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Convert MP3 Audio Free

An MP3 is thousands of tiny frames, each 26 milliseconds long.

There's no overall header that describes the file. Each frame starts with its own 4-byte label saying how it's encoded, then carries a slice of sound. That's why you can cut an MP3 anywhere, start a stream mid-way, and why a player can resume after a glitch. Click a frame to read its label.

song.mp3 · first second · variable bitrate38 frames shown · about 38 per second

sellID3 tag

frame width = size in bytes · orange cap = the 4-byte header

header of frame 25 · 32 bits

11111111111110111100000001100100

bitrate224 kbpsframe size731 bytessample rate44.1 kHzchannelsjoint stereopaddingnoneduration26.1 ms

infoA moderate passage: 224 kbps, 731 bytes for 26 ms of sound.

encoding

Variable (VBR)bitrate follows the musicConstant (CBR)every frame the same

Quiet, simple passages get small frames; the loud section in the middle gets 256-320 kbps frames. Better quality per megabyte, but older players sometimes show the wrong length.

average bitrate178 kbps3:34 song4.8 MB

~/sounds-you-wont-miss

MP3 throws away sound your ears were going to ignore.

The encoder uses a model of human hearing to decide what to discard. It doesn't cut evenly; it removes the parts other sounds would drown out anyway. These are the three main tricks, and what each saves.

Beyond hearingsaves ~10%Frequencies above about 16-19 kHz are cut. Most adults can’t hear them, and at 128 kbps the cut-off is lower still.

Maskingsaves the mostA loud sound hides quieter ones at nearby pitches. Those hidden sounds are removed or stored roughly.

Joint stereosaves ~15%Left and right are mostly the same, so the encoder stores what they share once, plus the small difference.

keptremoved by the encoder

~/the-id3-block

Song title and album art live in a block bolted to the front.

The MP3 standard had nowhere to store the title or artist, so a separate format called ID3 was added on. Players skip it before reading frames. Here's what a typical block holds, and why a small song can have a surprisingly big file.

TIT2TitleMidnight Train32 B

TPE1ArtistThe Lanterns30 B

TALBAlbumNorthern Lines32 B

TYERYear202416 B

TRCKTrack4/1116 B

USLTLyricsfull song lyrics2.1 KB

APICAlbum art1400 × 1400 JPEG412 KB

whole tagabout 9% of this 4.8 MB file≈ 414 KB

~/inspect-an-mp3

Every frame, tag and hidden delay in your MP3.

Drop an MP3 to walk it frame by frame: the ID3 tags at the front, the Xing or LAME header that makes seeking and gapless playback work, every frame header, and anything stuck on the end. The sample is a real MP3 built byte by byte in your browser, variable-bitrate silent frames with ID3v2, LAME and ID3v1 tags. Everything happens on your device.

uploadOpen MP3Download sample

sample-vbr.mp3 (built in your browser) · 138.5 KB · 231 frames

audio frames

ID3v2 159 Baudio frames 138.2 KBID3v1 128 B

First frame header

11111111 11111011 10010000 01000100

sync11 bits set

versionMPEG-1

layerIII

CRC protectionno

bitrate index9 → 128 kbps

sample rate44,100 Hz

paddingnone

channel modejoint stereo

frame size417 bytes · 1152 samples

ID3v2.4.0 · 159 B

TIT2Six seconds of silence

TPE1FileFormer

TALBTest tones · Vol. 1

TDRC2026

TSSELAME 3.100 -V2 (built in your browser)

ID3v1“Six seconds of silence” · 128 bytes at the end

Xing header + LAME3.100 tag

frames (excl. this one)230

audio bytes138.2 KB

seek table (TOC)100 entries

VBR quality78

encoderLAME3.100

methodVBR mtrh

low-pass16.0 kHz

encoder delay576 samples

end padding1000 samples

tag CRCvalid

bitrate of every framevariable · avg 184 kbps

128 kbps × 79160 kbps × 38192 kbps × 57224 kbps × 19256 kbps × 19320 kbps × 19

Three ways to measure the length

Frames × samples per frame6.008 s230 audio frames × 1152 ÷ 44,100 Hz. Includes the encoder’s silent priming and padding.

