What Is an MP3 File? Compressed Audio, and What It Throws Away
MP3 is compressed audio. Jump to your question below, or work through it in order.
MP3 is compressed audio. Jump to your question below, or work through it in order.
An MP3 file is an audio file, compressed. Its full name is MPEG-1 Audio Layer III, or MPEG-2 Audio Layer III at the lower rates. The file holds sound alone: no video, no pictures, no subtitles.
Encoding drops sound that a louder sound is already covering. What comes out lands near a tenth the size of its WAV original, and dropped sound never returns.
Every system plays one already. What breaks is rarely the audio inside an MP3: it is a renamed download, a half-finished transfer or a missing tag.
MP3 stands for MPEG-1 Audio Layer III. Layer III is one of three audio codecs defined in one standard, and it is the complicated one. Layer I and Layer II are its simpler siblings, saved as .mp1 and .mp2, and almost nobody uses either now.
An MP3 is an audio file format and nothing else, and it is not a container. That sets it apart from most formats on this site. Matroska and MP4 are containers, boxes carrying the video, the audio and the subtitle streams together. An MP3 is not a box. It is one coded audio stream written straight to disk.
Lossy is the word that matters. Encoding decides what listeners will not notice and discards it permanently. Decoding rebuilds an approximation, which is why an MP3 can never become its original recording, whatever you convert it to afterwards.
Roughly a tenth the size, and loss runs one way. Three minutes of music is about 3 MB at 128 kbps, against the size of an uncompressed WAV or AIF file at about 30 MB. Nothing brings those 27 MB back.
.mp3, with .mp1 and .mp2 for Layer I and Layer IIaudio/mpeg, registered by RFC 3003Wrong page? If your file holds video rather than MP3 audio, read the MP4 format or MKV files instead. For Windows Media audio, read WMA files.
Not MPEG. Fraunhofer IIS in Erlangen built MP3, and MPEG published that work as a standard. Every directory ranking for this term credits the wrong one.
Look up MP3 anywhere and one line keeps coming back: developed by the Moving Picture Experts Group. FileInfo prints it in its Developer field. How-To Geek opens with it. docs.fileformat.com opens with it, then contradicts itself four paragraphs later by crediting Fraunhofer. Our own older page said it twice.
The work happened at the Fraunhofer Institute for Integrated Circuits in Erlangen, Germany. It began with Prof. Dieter Seitzer, who wanted to send music down a telephone line, and was carried by Karlheinz Brandenburg, Bernhard Grill, Harald Popp and Jürgen Herre.
The Moving Picture Experts Group, MPEG for short, was founded in January 1988 and is a standards committee. Committees take the work that already exists, test it against the rival submissions and publish whichever wins. That is what happened here, and standardizing is not inventing.
| When | What happened |
|---|---|
| 1987 | Fraunhofer IIS starts work on low bit rate audio coding, under EUREKA project EU147 |
| Jan 1988 | The Moving Picture Experts Group is founded |
| Apr 1989 | Fraunhofer receives a German patent |
| 1993 | ISO/IEC 11172-3 published. Layer I, Layer II and Layer III |
| 14 Jul 1995 | The name mp3 is chosen by an internal Fraunhofer email poll |
| 1996 | ID3v1 arrives, so a file can carry a song title |
| Nov 2000 | RFC 3003 registers audio/mpeg as a media type |
| Apr 2017 | The last US patent expires. Licensing ends and MP3 becomes free |
One detail settles this. Fraunhofer says in its own words that its name was “chosen on July 14, 1995, through an internal email poll”. An internal poll, inside the institute. Committees do not name a format by emailing around the office.
Why that credit keeps getting copied. MPEG is in this format's name, so MPEG looks like its author. One directory wrote it that way years ago and everybody else copied them. Now four of the first results agree with one another and disagree with Fraunhofer.
A genuine MP3 almost always plays, because every system ships a player. When one refuses, usually the file is not an MP3 at all.
MP3 sits in a rare position. There is no codec to install and no player to hunt for, because the support arrived on every platform twenty years ago. So a refusal points at your MP3 rather than at your software.
Open it in VLC first. VLC reads almost anything and names the format it found, which turns guesswork into an answer in ten seconds.
