How to Turn an MP3 into SRT Captions

You have an MP3 and you need captions — timed lines of text that appear on screen at the right moment, not a plain transcript. Maybe you recorded a talk separately and want to caption the video later. Maybe a client sent audio and asked for a subtitle file. Either way, going from MP3 to SRT is a specific job with specific rules, and the output is very different from a regular transcript. This walks through the whole thing: what SRT is, how to make one from an MP3, and where the format helps or gets in your way.

What an SRT file actually is

An SRT file isn't a document. It's a plain-text file full of numbered, timed caption blocks that a video player reads to know what to show and when.

The anatomy of a caption block

Every block in an SRT has the same four parts: a sequence number, a start-and-end timestamp, one or two lines of text, and a blank line separating it from the next block. It looks like this:

1 00:00:02,000 --> 00:00:05,400 Thanks for coming. Let's get started.

2 00:00:05,400 --> 00:00:08,900 First, a quick recap of last week.

The timestamps are hours, minutes, seconds, and milliseconds. That millisecond precision is what lets a caption land exactly when the words are spoken. The format was popularized by the SubRip program, and the SubRip reference covers its history and the small syntax details if you ever need to hand-edit one.

Why captions come from audio at all

Here's the part that surprises people: captions are built from the audio, not the picture. The tool listens to the speech, writes down the words, and stamps each phrase with the time it was spoken. So an MP3 — which is pure audio — is a perfectly good source. You don't need the video file to generate the caption file, as long as the timing of the MP3 matches the timing of the video it'll sit on top of.

That last condition matters. If your MP3 was trimmed or starts at a different point than the video, the captions will drift. Keep the audio and video aligned from the same origin and the timestamps line up.

How to convert an MP3 to SRT

The process mirrors normal transcription, with one difference: you ask for a subtitle file at the end instead of a document.

Step by step

With Transkio this runs in the browser.

The workflow

1. Open the mp3 to srt tool.
2. Upload your MP3. Watch the size limit — free uploads go up to 100 MB, which covers most spoken-word audio.
3. Set the spoken language so the model isn't guessing at words; Transkio supports 50+ languages.
4. Transcribe, then review the text against the audio. This step is not optional for captions — an error is far more visible burned onto a screen than buried in a document.
5. Export as SRT.

If you're producing captions for a video specifically, the video subtitle generator handles the same job starting from a video file, which saves you extracting the audio yourself.

Timing and readability checklist

Good captions aren't just accurate words — they're readable at speed. Before you ship an SRT, check:

• No caption stays on screen for less than about one second, or viewers can't read it.
• No single line runs so long it wraps awkwardly; two shorter lines beat one wide one.
• Captions change at natural phrase breaks, not mid-word.
• The first caption isn't clipped at the very start of the file.
• Numbers and names are spelled right — they're the usual errors, and they're glaring on screen.

AI-generated transcripts may contain errors — please review before relying on them.

Editing the file after export

An SRT is plain text, so you can open it in any basic text editor and fix a typo or nudge a timestamp by hand. If one caption appears a beat too early, adjust its start time. If a name is wrong across ten blocks, a find-and-replace fixes them all at once. This is one of the quiet advantages of SRT: nothing is locked away, and you never need special software to make a small correction.

SRT versus VTT: which to pick

SRT isn't the only caption format. VTT is the other one you'll hit constantly, especially on the web.

The practical difference

SRT is older, simpler, and accepted almost everywhere — social platforms, most video editors, most players. VTT is the web-native format, with support for styling and positioning that SRT lacks, and it's what browsers expect for HTML5 video.

For most uploads to social or standard players, SRT is the safe default. For a website where you control the player and might want styled captions, VTT can be the better fit. If you want the same source as a web caption file instead, the mp3 to vtt route produces VTT directly. And if you're genuinely unsure which one a given platform wants, the breakdown of SRT vs VTT subtitle formats compares them side by side so you pick once and move on.

A simple rule of thumb

Uploading to YouTube, Instagram, LinkedIn, or a video editor? Use SRT. Embedding video on your own site with a custom player? Consider VTT. When both are accepted, SRT's broader compatibility makes it the lower-risk choice.

Uploading captions to where they'll be seen

Making the SRT is half the job. Getting it attached to your video correctly is the other half, and it varies by platform.

How platforms handle SRT

Most video platforms let you upload an SRT alongside the video, and they match the timestamps to the footage automatically. On YouTube, you add the SRT in the subtitles section of a video's settings and it becomes a selectable caption track. On social platforms, uploading the SRT with the post means captions display even when viewers watch on mute — which is how most people watch on a phone in public, so it's not a small detail.

The key thing is that the SRT and the video have to share the same timeline. If you generated captions from an MP3 that matches the final cut, they'll line up. If you later trim the video, the captions won't move with it, and you'll need to regenerate or shift the timestamps.

Burned-in versus uploaded captions

There are two ways captions reach a viewer. Uploaded SRT files are optional — the viewer toggles them on, and they can be turned off or translated by the platform. Burned-in captions are rendered permanently into the video pixels and can't be switched off. An SRT gives you the flexible, toggleable kind. If you want captions burned in, you'd take the SRT into a video editor and render it onto the footage. Either way, the SRT is the source you start from.

Limits and honest expectations

Captions expose transcription errors more than any other output, because they sit right on the screen where every viewer reads them.

What to watch for

Auto-generated captions get the timing broadly right and the common words right, but they stumble on the same things every transcript does: overlapping speakers, heavy accents, background noise, and proper nouns. A flipped name in a caption is embarrassing in a way the same error in a private note never is. So budget real time for the review pass.

There's also a scope limit worth stating plainly. Transkio generates AI captions you review and correct yourself. It doesn't offer a certified transcript service, and neither does any purely AI tool, so if a project demands a guaranteed-accurate, verified caption track, the output is your starting draft, not a certified deliverable. For the vast majority of video — social clips, tutorials, talks, courses — an AI SRT you've proofed is exactly the right tool and a fraction of the time of captioning by hand.

Free versus paid, briefly

The free plan lets you generate and export captions to get a feel for the workflow, with the usual limits on minutes and file size. Heavier features — translating the captions into another language, labeling who's speaking — sit on paid tiers. Translated subtitle export, in particular, isn't part of the free entry point, so don't assume a foreign-language caption track drops out for nothing.

Wrapping up

Going from MP3 to SRT comes down to three moves: transcribe the audio, proof it carefully against what's spoken, and export the timed file. The format is refreshingly simple — plain text, editable anywhere, accepted almost everywhere — which is exactly why it's stuck around. Run one short clip through the full loop first, open the resulting SRT in a text editor to see how the blocks are built, and you'll understand the format better in five minutes than any spec could teach you. After that, captioning becomes routine.