How to Add Subtitles to Video: SRT, VTT, and Hardcoded Captions

Subtitles are no longer optional. Whether you are publishing on YouTube, distributing training videos internally, posting short clips on social media, or uploading a film to a streaming platform, captions have gone from a nice-to-have to a fundamental requirement. Viewers watch with sound off on their phones. Accessibility regulations require captions for public-facing video content. Search engines cannot watch your video, but they can index your caption text. And audiences in multilingual markets expect subtitle tracks in their language.

Yet the process of adding subtitles to video remains confusing for most creators. There are multiple subtitle formats with different capabilities. There are two fundamentally different approaches to embedding captions — soft subtitles and hard-burned captions — each with trade-offs. There are timing and synchronization challenges that can derail an otherwise well-produced video.

This guide covers everything: the major subtitle formats and when to use each one, the difference between soft and hard subtitles, step-by-step methods for adding captions using FFmpeg and online tools, and best practices for timing, styling, and accessibility compliance.

Understanding Subtitle Formats

Three subtitle formats dominate the landscape. Each was designed for different contexts, and understanding their strengths helps you choose the right one.

SRT (SubRip Subtitle)

SRT is the oldest and most universally supported subtitle format. It stores subtitle entries as numbered blocks with a timestamp range and plain text. An SRT file looks like this:

1
00:00:01,000 --> 00:00:04,500
Welcome to our tutorial on video conversion.

2
00:00:05,000 --> 00:00:09,200
Today we will cover the most popular output formats.

SRT supports basic formatting through HTML-like tags (<b>, <i>, <u>), but most players ignore styling beyond bold and italic. There is no native support for positioning, colors, fonts, or animation. Its simplicity is both its strength and its limitation — every video player, editing tool, and platform on earth supports SRT files.

VTT (WebVTT)

WebVTT was designed specifically for the web and is the native subtitle format for HTML5 <video> elements. It extends SRT with several capabilities:

WEBVTT

00:00:01.000 --> 00:00:04.500 position:10% align:start
Welcome to our tutorial on video conversion.

00:00:05.000 --> 00:00:09.200
Today we will cover the most popular
<b>output formats</b>.

VTT supports CSS-like styling, positioning controls, vertical text, karaoke-style highlighting, and metadata headers. It uses periods for millisecond separators (instead of SRT's commas) and does not require sequence numbers. YouTube, Vimeo, and all modern browsers support VTT natively.

ASS/SSA (SubStation Alpha)

ASS is the powerhouse format for styled subtitles. It defines a full styling system with fonts, colors, outlines, shadows, animations, and precise positioning. Anime fansub communities popularized ASS because it enables typesetting that matches the original Japanese text placement and styling.

[Script Info]
Title: Example
ScriptType: v4.00+

[V4+ Styles]
Style: Default,Arial,20,&H00FFFFFF,&H000000FF,&H00000000,&H80000000,-1,0,0,0,100,100,0,0,1,2,0,2,10,10,10,1

[Events]
Dialogue: 0,0:00:01.00,0:00:04.50,Default,,0,0,0,,Welcome to our tutorial.

ASS is overkill for most subtitle needs, but indispensable when you need advanced styling control. MKV containers handle ASS subtitles particularly well, while MP4 containers have limited ASS support.

Format Comparison

Soft Subtitles vs Hard Subtitles

This is the most important decision you will make when adding subtitles to video. The two approaches are fundamentally different.

Soft Subtitles (Closed Captions)

Soft subtitles are stored as a separate stream inside the video container. The viewer can toggle them on or off, switch between languages, and sometimes adjust their appearance. The subtitle text remains editable and extractable.

Advantages:

• Viewers control visibility
• Multiple language tracks in one file
• Text is searchable and indexable
• File can be updated without re-encoding video
• Smaller file size than hardcoded captions

Disadvantages:

• Player must support subtitle rendering
• Some platforms strip soft subs during processing
• Styling may render differently across players
• Social media platforms generally cannot display soft subs

Hard Subtitles (Hardcoded / Burned-In)

Hard subtitles are rendered directly onto the video frames. They become part of the image data — permanent, unremovable, and visible regardless of player capabilities.

Advantages:

• Guaranteed to display on every player and platform
• Consistent styling across all devices
• Work on social media where soft subs are not supported
• Cannot be accidentally disabled by viewers

Disadvantages:

• Cannot be turned off or changed
• Only one language per video file
• Requires full video re-encoding
• Text is not extractable or searchable
• Larger processing time

For YouTube and web distribution, use soft subtitles — they provide the best viewer experience and SEO benefits. For social media platforms like Instagram Reels and TikTok, hardcoded captions are the only reliable option since these platforms do not support subtitle tracks.

Adding Soft Subtitles with FFmpeg

FFmpeg is the most powerful and flexible tool for subtitle operations. Here is how to add soft subtitles to different container formats.