.mp3 on the end will not play as one. VLC names the real format.Notice what is missing from that list. Nothing about codec packs, nothing about drivers, nothing about the registry. Guides still print all three, and they were written for the video formats where a missing decoder is real.
Do not install a codec pack for an MP3. Windows has decoded MP3 since Windows 95 with an add-on and every version since as standard. A page telling you to download the codec for it is either very old or selling something.
Windows or Mac giving you trouble? Our Windows error guides and macOS fixes cover the file association problems behind most double-click failures.
Nothing to install. Windows, macOS, Android, iPhone and every browser play MP3 already. The free downloads below are for doing more than listening.
MP3 is the most widely supported audio file format ever made, so an honest answer to how to open MP3 files is: with whatever you already have. Every system on this list will open MP3 audio files without a download. Paid MP3 players advertised against this search sell you a feature your operating system shipped with.
| Software | Runs on | Why choose it |
|---|---|---|
| Windows Media Player | Windows | Built in. Double-click and it plays |
| Apple Music | Mac | Already there. It replaced iTunes in macOS Catalina |
| Chrome, Firefox, Edge | Everywhere | Drag the file into a tab. No install, no upload |
| VLC media player | Windows, Mac, Linux, Android, iOS | The troubleshooter. Plays odd files and names what it found |
| Audacity | Windows, Mac, Linux | Not a player. Edits, trims and exports audio free |
| foobar2000 | Windows, Mac | Light, fast, and honest about what is in a file |
| Winamp | Windows | Still going, and still what many people picture |
| Mp3tag | Windows, Mac | Fixes song titles and album art in batches |
Two names on old lists are gone. Microsoft Groove Music stopped streaming in 2017 and no longer ships with the operating system. XMMS on Linux has been unmaintained for years, and Rhythmbox, Clementine or Audacious replaced it.
Double-click. Your system plays MP3 with nothing added.
Install Audacity. Free, and it trims, joins and exports.
Drag an MP3 onto an open browser tab. Chrome, Firefox, Edge and Safari all decode MP3 natively, so playback starts at once and the file never leaves your computer.
That is worth knowing because the first page of results for opening audio is full of upload-and-convert sites. None of them is needed to hear an MP3 you already have.
A car stereo that refuses your file is usually reading the folder, not the format. Older head units cap how many files a folder may hold, want plain ASCII names or read FAT32 only. The MP3 files are fine; rename them and try again.
An MP3 has no file header. It is a run of frames, every one carrying its own 32-bit header that opens with eleven sync bits. Almost everything odd about the format follows from that.
Most formats open with a signature and MP3 does not. PDF opens with %PDF, PNG with eight fixed bytes. MP3 opens with whatever the first frame happens to be, because the format was designed for broadcast, where listeners tune in halfway through and there is no beginning to read.
So players hunt. One scans until it meets 11 bits in a row, all set. It then checks that the bits after them make sense and starts decoding there.
| Part | What it holds |
|---|---|
| ID3v2 tag | Optional. Title, artist, album and cover art, at the front |
| Frames | The audio. Everything else on this list is optional |
| Frame header | 32 bits opening every frame: sync, version, layer, bit rate, rate |
| CRC | Optional. 16 bits after the header when the protection bit says so |
| Side information | How the encoder packed this frame, and where its bits sit |
| Xing or Info tag | Optional. A frame with no audio, holding the frame count |
| ID3v1 tag | Optional. The last 128 bytes, beginning with the letters TAG |
Every MP3 frame carries 1,152 samples of audio in MPEG-1 Layer III, which at 44,100 Hz is about 26 milliseconds of sound. The lower-rate versions carry 576 samples.
Frames are not fully independent. Layer III lets one borrow the spare room in the frames before it, so cutting an MP3 at an arbitrary byte gives a click at the join.
| Field | Bits | Values |
|---|---|---|
| Frame sync | 11 | All set. 1111 1111 111 |
| Version | 2 | 11 MPEG-1, 10 MPEG-2, 00 MPEG-2.5 |
| Layer | 2 | 01 Layer III, 10 Layer II, 11 Layer I |
| Protection | 1 | 0 means a CRC follows. 1 means none |
| Bit rate index | 4 | 32 to 320 kbit/s, or free |
| Sampling rate | 2 | 44,100, 48,000 or 32,000 Hz on MPEG-1 |
| Padding | 1 | Set means this frame is one slot longer |
| Channel mode | 2 | Stereo, joint stereo, dual channel or mono |
The protection bit reads backwards and catches people out. A 0 means the frame is protected and a CRC follows the header. A 1 means no CRC at all, which is what nearly every encoder writes.
