- Mastodon's default character limit is 500 per post — nearly 2x Twitter/X's 280.
- Each instance can customize the limit: some allow 1,000, 2,500, or even 65,535 characters per post.
- Bio (Description): 500 chars. Display name: 30 chars. Profile metadata fields: 4 fields, 255 chars each.
Mastodon Character Limit by Field
| Field | Default limit | Notes |
|---|---|---|
| Single post (toot) | 500 | Instance can customize |
| Reply | 500 | Same as post |
| Bio (Description) | 500 | Plain text + clickable URLs |
| Display name | 30 | Plus emoji |
| Profile metadata fields | 4 fields | Each: 255 char label + 255 char content |
| Username | 30 | Set when creating account |
| Hashtag length | 1-100 | Some instances limit this |
| Content warning (CW) | 500 | Spoiler text above the post |
| Image alt text | 1,500 | Per image (instance can adjust) |
| Video alt text | 1,500 | Per video |
Why 500 Chars? (Why Different from Twitter)
Mastodon was built to be different from Twitter. The 500-char limit reflects:
- More breathing room for nuanced takes.
- Fewer threads required.
- Less reliance on emoji shortcuts.
- Aligns with natural speech length (most thoughts fit in 500 chars).
Instances With Custom Character Limits
Each Mastodon instance can set its own limit. Some examples:
| Instance | Character limit |
|---|---|
| mastodon.social (default) | 500 |
| hachyderm.io | 500 |
| infosec.exchange | 11,000 (very long) |
| cybre.space | 1,500 |
| mstdn.io | 2,500 |
| mastodon.cloud | 500 |
| some niche instances | up to 65,535 (full novella per post) |
If you need long-form posts, pick an instance with a higher limit.
How Long Posts Display Across Instances
This is important: when you post 5,000 chars on a long-limit instance and someone on a 500-char instance follows you:
- Your post displays in full on their instance.
- They can read the entire post.
- They cannot reply with more than their instance's limit (so they're capped at 500 chars in reply).
Multi-Post Threads on Mastodon
If you exceed your instance's limit, Mastodon supports self-replies for threading:
- Post your first 500 chars.
- Reply to your own post with the next 500 chars.
- Continue chaining replies.
- Each post is independent but visually grouped.
This is the standard way to do "thread tweets" on Mastodon.
Content Warnings (CW) — Mastodon's Unique Feature
Content warnings are spoiler-style hide-by-default text above your post:
- Useful for: NSFW, sensitive topics (politics, mental health), spoilers.
- 500-char limit on the CW text itself.
- Posts with CWs collapse to just the warning; users tap to expand.
- Mastodon culture: use CWs liberally for sensitive content.
How to Add Image Alt Text
Mastodon supports up to 1,500 chars of alt text per image — much more than most platforms:
- When uploading an image, click "Edit" on the image preview.
- Type your alt text description.
- Save.
Mastodon culture strongly encourages alt text for accessibility. Some instances require it.
Bio Character Limit (500)
500 chars in bio is generous. Best structure:
- Who you are (~80 chars): role + niche + location.
- What you write about (~100 chars): topics, expertise.
- Personality (~80 chars): a quote, joke, or hot take.
- Pronouns + accessibility note (~80 chars): common on Mastodon.
- Optional URLs (~100 chars): your domain, link-in-bio.
Frequently Asked Questions
What is Mastodon's character limit?
500 by default. Some instances set higher limits (up to 65,535).
Why is Mastodon's limit 500 instead of 280?
Mastodon was designed to give more space than Twitter, encouraging nuanced posts. Each instance can adjust its own limit.
Can I post longer than 500 chars?
Yes, if your instance allows it (some go up to 65,535). Otherwise, use self-reply chains for longer threads.
How long is Mastodon bio?
500 characters. Plus 30-char display name and 4 verified Profile metadata fields (255 chars each label + 255 URL).
What's a Content Warning?
A spoiler-style hidden text above your post. Useful for sensitive content, NSFW, politics, spoilers. Up to 500 chars on the CW itself.
Key Takeaways
- Default post limit: 500 chars. Bio: 500 chars. Display name: 30.
- Each instance can customize the post limit (up to 65,535).
- Posts beyond your instance's limit display fully on viewers' instances.
- Multi-post threads via self-replies for long content.
- Image alt text limit: 1,500 chars (encouraged for accessibility).
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