Free Online Converters
A converter writes the same value in a different notation: kilograms as pounds, 255 as FF, 1984 as MCMLXXXIV, 3pm in Tokyo as the time in London. The value never changes, only how it is written.
Showing 12 tools
Video Compressor
Make a video smaller in your browser. Choose how hard to compress, drop the resolution if you want, and download an MP4 or WebM. Nothing is uploaded.
Video Converter
Convert MOV, MP4, WebM and GIF in your browser, with MOV to MP4 done losslessly
Audio Extractor
Pull the audio track out of a video in your browser, losslessly where the format allows
Audio Converter
Convert M4A, MP3, WAV, OGG and FLAC in your browser, with nothing uploaded
Unit Converter
Convert length, weight, temperature, area, volume, speed, time, data, pressure, energy, and angle units instantly with exact conversion factors
Time Zone Converter
Convert any date and time between IANA time zones with DST-accurate offsets, see the gap between two cities, and plan meetings across the world
Number Base Converter
Convert integers between binary, octal, decimal, hexadecimal, and any base from 2 to 36 with exact BigInt precision and live bit and byte counts
Roman Numeral Converter
Convert numbers to Roman numerals and Roman numerals back to numbers, with instant validation, a symbol breakdown, and full support for the standard 1 to 3999 range.
Number to Words Converter
Convert numbers to words and words back to numbers in English - with cardinal, ordinal, currency, and decimal support for values from small integers up to decillions.
NATO Phonetic Alphabet Translator
Translate text into the NATO phonetic alphabet (Alfa, Bravo, Charlie) and decode code words back to letters
Scientific Notation Converter
Convert numbers between scientific notation, E-notation, engineering notation, and plain decimal with exact digit-by-digit precision, plus order of magnitude and significant-figure rounding.
EPUB to PDF
Convert an EPUB ebook into a PDF you can print or read anywhere, or into plain text. Runs in your browser, so the book is never uploaded.
An online converter restates the same quantity in a different notation. The number does not change when 255 becomes FF, when 1984 becomes MCMLXXXIV or when a kilogram becomes 2.205 pounds; only the way of writing it does. That is what separates this category from a calculator, which produces a value that was not there before.
Two of these online converters carry more subtlety than the rest. The time zone converter has to know not just the offset of a place but the offset it had on the date in question, since daylight saving rules change and countries change them at short notice. The unit converter has to know which conversions are exact definitions and which are measured, because an inch has been exactly 25.4 millimetres since 1959 while a calorie has several competing definitions still in circulation. Scientific notation is the same problem at the far ends of the scale: 6.022e23 and 602,200,000,000,000,000,000,000 are one number, and the converter moves it between scientific, E-notation, engineering and plain decimal form.
Two tools in here are doing something else, and it is worth saying so: the video compressor does not restate a quantity, it re-encodes a file so that it takes up less room, and the video converter changes the format a clip is stored in, turning a MOV into an MP4 or a clip into a GIF. The audio extractor pulls the sound track out of a video on its own, and the audio converter turns an M4A voice memo into an MP3 that other software will accept. All four are closer to the rest of the category than they look, because a MOV and an MP4 usually hold identical video, and the audio inside an MP4 is already the AAC that an M4A carries: in both cases the conversion is a relabelling rather than a re-encode. It sits in this category because the question people arrive with is the same shape - take what I have and give it back in a form I can use - and because the answer is the same promise: the work happens in your browser and the file is never uploaded. Books work the same way: an EPUB is a zip of web pages, so converting one to PDF means typesetting it afresh rather than copying a layout it never had. Still images have a set of their own in image conversion, and when the job is cutting or joining the video rather than changing its container, that is video editing.
