Image Specs
Post Cover Image
| Dimensions | 1200 × 630 px recommended |
|---|---|
| Aspect Ratio | 1.91:1 |
| Max File Size | 25 MB |
| File Formats | JPG, PNG, GIF, WEBP |
| Notes | Shown at top of post and in email preview; also used for Open Graph social sharing |
Publication Logo
| Dimensions | 256 × 256 px recommended (square) |
|---|---|
| Aspect Ratio | 1:1 |
| Max File Size | 5 MB |
| File Formats | JPG, PNG |
| Notes | Displayed in subscriber inboxes and the publication header |
Publication Cover Photo
| Dimensions | 1920 × 1080 px recommended |
|---|---|
| Aspect Ratio | 16:9 |
| Max File Size | 25 MB |
| File Formats | JPG, PNG |
| Notes | Banner image shown on your Substack publication homepage |
Inline Post Image
| Dimensions | Up to 1200 px wide |
|---|---|
| Max File Size | 25 MB |
| File Formats | JPG, PNG, GIF, WEBP |
| Notes | Images embedded in the body of a post or newsletter |
[Ad]
Video Specs
Video Post
| Max File Size | 4 GB |
|---|---|
| File Formats | MP4, MOV, AVI, WEBM |
| Resolution | Up to 1920 × 1080 px |
| Notes | Hosted video for paid subscribers; H.264 + AAC recommended |
Podcast / Audio Post
| Max File Size | 4 GB |
|---|---|
| File Formats | MP3, AAC, M4A, WAV |
| Notes | Audio-first posts with waveform player; 192 kbps MP3 recommended |
Text & Character Limits
Post / Newsletter Title
| Character Limit | 100 characters |
|---|---|
| Notes | Used as email subject line when sent to subscribers |
Post Subtitle
| Character Limit | 250 characters |
|---|---|
| Notes | Appears under the title in the post header |
Post Body
| Character Limit | No hard limit |
|---|---|
| Notes | Rich text editor with Markdown; no character cap |
Publication Name
| Character Limit | 50 characters |
|---|
Publication Description
| Character Limit | 250 characters |
|---|---|
| Notes | Shown on the publication page and in the Substack directory |
Author Bio
| Character Limit | 500 characters |
|---|
Profile & Publication Assets
Author Photo
| Dimensions | 256 × 256 px minimum |
|---|---|
| Max File Size | 5 MB |
| File Formats | JPG, PNG |
| Notes | Displayed as a circle in posts and author bylines |
Publication Logo
| Dimensions | 256 × 256 px |
|---|---|
| Max File Size | 5 MB |
| File Formats | JPG, PNG |
| Notes | Square format; appears in subscriber email clients |
Publication Cover
| Dimensions | 1920 × 1080 px |
|---|---|
| Max File Size | 25 MB |
| File Formats | JPG, PNG |
Subscription & Monetisation
Pledge / Paid Post Preview
| Notes | Free preview shown to non-subscribers before paywall; no specific image size requirement |
|---|
Referral Programme Image
| Dimensions | 1200 × 630 px |
|---|---|
| File Formats | JPG, PNG |
| Notes | Promotional image shared when readers refer new subscribers |