Why upload sites refuse big files, and why this editor doesn't need to

An upload-based subtitle site has to receive your whole video before it can do anything, so it sets a ceiling on file size and you wait while every gigabyte crosses your connection. That's where a 6 GB conference recording or a camera's 4K master usually gets turned away.

The editor here never sends the file. When you choose a video, the browser gains permission to read it where it sits, and the editor pulls in only what it needs: enough to learn the format, duration and bitrate when it opens, then frame after frame during the render. A 20 GB source isn't copied into the page first. What does need room is the new file the render produces, and that's the number to watch. Processing large video files in the browser explains the general technique.

Where the finished file goes

Before rendering, the editor picks one of two destinations for the output, or declines:

Private browser storage on disk: Used when the browser offers this kind of storage (Chrome and Edge do), the estimated output is 8 GB or less on a computer, and the browser reports room for the estimate plus 10 percent and another 64 MB. The MP4 is written out in chunks while it's encoded, so it never has to sit in memory whole.
Memory: Used when the disk route is unavailable or short of room. The entire MP4 is built inside the tab, so it must fit a budget based on what the browser reports about the computer: 500 MB when it reports 4 GB of RAM or less, 1.5 GB when it reports more, and 1 GB when it reports nothing.
Neither: If the estimate fits neither, the summary explains why and its Render video button stays greyed out.

The space that counts for the disk route isn't quite your drive's free space. Browsers grant each site a share of the disk and report how much of that share is unused, which is normally less than your file manager shows. The storage sits on the drive holding your user profile, usually the system drive, not on an external disk.

Each render replaces the last one there, and the editor clears it when the page loads or you start a new video. Clicking Download video copies the file into your Downloads folder, so for a while it exists twice; leave room for both.

How the output size is estimated

The estimate starts from the bitrate the editor will encode at, worked out like this:

Reference: 3 Mbps for a 1920×1080 frame, scaled by pixel count slightly less than in proportion, so a 4K frame's reference is about 11.2 Mbps rather than 12.
Frame rate: The reference is multiplied by (frame rate ÷ 30) to the power 0.6, counting anything under 24 as 24 and anything over 120 as 120.
High-quality level: That figure times 3.86: about 11.6 Mbps for 1080p at 30 frames per second, about 43 Mbps for 4K at 30.
Target: A quarter above the source's own average bitrate when the file reports one, or the high-quality level when it doesn't.
Bounds: Never below half the high-quality level, never above double it, and never above 150 Mbps for any video.

The size estimate is that video bitrate plus 320 kbps for audio (dropped when there's no audio track), multiplied by the duration, divided by eight to turn bits into bytes, plus 8 percent headroom. The "Ready to render" summary shows the result on its Output line as the codec, the bitrate and a rough size, before a single frame is encoded.

So the output follows the source's bitrate, a little above it, so the subtitled copy usually comes out somewhat larger than the original. A source with an unusually low bitrate is lifted to the floor and can grow by more than a quarter. Video bitrate explained covers why bitrate drives file size.

Four files, worked through

One-hour 1080p recording at 8 Mbps and 30 fps, about 3.4 GB

Target 10 Mbps, estimate about 4.7 GB. That's beyond every memory budget, so it renders only through the disk route, which needs roughly 5.2 GB of the browser's share free.

Ten-minute 1080p screen recording at 9 Mbps

Target about 11.3 Mbps, estimate about 894 MB. In memory it renders without comment on the 1.5 GB budget, with the close-other-tabs note on the 1 GB budget, and is refused on the 500 MB budget.

Twenty-minute 4K phone clip at 50 Mbps

Target 62.5 Mbps, estimate about 9.5 GB. That's over the 8 GB ceiling, so it's refused however empty the drive is. Split in two, each half comes to about 4.7 GB and goes through the disk route.

4K at 120 fps from a 200 Mbps camera file

A quarter above the source would be 250 Mbps, so the 150 Mbps cap applies, if the browser's encoder accepts that size and rate at all. Each minute then adds about 1.1 GB, and roughly seven minutes reach the 8 GB ceiling.

Messages you may see

In the summary, before rendering:

"This is a large video for this device. Close other tabs before rendering." The output fits the memory budget but takes more than about 60 percent of it.
"The finished video would be about N GB, more than this browser can hold. Trim the video into shorter parts, or use Chrome or Edge on a computer with free disk space." Neither route fits, and rendering is blocked.
"This browser can't encode video at" the given width and height. No encoder in this browser accepts the frame size; very large frames are the usual cause.

