Add Subtitles to 4K Video

Same frame in, same frame out

The editor renders at the source's exact frame size. A 3840×2160 clip comes back as 3840×2160, a 4096×2160 cinema master stays that width, and an upright 2160×3840 phone clip stays upright. There is no scaling step and no resolution menu to get wrong. Before rendering, the summary's Resolution row shows the size and frame rate followed by "(kept as is)".

Subtitles are laid out in the video's own pixels, with the font size measured against the frame's shorter side. The default size of 5.2 sets the font at roughly 112 pixels on 2160-line footage, twice what the same setting produces at 1080p, so the text covers the same share of the picture at any resolution. Outlines, shadows and box padding are relative to the font size and grow with it. The preview on your screen is smaller, but it uses the same layout code, so line breaks and positions match the render.

If a 4K preview stutters on a modest computer, that only affects playback in the page. The render decodes and encodes every frame on its own schedule, so dropped preview frames don't reach the file.

How much data each second gets

Drawing subtitles into the picture means encoding the video again, so the editor spends data generously to keep the loss small. These are its current settings:

Starting point

A quarter above the source's own average bitrate, when the file reports one.

Floor

Never below a minimum scaled to resolution and frame rate: about 22 Mbps for 4K at 30 frames per second, about 33 Mbps at 60.

Ceiling

Never above about 86 Mbps for 4K at 30 frames per second or about 131 Mbps at 60, and never above 150 Mbps for any video.

Rate control

Variable bitrate, so calm shots use less and busy ones more.

When the file doesn't report a bitrate, 4K at 30 frames per second gets about 43 Mbps. You don't have to work any of this out: the summary's Output row gives the codec, the bitrate and the expected size for your actual file, along the lines of "H.264 at about 45 Mbps, roughly 3.1 GB" for a nine-minute 4K clip. The result is a careful re-encode rather than an identical copy; burning subtitles without losing quality explains what survives, and video bitrate covers the theory.

Expect the subtitled file to be larger than the original, especially from HEVC sources such as phones and many mirrorless cameras. At 4K, hundreds of megabytes per minute is normal.

Which codec the file comes out in

The editor asks the browser, in order, whether it can encode H.264, HEVC (H.265), VP9 or AV1 at your exact size, frame rate and bitrate, and uses the first one that works. At 1080p H.264 almost always passes. At 4K the answer depends on the browser's encoder and your graphics hardware: many computers encode H.264 at 3840×2160, some can't. When H.264 isn't available, the summary says so and names the codec it will use instead, warning that some older players may not open it. If no codec can handle the size, it states that the browser can't encode video at that resolution, and the Render video button stays disabled; Chrome or Edge, or a newer computer, may get past that.

Decoding matters as well. A 4K HEVC source needs a browser that can decode HEVC before anything else happens, and an H.265 output needs players that can play it; HEVC compatibility lists where that tends to work. The editor sets no resolution cap of its own, so for 5K, 6K or 8K footage it is the browser's encoder that decides.

Space for the finished file

Before rendering, the editor estimates the output size from the bitrate and duration and decides where the file will go:

• Streamed to disk. When the browser offers private file storage that can be written as a stream, which Chrome and Edge do, the MP4 is written there while it's encoded, up to about 8 GB on a computer, provided the browser reports room for the estimate plus a margin.
• Held in memory. Otherwise the whole file must fit in the tab's memory, a much smaller budget of roughly half a gigabyte to 1.5 GB depending on what the browser reports about the device. As the estimate approaches that budget, the summary asks you to close other tabs before rendering.
• Refused. If neither fits, the editor gives the expected size and suggests trimming the video into shorter parts, or using Chrome or Edge on a computer with free disk space. There is no server to fall back to.

That private storage lives on your system drive, so free space there is what counts. If a render stops because the device ran out of storage, clear space and start again. A long 4K master can be cut into parts with the methods for splitting a video and subtitled piece by piece; large video files goes into the limits in more depth.

Rendering subtitles into 4K footage

1. Check the free space on your system drive before you start.
2. Open the editor and choose the 4K file. Its resolution and frame rate appear above the preview once the browser has read it.
3. Bring in the subtitles from AI, an SRT or VTT file, or typing. AI subtitles upload only compressed audio, so the size of the 4K picture doesn't slow that step.
4. Edit the lines, then set the style. Thin fonts and hairline outlines that look delicate at 4K can vanish if the video is later viewed small, so check the preview at the size most people will watch.
5. Click Render video and read the summary: codec, bitrate, expected size and any note about HDR or a non-H.264 codec.
6. Start the render with the laptop plugged in and the tab open, and stop the computer from sleeping until it finishes.
7. Click Download video and play the file on the screen it's meant for before you delete anything.

Habits for long 4K renders

• Close other tabs and heavy applications. Decoding and encoding 4K uses a lot of memory and graphics hardware, and both are shared.
• Expect 60 frames per second to mean twice the frames to process and a higher bitrate than 30.
• HDR footage comes out as standard dynamic range, and the editor warns you first. Grade to SDR in your editing software beforehand if you want control over the result.
• Keep the source on a local drive rather than a network share, so reading it doesn't hold the render back.
• If the master is going to YouTube or Vimeo, consider not re-encoding at all: upload the untouched 4K file with an SRT from Export SRT, as described in uploading subtitles to YouTube.
• For batch jobs or full control over encoder settings, FFmpeg can burn the exported SRT instead; burning subtitles with FFmpeg shows the commands.

If all you need is the subtitle file, the AI subtitle generator produces SRT and VTT without touching the picture. For upright 4K clips from a phone, subtitling iPhone video covers HEVC and HDR in more detail.

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