MPEG-2.5 is not a standard. The version field has a slot for it and the specification does not. Fraunhofer added it to reach 8,000, 11,025 and 12,000 Hz for speech, and the header documentation calls it an unofficial extension of MPEG-2.
Song title, artist and cover art live in an ID3 tag, which is not part of the MP3 audio. Losing the tag costs you the name and nothing else. The sound plays perfectly.
MP3 as published had nowhere to put a song title. It was designed to carry sound, and broadcast streams have no need for album names. MP3 tags were bolted on afterwards by people outside the standards process, which is why there are two of them and they sit at opposite ends of the file.
Eric Kemp introduced ID3v1 in 1996 in his program Studio3. It is the last 128 bytes, right at the end of the file, and it starts with three letters, TAG.
Its fields are fixed sizes. Thirty bytes for a title, 30 for an artist, 30 for an album, 4 for a year, 30 for a comment and 1 for a genre number.
Thirty bytes is short, which is why long titles arrive truncated on old players. ID3v2 answered that by moving to the front of the file with no such limit, and by making room for album art, lyrics and much more.
Martin Nilsson wrote that specification, and he is also the author of RFC 3003, which registered the media type.
| ID3v1 | ID3v2 | |
|---|---|---|
| Where | Last 128 bytes | The front of the file |
| Marker | TAG | ID3 |
| Arrived | 1996, Eric Kemp | 1998 |
| Title length | 30 bytes, hard cap | No practical limit |
| Cover art | No | Yes |
| Track number | Only in v1.1 | Yes |
| Non-English text | Unreliable | Unicode |
ID3v1.1 is a neat piece of salvage. Michael Mutschler noticed comment fields were padded with zeros, cut his to 28 bytes and used the second to last byte for a track number. One spare byte, and suddenly a CD ripped in order stayed in order.
The tag is gone, not the song. Mp3tag rewrites them in a batch.
Only ID3v2 carries art. An ID3v1 file has nowhere to put it.
One practical consequence: players reading only ID3v1 ignore everything you typed into ID3v2, and the reverse. Old car stereos and cheap players are usual culprits. Mp3tag writes both at once, which ends that argument.
Comparing audio formats? the WMA format was Microsoft's answer to MP3, and MP4 audio is what replaced MP3 in most music stores.
Bit rate sets how many bits of data go into every second of sound, and the quality of an MP3 is mostly what people mean when they talk about it. More bits means an MP3 throws less away. Above 192 kbps most listeners stop hearing the difference, and the encoder matters as much as the number.
One sentence appears in almost every guide, including the older version of this page, and it is wrong: MP3 removes the sounds your ears cannot hear. It does not. Your ears hear them fine.
The proper name for what MP3 does is perceptual audio coding, and the model behind it is psychoacoustic: a study of what people actually notice.
The compression algorithm does not delete the frequencies you cannot hear. What it removes is sound that another, louder sound is already covering. Hit a cymbal over a quiet guitar note and the note is still there, still inside your hearing range. You will not hear it.
This effect is called masking, and modeling it is the entire job. The encoder decides, moment by moment, what is safe to drop, and the decision is irreversible: once encoded, it cannot be undone.
That difference explains what people actually notice. A low bit rate does not make an MP3 quieter or duller at the edges. It makes the crowded moments smear, because the encoder ran out of room and guessed which parts were hidden.
It also explains why encoders matter. Masking is a judgment, so two programs at one bit rate produce different files. LAME won that argument in the early 2000s, and 128 kbps files from 1999 sound worse than 128 kbps files today because older encoders guessed badly.
| Bit rate | Per minute | Use it for |
|---|---|---|
| 32 kbit/s | 0.2 MB | The floor. Voice memos and nothing else |
| 64 kbps | 0.5 MB | Speech. Music falls apart here |
| 128 kbps | 0.9 MB | The old default. Audible on busy music |
| 192 kbps | 1.4 MB | The sensible floor for music |
| 256 kbps | 1.8 MB | Comfortable. Hard to fault on any system |
| 320 kbps | 2.3 MB | The ceiling. Nothing in MP3 goes higher |
| V0, variable | 1.6 MB | Better than 256 at a smaller size. Ask for this |
That last row is worth acting on. Constant bit rate, written CBR, spends equal bits on a drum solo and on silence, which is waste at one end and shortage at the other. Variable bit rate moves those bits to where music actually is.