Adding SRT to MP4

To embed an SRT file as a soft subtitle track in an MP4 file without re-encoding the video or audio:

ffmpeg -i input.mp4 -i subtitles.srt -c copy -c:s mov_text -metadata:s:s:0 language=eng output.mp4

The -c copy flag copies video and audio streams without re-encoding. The -c:s mov_text flag converts the subtitle format to MOV text, which is the subtitle codec that MP4 containers support. The metadata flag sets the language tag.

Adding Multiple Subtitle Tracks

To embed subtitles in multiple languages:

ffmpeg -i input.mp4 -i english.srt -i spanish.srt -i french.srt \
  -map 0:v -map 0:a -map 1 -map 2 -map 3 \
  -c copy -c:s mov_text \
  -metadata:s:s:0 language=eng -metadata:s:s:0 title="English" \
  -metadata:s:s:1 language=spa -metadata:s:s:1 title="Spanish" \
  -metadata:s:s:2 language=fre -metadata:s:s:2 title="French" \
  output.mp4

Adding Subtitles to MKV

MKV containers are more flexible with subtitle formats. You can embed SRT, VTT, or ASS directly:

ffmpeg -i input.mkv -i subtitles.ass -c copy -c:s ass output.mkv

For SRT in MKV:

ffmpeg -i input.mkv -i subtitles.srt -c copy -c:s srt output.mkv

Hardcoding Subtitles with FFmpeg

Burning subtitles into the video requires re-encoding the video stream. This takes longer but guarantees captions are visible everywhere.

Basic Hardcoded SRT

ffmpeg -i input.mp4 -vf "subtitles=subtitles.srt" -c:a copy output.mp4

This re-encodes the video with subtitles rendered onto each frame while copying the audio stream untouched.

Styled Hardcoded Subtitles

You can customize the appearance of burned-in subtitles:

ffmpeg -i input.mp4 -vf "subtitles=subtitles.srt:force_style='FontName=Arial,FontSize=24,PrimaryColour=&H00FFFFFF,OutlineColour=&H00000000,Outline=2,Shadow=1,MarginV=30'" -c:a copy output.mp4

This sets a white font with a black outline, which ensures readability against any background. The MarginV=30 parameter adds padding from the bottom edge.

Hardcoded ASS with Full Styling

For ASS files that contain their own styling, simply reference the file:

ffmpeg -i input.mp4 -vf "ass=subtitles.ass" -c:a copy output.mp4

FFmpeg will render the ASS styling faithfully, including fonts, colors, positioning, and animations defined in the file.

Recommended Encoding Settings for Hardcoded Subtitles

When re-encoding video for hardcoded subtitles, use settings that maintain quality. For an explanation of codec trade-offs, see our video codecs guide:

ffmpeg -i input.mp4 -vf "subtitles=subtitles.srt" \
  -c:v libx264 -crf 18 -preset slow \
  -c:a copy \
  output.mp4

A CRF value of 18 is visually lossless for H.264. For optimal YouTube settings, use CRF 17-18 with the slow preset.

Adding Subtitles Using Online Tools

Not everyone wants to wrestle with command-line tools. ConvertIntoMP4's video converter supports subtitle embedding during the conversion process. Upload your video, add your SRT or VTT file, and choose whether to embed as a soft track or hardcode the captions directly.

For batch subtitle operations — processing an entire season of episodes or a series of training videos — the online workflow is significantly faster than running individual FFmpeg commands. See our batch processing guide for tips on handling large volumes efficiently.

Creating Subtitle Files from Scratch

If you do not have a subtitle file yet, you need to create one. There are several approaches depending on your budget and timeline.

Manual Transcription

Writing subtitles by hand gives you complete control over accuracy and timing. Use a text editor for SRT files or a dedicated subtitle editor like Subtitle Edit or Aegisub for more complex formats.

A well-timed SRT file follows these rules:

• Each subtitle should display for between 1 and 7 seconds
• Maximum two lines per subtitle entry
• Maximum 42 characters per line (for readability)
• Subtitles should not overlap with scene changes
• Leave at least 200 milliseconds between consecutive subtitles

Auto-Generated Captions

YouTube, Vimeo, and many video platforms now generate captions automatically using speech recognition. These auto-generated captions are a useful starting point but almost always require manual correction. Accuracy rates vary from 80% to 95% depending on audio quality, accents, background noise, and technical vocabulary.

The most efficient workflow is:

1. Upload your video to YouTube (you can keep it unlisted)
2. Wait for auto-captions to generate
3. Download the auto-generated SRT or VTT file
4. Edit the file to correct errors, fix timing, and add proper punctuation
5. Use the corrected file for your final video

Subtitle Timing Tips

Accessibility and Legal Requirements

Subtitle accessibility is not just good practice — it is increasingly a legal requirement.

Who Benefits from Captions

• Deaf and hard-of-hearing viewers: Captions provide full access to spoken content, sound effects, and musical cues
• Non-native speakers: Reading along with speech dramatically improves comprehension
• Viewers in noisy environments: Public transit, open offices, waiting rooms
• Viewers in quiet environments: Late-night viewing, shared spaces, libraries
• Search engines: Caption text is indexable, improving discoverability

Studies consistently show that captioned videos have higher engagement metrics. Facebook reported that captioned video ads increased view time by 12% on average. Multiple accessibility studies have found that adding captions increases overall audience comprehension, even for hearing viewers.