Gapless (LAME delay and padding removed)5.972 sTrims 576 samples from the start and 1000 from the end, the true length of the original audio.

Your browser’s decoder5.972 sMatches the gapless length: this browser honours the LAME tag.

verifiedLAME tag CRC-16 verified (0xC2EA), so the delay and padding values can be trusted for gapless playback.

Bitrate32 kHz44.1 kHz48 kHz

32 kbps144104 / 10596

40 kbps180130 / 131120

48 kbps216156 / 157144

56 kbps252182 / 183168

64 kbps288208 / 209192

80 kbps360261 / 262240

96 kbps432313 / 314288

112 kbps504365 / 366336

128 kbps576417 / 418384

160 kbps720522 / 523480

192 kbps864626 / 627576

224 kbps1008731 / 732672

256 kbps1152835 / 836768

320 kbps14401044 / 1045960

MPEG-1 Layer III frame size in bytes = ⌊144 × bitrate ÷ sample rate⌋ + padding bit. At 44.1 kHz the division doesn’t come out even, so encoders set the padding bit on some frames to keep the average exact. Rows highlighted are bitrates in your file.

~/spec-sheet

MP3 at a glance: the technical reference.

Identification, limits and variants, from ISO/IEC 11172-3 and 13818-3.

Full nameMPEG-1 Audio Layer IIIdeveloped at Fraunhofer IIS, Germany

Extension.mp3one extension for every variant

MIME typeaudio/mpegaudio/mp3 is common but unregistered

Magic bytesFF FB · or 49 44 33frame sync, or “ID3” when a tag comes first

StandardISO/IEC 11172-3 (1993)extended by 13818-3 (MPEG-2, 1995)

Patentsexpired 2017free to use and implement since then

Sample rates8-48 kHznine rates across three MPEG versions

Bitrates8-320 kbpsconstant, variable or average

Channelsmono · stereo · joint · dualno surround in standard MP3

Frame length1152 samples26.1 ms at 44.1 kHz

Gapless playbacknot built inthe LAME tag adds encoder delay and padding

MetadataID3v1 (end) · ID3v2 (start)v2.3 is most compatible; v2.4 is newest

~/three-mpegs-two-tags

MPEG-1, MPEG-2, MPEG-2.5: the same "MP3" at different sample rates.

The two version bits in every frame header pick one of these. Lower sample rates exist for speech and low-bandwidth streams.

MPEG-1bits 1132 · 44.1 · 48 kHz32-320 kbpsThe music version. Almost every MP3 you own is this.

MPEG-2bits 1016 · 22.05 · 24 kHz8-160 kbpsHalf the sample rates, for speech, podcasts and early internet radio.

MPEG-2.5bits 008 · 11.025 · 12 kHz8-160 kbpsAn unofficial Fraunhofer extension for very low bitrates, like telephone audio.

~/mp3-converters

To MP3 for compatibility, from MP3 for editing.

Converting from MP3 never restores what the encoder removed, and each page says so plainly.

compressWAV to MP3The classic: shrink a recording about ten times.take.wav · 38 MBarrow_forward.mp3 · 3.4 MB graphic_eqMP3 to WAVFor editing software, or burning an audio CD.song.mp3arrow_forward.wav · 36 MB music_videoMP4 to MP3Pull the soundtrack out of a video.talk.mp4arrow_forwardtalk.mp3 phone_iphoneMP3 to M4AAAC in M4A, for Apple devices and ringtones.song.mp3arrow_forward.m4a

Sources

[1]MP3 (MPEG-1 Audio Layer III)History and how the encoder works.Wikipedia ↗[2]MPEG audio frame headerBit-by-bit layout of the frame header.mpgedit.org ↗[3]ID3v2.4 specificationThe tag format for title, artist and art.id3.org ↗