Ask for V0 or 320. Storage is cheap and re-encoding later is not free.
128 is fine for speech. An hour of talk fits in 55 MB.
320 kbps is not a quality guarantee. Encoding a 128 kbps file at 320 gives you a bigger copy of the same damage. The number describes what the encoder was allowed to spend, never what the source was worth.
AAC and Opus do more with equal bits. FLAC throws nothing away. MP3 loses on every technical measure and wins the only one deciding most transfers, which is that everything plays it.
Four formats, three jobs. MP3 is one of the two that throw sound away to make a smaller file, and the trade is quality sound against space. MP3 and AAC discard audio to save space. FLAC gives you a lossless file, compressed without discarding anything. WAV compresses nothing at all and stores the samples as recorded.
| MP3 | AAC | FLAC | WAV | |
|---|---|---|---|---|
| Reduces the data | Yes, by discarding | Yes, by discarding | Yes, safely | No |
| 3-minute song at 192 kbps | 4 MB | 3.5 MB | 18 MB | 30 MB |
| Quality per bit | Good | Better | Perfect | Perfect |
| Plays everywhere | Yes | Nearly | No | Yes |
| Old car stereo | Yes | Sometimes | Rarely | Sometimes |
| Good for archiving | No | No | Yes | Yes |
| Patents | All expired | Licensed | Free | Free |
Read down that MP3 column and it loses four rows out of seven, because AAC and Opus are more efficient with every bit they are given. Read one row, about playing everywhere, and everything changes. Being compatible with everything is what your file has to survive when you cannot see what is at the other end.
Apple moved the iTunes Store to AAC. Spotify streams Ogg Vorbis and AAC. YouTube uses Opus. All three chose efficiency, and all three still accept an MP3 upload, because refusing one would be commercial suicide.
Use FLAC. Half the size of WAV, and nothing is thrown away.
Send MP3. It plays on whatever they own, without asking first.
Working with other media formats? The MP4 container holds the M4A audio the music stores use. Windows Media Audio came from Microsoft, and FLV files carried early web audio.
Convert from the original recording, never from the MP3. Every re-encode throws away more, and the losses stack. Compressing an MP3 smaller means encoding it again, so do it once and keep your source.
Converting an MP3 is easy and most of the advice about it is backwards. One rule covers it: every conversion out of MP3 carries the damage along, and every conversion back adds more. Engineers call that stacking generation loss, and it is why you never convert back to MP3 from a copy.
Turning an MP3 into FLAC is the clearest example. FLAC is lossless, so people expect a rescue. What you actually get is a file three times bigger holding exactly what your MP3 had, damage included. Lossless means losing nothing from here, not recovering anything.
That line encodes with LAME at variable quality 2, which lands near 190 kbps and suits most people. Lower that number for better quality: -q:a 0 is the best LAME offers.
Watch what an online converter does. Many re-encode at a fixed 128 kbps whatever you fed them, so a 320 kbps file comes back visibly smaller and audibly worse. Check the bit rate of what you get back before deleting the original.
Cutting silence off the front of a track does not need a re-encode. Tools built for MP3, including mp3splt and mp3DirectCut, cut on frame boundaries and copy everything else untouched, so your audio is identical and it takes a second.
Most MP3 faults are not damage. A wrong length, a jumping seek bar and a click at one spot all have ordinary causes, and the audio around them is intact.
An MP3 stores no duration. Nothing inside an MP3 says how long the song runs, so a player works it out: divide the file size by the bit rate and you have an answer.
