Small businessDesign

Add Captions to a Video for Free — Trim a Vertical Short, Add Text and Music, Export MP4

SENULA COMPANY · 2026-10-18 · 7 min read

You have three phone clips from this morning: dough being shaped, trays going into the oven, and the counter after opening. You want a 30-second vertical Short with the product name and price on screen. If you want to add captions to a video for free, without a watermark and without sending your footage to a server, you can do the whole edit in a browser tab.

There is one limit to know before you start. No tool here writes captions for you. You type each line yourself, or you import an SRT subtitle file you wrote beforehand. A 30-second Short usually needs six to ten short lines, so typing them takes a few minutes.

Why a shop Short needs text on screen

Many people watch short videos with the sound off, on a bus or at a desk. If the price, the product name and your opening hours exist only in your voice, those viewers never hear them. A caption puts that information where the eye already is.

Format matters too. Clips filmed sideways look small in a vertical feed, with empty bands above and below. A 9:16 frame with a blurred copy of the clip filling those bands looks finished, and it takes one setting.

What to prepare

Get these together first so the edit goes in one sitting.

MP4 export depends on a browser feature called WebCodecs. Recent Chrome and Edge have it. If your browser lacks it, export will not be available, so check this before you spend time on the edit.

A note on music: a track you found online is not automatically free to use in a business video. Use music you made, bought with a licence that covers this use, or that is clearly offered for commercial use. Licensing rules differ by country and by platform, so check the terms that apply to you.

Step 1: Set the frame and add your clips

Open the video editor in Keepdesk Studio, choose the 9:16 aspect ratio, and set the fill to blurred background. Then drop or choose your clips. Nothing is uploaded; the files stay on your device.

The clips land on the timeline in the order you added them. Drag them to reorder if needed.

If you filmed some clips vertically and some sideways, the vertical ones fill the frame and the sideways ones get the blurred bands. That mix is normal for a shop Short and does not need fixing.

Step 2: Trim and split to hit 30 seconds

Trimming cuts from the start or end of a clip. Splitting cuts a clip in two so you can delete a bad section in the middle.

Here is the bakery example worked through:

• dough.mp4 is 14 seconds. The first 2 seconds show a hand reaching for the phone. Trim them: 12 seconds left.
• oven.mp4 is 11 seconds. In the middle, someone walks in front of the oven for about 2 seconds. Split before and after that moment, delete the piece: 9 seconds left.
• counter.mp4 is 9 seconds and stays as is.

The total is 12 + 9 + 9 = 30 seconds. Before trimming it was 14 + 11 + 9 = 34, so you removed 4 seconds.

Pace and look

If a clip runs slow, adjust its speed instead of cutting more. Keep in mind that changing speed also changes the clip's length, so recheck the timeline total afterwards. A clip of 10 seconds played faster ends up shorter than 10 seconds, and your 30-second plan can slip.

You can also add transitions between clips and apply a filter or brightness adjustment so the three clips look like they were shot in the same light. Morning clips filmed near a window and clips filmed under shop lights often differ in colour, and a small adjustment on one of them usually closes the gap.

Step 3: Add captions to the video

You have two ways to add captions: type them directly on the timeline, or import an SRT file. You can also combine them, for example a fixed header you type plus changing lines from an SRT.

Typing captions on the timeline

Pick a text preset: title, caption, Shorts header or name tag. A Shorts header works well as a fixed line at the top ("Fresh sourdough, 7 a.m."). The caption preset suits lines that change every few seconds.

Drag each text box to where it reads clearly, usually the lower third but above the area where the platform puts its own buttons. You can give each line an entrance effect, though one style used throughout looks calmer.

Importing an SRT file

This is faster if you already have the lines written, or if you made the same video in another language. An SRT is a plain text file: a number, a time range, the text, and a blank line.

The second worked example is a flower shop with six captions of 5 seconds each, covering 0 to 30 seconds. The first two entries look like this:

1
00:00:00,000 --> 00:00:05,000
Tulip bunches are in today

2
00:00:05,000 --> 00:00:10,000
10 stems for $12

Entries 3 to 6 continue at 10, 15, 20 and 25 seconds, with the last one ending at 00:00:30,000. Six entries times 5 seconds comes to 30 seconds, matching the video. Save the file with the .srt extension and import it.

If you write the SRT in a basic text editor, save it as plain text rather than a formatted document, otherwise the file may not import cleanly.

Keep each caption under about 40 characters and on screen for at least 2 seconds. A line people cannot finish reading is the same as no line.

Step 4: Add music that steps aside for speech

Add your music file to the timeline. Set the start point so the track begins on a strong part rather than a slow intro, then trim it to 30 seconds. Add a short fade in and a fade out of a second or two so the ending does not cut off mid-note. If the track is shorter than the video, turn on loop.

If any of your clips has speech, such as you saying the price, turn on ducking. The music then drops under the video's own sound and comes back up afterwards. Also lower the volume on clips with only background noise, like a humming fridge.

Step 5: Export MP4

Play the whole thing once from start to finish with sound on, then once with sound off. The silent pass tells you whether the captions carry the message alone.

Export as MP4 (H.264 video, AAC audio), which most platforms accept. WebM is also available. There is no watermark on the result. If you plan to post the same video elsewhere with subtitles as a separate file, export your captions as SRT from Keepdesk Studio at the same time so you do not have to retype them.

After exporting, send the file to your phone and play it there before posting. A caption that looks fine on a laptop screen can feel small on a phone.

Common mistakes

• Captions placed at the very bottom, where the platform's own text and buttons cover them.
• Captions that change faster than they can be read, often from typing an SRT end time wrong (00:00:05,000 written as 00:00:50,000 is a common slip).
• Music louder than speech because ducking was left off.
• A final length of 31 or 32 seconds when the plan was 30. Check the timeline total after every cut and every speed change.
• Trying to export in a browser without WebCodecs and only finding out at the end.

Final checklist

FAQ

Can it generate captions from what I say in the video?

No. There is no speech recognition or automatic captioning. Type the lines or import an SRT file you wrote.

Do my clips leave my computer?

No. Everything is processed in the browser, and the files stay on your device.

Why is MP4 export missing for me?

Export needs WebCodecs, which recent Chrome and Edge include. Update your browser or switch to one of those.

Is there a watermark or a length limit?

There is no watermark, and the tool is free with no sign-up. Longer videos simply take longer to export on your own device.

Wrapping up

A 30-second shop Short comes down to three clips cut to length, six to ten readable lines, and music that stays out of the way. Once you have made one, the next takes much less time, because you can reuse the same caption style and swap in new clips. When your clips are ready, open Keepdesk Studio and set the frame to 9:16 first.