Two families, and they fail differently
The tools here look alike but split into two groups that go wrong in completely different ways.
Physical quantities — length, weight, area, volume, speed, pressure, energy, power, angle, time and temperature. These are arithmetic. One multiplication, occasionally an offset. If you picked the right two units, the answer is right — and if you picked the wrong ones, the answer is obviously wrong by a factor of a thousand and you notice immediately.
Representations — Base64, Base64 back to an image, text to binary, data units, number systems and Roman numerals. These are rewrites, and they fail quietly. A representation converter can hand you output that is perfectly well-formed and still wrong, because the mistake was in an assumption neither of you stated.
The quiet failures worth knowing about
Which characters counted as characters. Encoding text to Base64 or binary means encoding bytes, and text only becomes bytes once you choose an encoding. Anything outside plain ASCII — accents, emoji, Cyrillic, CJK — is one character but several bytes in UTF-8. Tools that assume one byte per character corrupt exactly those characters and pass everything else through cleanly, which is why the damage often surfaces days later in one customer’s name.
Which sign convention. Negative numbers have no single binary spelling. A tool can show you a minus sign and a magnitude, or two’s complement, and both are “correct” — but only one matches what your program is holding. This is the whole reason the number system converter exists as a separate tool rather than a checkbox.
Which definition of the unit. Data units are the one place where the two families collide: a “megabyte” is either 1,000,000 or 1,048,576 bytes depending on who is counting, so the same file has two true sizes. The data unit converter keeps both columns visible instead of picking for you.
What lives elsewhere
Colour values (HEX, RGB, OKLCH) are conversions too, but they sit under Generators with the colour wheel. Timestamps — epoch seconds to a readable date — are under Developers. Image format conversion is under Images.
Every conversion on this page runs as you type, in the page. Nothing is sent anywhere, which is also why there is no rate limit.