That arithmetic holds at constant bit rate, where every frame is the same size. At variable bit rate it falls apart, because the encoder spent more bits on the chorus than on the intro, so any average is a guess.
| Symptom | Usually | Fix |
|---|---|---|
| MP3 length is wrong | A VBR file with no frame count | Rebuild the tag in foobar2000 |
| Seek bar jumps around | The same missing frame count | The same fix |
| Plays then stops early | A truncated download | Fetch it again |
| A click at one spot | Bytes damaged in one frame | 26 milliseconds lost. Ignore it |
| Silence at the start | Encoder padding | Normal. Gapless players hide it |
| Nothing plays at all | It is not an MP3 | Open it in VLC. See section 3 |
Both top rows have one fix. Xing solved this in the 1990s with a Xing header, a frame count written into the first frame, a frame holding no audio, there only to carry the number. LAME kept the idea and called its version the Info tag.
MP3 files made without one, or files somebody cut with a blunt tool, lost it. foobar2000 rebuilds it. Select the tracks, run Utilities, then Fix VBR MP3 Header. Both the length and the seek bar come right.
A damaged MP3 still plays. Corrupt a few bytes and the decoder loses one frame, hears the next sync word and carries on. You get a click. Formats with one index at the front lose the entire file to the same accident.
Keep MP3 for anything you send to other people. Use FLAC, not MP3, for anything you are storing. The two jobs pull in opposite directions, and reading them as one question is how libraries get ruined.
MP3 is the format that put music on a hard drive, filled portable players and made services like Napster possible. In May 2017 the technology press buried it. The headlines said MP3 was dead, and that its own creators had killed it.
What actually happened: the last United States patent expired in April 2017, so Fraunhofer and Technicolor ended a licensing program that had collected fees on it. Fraunhofer's own wording was that its “licensing program for certain mp3 related patents and software of Technicolor and Fraunhofer IIS has been terminated”.
A patent expiring does not kill a format. It frees it. MP3 went from a technology you had to license to one anybody can encode, decode and ship without asking, and the reporting turned that into a funeral.
Use MP3. It plays on whatever the other person owns.
Use FLAC. Nothing is discarded, and you can make MP3s from it later.
Everything on this page in one block: names, rates, the media type and the standards that define this format.
Directories file this under the MP3 file type, and the file type is what decides your answer. That MP3 file extension your file manager shows has meant one thing since 1995: MPEG-1 Audio Layer III wrote the sound inside.
| Item | Value |
|---|---|
| Extension | .mp3, plus .mp1 and .mp2 for the two simpler layers |
| Full name | MPEG-1 Audio Layer III |
| Kind | Lossy audio coding. Not a container |
| Built by | Fraunhofer IIS, Erlangen, from 1987 |
| Named | 14 July 1995, by an internal email poll |
| Standard | ISO/IEC 11172-3, 1993. MPEG-2 rates in ISO/IEC 13818-3 |
| MPEG-2.5 | A Fraunhofer extension. Not in any standard |
| Media type | audio/mpeg, registered by RFC 3003, November 2000 |
| Signature | None. Frames start with 11 sync bits, all set |
| Bit rates | 32, 40, 48, 56, 64, 80, 96, 112, 128, 160, 192, 224, 256, 320 kbit/s |
| Sampling rates | 32,000, 44,100 and 48,000 Hz on MPEG-1 |
| Samples per frame | 1,152 on MPEG-1, 576 on MPEG-2 and MPEG-2.5 |
| Channels | Stereo, joint stereo, dual channel or mono |
| Patents | All expired. Licensing ended April 2017 |
The frame header, the bit rates and the sampling rates come from the MPEG audio frame header specification and ISO/IEC 11172-3. The media type comes from RFC 3003, published by the IETF in November 2000. The founding dates and the 14 July 1995 naming come from Fraunhofer IIS in its own words. The ID3v1 layout comes from the ID3 specification.
The patent expiry and the licensing statement come from Fraunhofer, quoted by NPR in May 2017.
Try VLC. Free everywhere, and it names the real format. See section 4.
Rebuild the VBR header. Section 10 has the steps.
Comparing audio formats? the Windows Media codec was Microsoft's answer to this one, and MP4 files hold the M4A audio the music stores moved to.
Working with video instead? the Matroska container and AVI files carry picture and sound together, and Flash video files did the same job on early web pages.
Stuck on your computer rather than your file? Our Windows fixes and macOS guides cover the system side, including file associations.
New to all of this? What a file format is covers the ground under it, and our file format guides list the rest.
Found an MP3 file that behaves in a way nothing here explains? Send us what your player reports and we will tell you what is wrong with it.