MP3 to MP4, with Your Cover Image

Because YouTube has no audio-only upload. Its uploader accepts video containers — MP4, MOV, WebM and the rest — and turns a bare MP3 away before the upload starts. A still image over your audio is a perfectly ordinary upload: most of the lyric and "topic" videos on the platform are exactly that, and they are indexed, monetised and recommended like anything else. Instagram, TikTok and X work the same way.

Free, with no account, no quota and nothing stamped in the corner. A track can be up to 200 MB and 60 minutes. The whole render runs on your own hardware, so there is no server bill for us to pass on to you — which is also why there is no queue to wait in.

No. The audio and the image are read straight off your disk, decoded by the browser and re-encoded by WebCodecs, the video encoder that is already built into it. You can check it yourself: open DevTools, switch to the Network panel and make a video — you will not see your files going anywhere. Once the page has loaded, the tool keeps working with the network switched off.

Audio: MP3, WAV, M4A/AAC, OGG, Opus and FLAC, plus the audio track of an MP4 if your browser can read it. Images: JPG, PNG and WebP. The output is always an MP4 with H.264 video and AAC audio at 192 kbps, which is the combination every platform is happiest with. If your browser cannot encode AAC at all — some Firefox builds — you get Opus inside the MP4 instead and the result panel says so.

16:9 at 1920×1080 for a normal YouTube upload. 1:1 at 1080×1080 for an Instagram feed post. 9:16 at 1080×1920 for YouTube Shorts, Instagram Reels and TikTok. If you are not sure, render the 16:9 first — it plays correctly everywhere — then load the same song again and render a 9:16 for the short-form platforms. The 720-class option exists for when you want the file to be small and quick rather than perfect.

With a still image the frame rate is deliberately 2 fps, so a four-minute song is around 480 frames and a recent laptop is done in a few seconds; the file usually lands between 6 and 15 MB. The waveform and progress-bar modes run at 30 fps — about 7 200 frames for that same song — and take a noticeable slice of the song's own length. The estimate above the button tells you which of the two you are about to start, and a very long track automatically gets a lower frame rate so the wait stays sane.

The audio is re-encoded once, to AAC at 192 kbps. That is transparent for a finished mix and unavoidable: an MP4 has to carry a codec it is allowed to carry. Nothing is resampled twice and no silence is padded on. The length is exact to the sample — the frame table is built from the decoded track's own end timestamp, and the final frame's duration is trimmed to cover whatever tail is left, so the picture and the sound stop together instead of a frame apart.

One song at a time, on purpose: the encoder holds the finished MP4 in browser memory, and two long jobs side by side would push most tabs over their limit. When one is finished, press "Do another track" and load the next. On mobile it runs in Safari on iPhone and Chrome on Android — pick the 720-class resolution and the still image there, because a phone encodes more slowly and the tab has far less memory to work with.