Legal Requirements

In the United States, the Americans with Disabilities Act (ADA) and Section 508 of the Rehabilitation Act require captions for video content produced by or on behalf of government agencies, educational institutions receiving federal funding, and in many cases, businesses operating as public accommodations. The FCC mandates captions for television content distributed online.

In the European Union, the European Accessibility Act (coming into full effect in 2025) requires accessibility for digital products and services, including video captions.

SDH vs Standard Subtitles

Standard subtitles transcribe dialogue only. Subtitles for the Deaf and Hard of Hearing (SDH) also describe:

• Sound effects: [door creaking], [phone ringing]
• Music descriptions: [upbeat jazz music playing]
• Speaker identification: NARRATOR: / SARAH:
• Tone indicators: (sarcastically), (whispering)

For maximum accessibility, use SDH-style captions that include non-speech audio information.

SEO Impact of Subtitles

Captions directly improve your video's search engine optimization in several ways.

Indexable text content. Google and other search engines cannot watch or listen to video content. They rely on text signals to understand what a video is about. A subtitle file or transcript provides a complete text representation of your spoken content, packed with natural keywords and phrases.

Featured snippets. Google increasingly pulls answers from video content for featured snippets. Having accurate captions makes it significantly more likely that your video content will be selected for these prominent search positions.

YouTube search ranking. YouTube's algorithm uses caption data to understand video content and match it to search queries. Videos with accurate captions consistently rank higher than identical videos without captions. For more on optimizing video for YouTube, see our YouTube settings guide.

International reach. Multi-language subtitle tracks enable your content to rank in searches across different languages without creating separate videos. Upload your video once with English, Spanish, and French subtitle tracks, and you are eligible to appear in search results across three languages.

Converting Between Subtitle Formats

Sometimes you have subtitles in one format but need another. FFmpeg handles format conversion cleanly:

SRT to VTT

ffmpeg -i subtitles.srt subtitles.vtt

VTT to SRT

ffmpeg -i subtitles.vtt subtitles.srt

SRT to ASS (with default styling)

ffmpeg -i subtitles.srt subtitles.ass

You can then edit the ASS file to customize fonts, colors, and positioning.

Platform-Specific Subtitle Requirements

Different platforms have specific requirements for caption files:

For social media platforms that require hardcoded subtitles, check out our social media video specs guide for the exact resolution, aspect ratio, and format requirements for each platform.

Batch Subtitle Processing

If you need to add the same subtitle file to multiple video files, or add different subtitle files to a series of videos, scripting the process saves enormous time:

for video in *.mp4; do
  base="${video%.mp4}"
  if [ -f "${base}.srt" ]; then
    ffmpeg -i "$video" -i "${base}.srt" -c copy -c:s mov_text "${base}_subtitled.mp4"
  fi
done

This script looks for matching SRT files (same name as the video) and embeds them as soft subtitles. For more batch processing strategies, see our batch conversion guide.

Troubleshooting Common Subtitle Issues

Character Encoding Problems

If your subtitles display garbled characters (especially for non-Latin scripts), the file encoding is wrong. Convert to UTF-8:

iconv -f ISO-8859-1 -t UTF-8 subtitles_latin.srt > subtitles_utf8.srt

Always save subtitle files in UTF-8 encoding with BOM (Byte Order Mark) for maximum compatibility.

Sync Offset Fix

If all subtitles are offset by a consistent amount (for example, 2.5 seconds late), adjust globally:

ffmpeg -itsoffset 2.5 -i subtitles.srt -c copy subtitles_shifted.srt

For more complex sync issues where the offset varies over time (drift), tools like Subtitle Edit can perform linear or curve-based timing adjustments.

Font Rendering in Hardcoded Subtitles

If hardcoded subtitles look blurry or poorly rendered, ensure you are using a font that is installed on your system and specify it explicitly:

ffmpeg -i input.mp4 -vf "subtitles=subs.srt:fontsdir=/path/to/fonts:force_style='FontName=Roboto'" -c:a copy output.mp4

Recommended Workflow Summary

For most creators, this workflow covers all scenarios:

1. Create or obtain SRT file — Manual transcription, auto-generated, or professional service
2. Edit and verify timing — Correct errors, adjust sync, ensure readability
3. Convert format if needed — SRT for maximum compatibility, VTT for web, ASS for styled captions
4. Embed soft subtitles — For YouTube, Vimeo, and web players
5. Hardcode for social media — Burn in captions for Instagram, TikTok, and Twitter
6. Verify playback — Test on target platforms before publishing

Use our video converter to handle subtitle embedding alongside format conversion, or the video trimmer if you need to cut your video to length before adding captions. For converting your final output to the right format, our MP4 converter ensures compatibility with every major platform.

Whether you are adding subtitles for accessibility, SEO, or international reach, the effort pays dividends. Captioned videos perform better on every metric that matters — watch time, engagement, search ranking, and audience satisfaction.