Free MIDI to MP3 Converter

Worked example

Press "Try a sample" to load a built-in arrangement of Ode to Joy with piano, strings, bass and drums: 32 beats at 112 BPM, about 17 seconds. The first "Convert to audio" downloads the 22.6 MB soundfont, then renders about 19 seconds of audio, which is the song plus a 2-second tail for the reverb. As WAV that is about 3.2 MB (44.1 kHz, 16-bit stereo). A 3-minute song at the default tempo comes out 3:02 long, 30.6 MB as WAV and about 4.2 MB as MP3 at 192 kbps. At "Tempo" 150% the same song is 2:02.

Which settings to use, and the limits

• Pick WAV for speed, MP3 for size. After converting, "Output format" offers WAV and MP3. WAV is written at once and is about 10 MB per minute of audio. MP3 is encoded at 192 kbps, about 1.4 MB per minute, by the site's ffmpeg encoder, a one-time download of about 30 MB that is fetched when you save as MP3.
• Keep the original instruments, or force one. "As in the file" plays each part with the instrument the file asks for. Any other choice under "Instrument" (Grand piano, Electric piano, Church organ, Nylon guitar, Strings, Choir, Brass section, Flute, Synth lead or Music box) plays every part on that one instrument. Drums always stay drums, since a melody instrument on a drum track gives random pitches.
• Tempo and Transpose are separate. "Tempo" runs from 50% to 200% in steps of 5 and does not change the pitch. "Transpose" moves every part from -12 to +12 semitones and does not change the speed; drums are not moved. Because tempo changes the length, 200% halves the audio and 50% doubles it.
• Turn off reverb and chorus to render faster. "Reverb and chorus" is on by default. With it off the sound is drier and the render is about three times faster. In our own test, a 17-second piece took about 6 seconds with the effects and about 2 seconds without. The page shows a progress percentage and an estimate of the time left, and "Cancel" stops the render at once.
• Know the length limit. A render can be at most 20 minutes of audio on a computer and 8 minutes on a phone or tablet, because the whole song is held in memory (about 10 MB a minute). The limit counts the 2-second tail and depends on Tempo. For a 10:00 song on a phone the page says "Only the first 8:00 of this 10:02 song were rendered on this device. Raise the tempo to fit more in, or use a computer for the whole song." A file with loop markers, such as game music, plays through once.
• Check the file and the soundfont. The tool opens .mid, .midi, .rmi and .kar files up to 16 MB and checks the first bytes, so a file that is not MIDI is refused before anything downloads. The sound comes from a 22.6 MB General MIDI soundfont (FluidR3 Mono, 128 instruments and drum kits) that downloads once from our own server when you first press convert, and it is checked against a fixed size and checksum. Your MIDI file itself is not uploaded.

Common problems and how to fix them

It says "This is not a MIDI file this tool can read. Use a standard .mid, .midi, .rmi or .kar file."

The file does not start like a MIDI file: it must begin with "MThd", or be a Windows .rmi wrapper. For example, an audio file or a web page saved as .mid, or a damaged download, fails this check. Download the MIDI file again. If you have a recording and want its notes, use the Audio to MIDI Converter instead.

I changed Tempo or Instrument and the audio did not change.

The audio is rendered when you press the button, not live. After a change the page says "Settings changed. Press Convert again to hear them." Press "Convert again". The player, the piano roll and the saved file always match the last render.

It says "The soundfont could not be loaded. Check your connection and try again."

The 22.6 MB soundfont did not download, or the downloaded copy failed its size and checksum test, in which case it is deleted so a retry starts fresh. Check your connection and press "Convert again"; the model box can also show a "Try again" button. If your device is set to save data, the download does not start; turn off the data saver or switch to Wi-Fi.

It says "Rendering stopped in this browser. Reload the page and try again, or use another browser."

The synthesizer failed, or it sent no progress for 60 seconds. Reload the page and try again. A shorter song, a faster Tempo or "Reverb and chorus" turned off each reduce the work, and a computer has a higher length limit than a phone.

Saving as MP3 says "The file could not be made. Try WAV instead."

The MP3 encoder did not load or failed while encoding. Choose WAV, which needs no encoder and saves at once, or check your connection and try MP3 again. The WAV is the full rendered audio, so nothing is lost by saving it first.

How it compares

A MIDI file stores notes, not sound, so the result sounds like whichever soundfont plays it. This tool always uses one General MIDI soundfont, FluidR3 Mono, so it will not match the sampled instruments in a music program or the built-in MIDI player of a phone or operating system. You cannot load your own SoundFont, mute or re-balance single tracks, or give one part a different instrument: you keep the file's instruments or put every part on one. In return you get a quick conversion in the browser tab with nothing to install, and the MIDI file stays on your device. For a specific sound or per-track control, use a music program with the instruments you want.