During the render:

"Your device ran out of storage space while saving the video." The estimate fitted, but space ran out anyway. Free some and render again.
"Your browser ran out of memory. Close other tabs or try a shorter video."

Cancelling at any point deletes the half-written file from private storage.

Getting an oversized file through

Switch to Chrome or Edge on a computer and free space on the drive with your user profile. That opens the disk route and its 8 GB ceiling.
Cut the video into parts below the limit and render each one, as in the third example. Splitting a video file lists free ways to do it, and for one continuous set of subtitles across the parts, the long-video guide shows how to offset and join SRT files.
Start from a lighter source. The render keeps the source's resolution and nudges its bitrate up, so a 4K master yields a 4K-sized result. If the destination only needs 1080p, make a 1080p copy with a desktop tool such as HandBrake first, and the estimate drops with it. For footage that has to stay 4K, subtitling 4K video covers codecs and settings at that resolution.
Skip the re-encode. Where the platform accepts subtitle files, Export SRT and upload it beside the untouched original. For a switchable track inside the file, MKVToolNix can add the SRT to an MKV, and FFmpeg to an MP4 or MKV, without re-encoding the picture; embedding subtitles explains the difference.

AI subtitles on a big file

For the AI step, length matters and file size barely does. AI subtitles accept videos up to 2 hours, and a longer one is turned away before any audio is sent. The browser decodes the soundtrack from the file in place, compresses it and uploads only that audio over HTTPS, so a 15 GB video and a 500 MB video of the same length send about the same amount. The picture never leaves your computer. A one-hour file costs 60 credits (6¢) whatever its size, and the AI subtitle generator does the same transcription if an SRT is all you want.

Checklist for a multi-gigabyte render

Read the summary's size estimate and make sure the drive has comfortably more free, plus room for the copy in Downloads.
Use Chrome or Edge on a computer, so the disk route is available.
Close other tabs, especially if the summary mentions memory.
Plug the laptop in, stop it sleeping and leave the tab open.
Export SRT before rendering, so the subtitles are safe whatever happens.

About the subtitle editor on this page

Opens: MP4, MOV, M4V, WebM, MKV videos your browser can decode
Subtitles from: AI (from the speech, optionally translated into one of 21 languages), an SRT or VTT file, or typing them in
You download: An MP4 with the subtitles drawn into the picture at the original resolution and frame rate, plus SRT and VTT files
Cost: Editing, styling, rendering and SRT/VTT export are free, with no account and no watermark. AI subtitles cost 1 credit per minute (5-credit minimum per video); signing in with Google gives 100 free credits and accounts get 10 free credits a day
Privacy: The video never leaves your device and is rendered in your browser. Only for AI subtitles is the audio sent, over HTTPS, and it is deleted within 30 minutes of delivery
Rendering needs: Chrome or Edge 94+, Safari 16.4+, or Firefox 130+ on a computer. Editing and SRT/VTT export work in any current browser

More: free, private and in the browser

Subtitle tools

Frequently asked questions

Is there a file size limit on the video I open?

Not on the file you open, because it's read from your disk rather than uploaded. The limit applies to the rendered output: up to 8 GB through the browser's private storage on a computer with room, otherwise 500 MB, 1 GB or 1.5 GB in memory depending on the device.

Why is my subtitled video bigger than the original?

The editor encodes a quarter above the source's bitrate, and lifts very low bitrates to a quality floor, to keep re-encoding losses small.

Why does the editor say the video is too large when my drive has plenty of space?

Either the estimate is above the 8 GB ceiling for disk streaming, the browser doesn't offer the storage the disk route needs, or the share of the disk it grants the site is smaller than the estimate. Chrome or Edge on a computer, or splitting the video, usually gets past it.

Can I pick a lower bitrate to make the output smaller?

No, there's no bitrate setting; it follows the source automatically. Make a smaller source first, such as a 1080p copy of a 4K master, or split the video into parts.

Does a huge file mean a huge upload for AI subtitles?

No. Only the compressed audio is sent, so the upload depends on the video's length, which is capped at 2 hours for AI subtitles, not on the size of the picture.