What each converter translates between
| Converter | From | To |
|---|---|---|
| Unit Converter | Length, weight, temperature, area, volume | Speed, time, data, pressure, energy |
| Number Base Converter | Binary, octal, decimal, hex | Any base from 2 to 36, with bit and byte counts |
| Time Zone Converter | A date and time in one place | The same instant in 50+ cities, DST-accurate |
| Roman Numeral Converter | Numbers from 1 to 3999 | Standard subtractive numerals, and back |
| Number to Words | Any number | Cardinal, ordinal, currency or decimal English |
| NATO Phonetic Alphabet | Text | ICAO spelling alphabet, with aviation digits |
Time zones are a database, not arithmetic
Converting a time between two places is not subtraction, because an offset is a property of a place on a date rather than of the place itself. Most of Europe and North America shift by an hour twice a year, on different dates from each other; some countries have abolished the practice recently; a few use offsets that are not whole hours, such as India at UTC+5:30 and Nepal at UTC+5:45. All of this lives in the IANA time zone database, which browsers ship and update, and which the time zone converter reads through the Intl API so that a date in March gets March rules.
The practical failure this prevents is a meeting scheduled across a week when one side changes clocks and the other has not yet. If you take one thing from this: convert with the date attached, never with a remembered offset, and prefer naming a city over naming an abbreviation, since CST alone is ambiguous between North America, China and Cuba.
Bases, and where developers get bitten
Hexadecimal is base 16, so each digit stands for exactly four bits and each pair for one byte, which is the only reason it is used for colours, memory addresses and byte dumps rather than because it is inherently readable. Octal is base 8 and survives mostly in Unix file permissions. Binary is base 2, and the count of digits is the count of bits, which is why the base converter reports bit and byte lengths alongside the digits.
The trap in most converters is precision. A value parsed into a double-precision float loses exactness above 2^53, so a 64-bit identifier or hash converted through one comes back subtly wrong, and nothing warns you. This converter uses arbitrary-precision integers, so a value of any length converts exactly across any base from 2 to 36.
The notations that come with rules
Roman numerals stop at 3999 because the standard ASCII symbols stop at M for a thousand, and larger values need a vinculum, the overbar meaning multiply by a thousand, which is not plain text. Within that range the rules are the subtractive ones: only I, X and C may be subtracted, and only from the next two values up, so 4 is IV and never IIII, and 99 is XCIX rather than IC. The Roman numeral converter validates in both directions, which is how it catches numerals that look plausible and are not.
Spelling numbers out has a rule that changes by geography. English now uses the short scale, in which a billion is a thousand million, and this is what the number to words converter produces; the long scale, where a billion means a million million, is still in use across much of Europe and is the reason a translated financial figure can be out by a factor of a thousand. The NATO phonetic alphabet has a similar split: the ICAO spelling alphabet is standard, and aviation and military radio additionally replace several digits with Tree, Fower, Fife and Niner, which exist because those numbers are the ones that sound alike over a noisy channel.
Frequently asked questions
Why do Roman numerals stop at 3999?
Because M is the largest standard symbol, so 3999 is MMMCMXCIX and 4000 would need four thousands in a row. Larger numbers historically used a vinculum, a bar drawn over a numeral to multiply it by a thousand, which has no plain-text equivalent. Every converter that works in ASCII has the same ceiling.
Is a billion the same everywhere?
No. English uses the short scale, where a billion is a thousand million, and the number to words converter produces that. Much of continental Europe uses the long scale, where a billion is a million million and the thousand-million step is called a milliard. The difference is a factor of a thousand, which is why translated financial figures are worth checking.
Does the time zone converter handle daylight saving?
Yes, per date. It reads the IANA time zone database through the browser Intl API, so a conversion in March uses March rules and one in November uses November rules, including places that abolished the switch or never had it. Converting with a remembered offset instead is what produces meetings that are right this week and an hour out next week.
Can these converters handle very large numbers?
Yes. The base converter uses arbitrary-precision integers, so values beyond the 2^53 limit of a double-precision float convert exactly. That limit is where most converters quietly break: a 64-bit identifier or hash passed through one comes back with its low digits altered and no warning that anything happened.
Why does aviation say Niner instead of Nine?
Because nine and the German nein sound alike over a noisy radio channel, and the same reasoning gives Tree for three, Fower for four and Fife for five, all of which are hard to confuse with anything else. The standard ICAO letter words are used everywhere; the modified digits are specific to aviation and military radio procedure.
Do these converters work offline?
Yes. Every conversion here is computed client-side from tables and factors held in the page, with no lookup call to a currency or measurement service, so all six keep working with the network disconnected. Nothing you enter is transmitted or logged.