelvin - and every conversion happens in two steps: multiply into the base, then divide out of it. Nothing on the page converts directly from one unit to another.
That is why the table under the answer holds together. Every row is produced from the same base reading, so no row is derived from another row and a drift cannot creep in between the third line and the ninth. It is also why four gallons of paint converts to liters and to cubic meters with the same number of liters at the center.
It makes checking easy too. If the answer for feet looks wrong, the row for meters is the row to check, because everything else is that one number multiplied or divided.
The factors are exact, and the page says which exact amount
An inch is 25.4 mm exactly, so a foot is 304.8 mm, a yard is 914.4 mm and a mile is 1609.344 m - all exact, none rounded. A pound is 0.45359237 kg exactly, an ounce is that divided by 16, and a stone is exactly 14 pounds. An acre is exactly 4840 square yards and a hectare is exactly 10,000 square meters.
Two factors are ratios rather than decimals: one kilometer per hour is 1/3.6 meters per second and one knot is 1852/3600 meters per second. The page prints those as divisions, because 0.2777777777777778 is a rounded value wearing the clothes of a definition, and a factor is the one place where the difference shows.
Data is the exception that proves the rule. A kilobyte is 1000 bytes and a kibibyte is 1024 bytes, both by definition, so both are in the table. Neither is "the real one": a drive sold as 1 TB holds 1000 GB, which is 931.32 GiB, and that 7% difference is the whole of the "my drive is smaller than advertised" story.
Temperature is an offset as well as a scale
Every other category here converts by multiplying. Temperature does not: 0 °C is 32 °F and a Fahrenheit degree is 5/9 of a Celsius degree, so one reading is an offset and a scale together. The conversion goes through kelvin, and its two anchors are 0 °C = 273.15 K and 0 °C = 32 °F.
That is why -40 is the same number in both scales (the offset and the scale happen to cancel there) and why 0 °C is not 0 °F. It is also the only place this tool refuses a value: -273.15 °C is absolute zero, so -300 °C is a typo rather than a temperature, and the page says which reading would be the coldest possible instead of printing a negative kelvin.
What is not in any list, and where to go instead
Months and years are not units of time in this tool, because a month is 28 to 31 days and a year is 365 or 366: any single factor would be wrong most of the time. "Three months from today" is a question for the date calculator, which knows which month it is in, and "how long is 90 days" is answered exactly here, because a day is always 86400 seconds.
Calories are missing because the calorie on a food label is a kilocalorie: 1 kcal = 1000 cal = 4184 J exactly, and both names are used interchangeably, often in the same paragraph. The calorie calculator is where energy belongs.
Currency is missing because an exchange rate is not a unit: it changes, it has a date and a spread, and it depends on who is quoting it. The currency converter is where that lives.
Angle is missing for a cleaner reason. A degree and a radian are a different kind of quantity rather than a different unit of the same one, and putting them in a list beside lengths would invite exactly the mistake this page exists to prevent.
How the answer is rounded, and where the unrounded one is
Answers are shown to ten significant digits, not to a fixed number of decimal places, because a conversion produces numbers of every size: 1 mile is 1609344 mm and 0.8689762419 nautical miles. A value that comes out as an exact integer - 86400 seconds in a day, 1609344 millimeters in a mile - is printed in full rather than rounded to something tidier.
Values below a millionth or above a quadrillion switch to exponential notation (6.213711922e-7), where the decimal form stops being readable. Rounding noise from the browser’s number format is removed rather than shown: 1609344.0000000002 is 1609344.
The download holds exactly the numbers on screen, because the same rule that renders the table writes the file, so nothing is rounded a second time on the way out. The CSV keeps the factor column as well, so a spreadsheet can redo the arithmetic and check it.
Danfio