Image Specs
Post Cover Image
| Dimensions | 1200 × 630 px recommended |
|---|---|
| Aspect Ratio | 1.91:1 |
| Max File Size | 10 MB |
| File Formats | JPG, PNG, GIF, WEBP, AVIF |
| Notes | Shown at top of post and in email preview; also used for Open Graph social sharing |
Publication Logo
| Dimensions | 256 × 256 px minimum (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 | 600 × 600 px minimum |
|---|---|
| Aspect Ratio | Flexible — no fixed ratio enforced (square or landscape both used in practice) |
| Max File Size | 10 MB |
| File Formats | JPG, PNG |
| Notes | Shown on your Substack welcome page (yourpub.substack.com/welcome); Substack scales it down to a max of ~324 × 324 px on display, so keep key content centered |
Inline Post Image
| Dimensions | Up to 1200 px wide |
|---|---|
| Max File Size | 10 MB |
| File Formats | JPG, PNG, GIF, WEBP, AVIF |
| Notes | Images embedded in the body of a post or newsletter |
[Ad]
Video Specs
Video Post
| Max File Size | 20 GB |
|---|---|
| File Formats | MP4, MOV, AVI, 3GP, FLV, MPEG-2 |
| Resolution | Up to 4K supported; 1080p recommended for fastest processing |
| Notes | Hosted video for paid subscribers; MP4 container with H.264 video + AAC audio recommended |
Podcast / Audio Post
| Max File Size | 250 MB |
|---|---|
| File Formats | MP3, WAV, AAC |
| Notes | Audio-first posts with waveform player |
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 | 255 characters |
|---|---|
| Notes | Shown on the publication page and in the Substack directory |
Author Bio
| Character Limit | 250 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 minimum |
|---|---|
| Max File Size | 5 MB |
| File Formats | JPG, PNG |
| Notes | Square format; appears in subscriber email clients |
Publication Cover
| Dimensions | 600 × 600 px minimum |
|---|---|
| Max File Size | 10 MB |
| File Formats | JPG, PNG |
| Notes | Welcome page cover photo; no fixed aspect ratio enforced |
Publication Wordmark
| Dimensions | 1344 × 256 px minimum |
|---|---|
| Aspect Ratio | 21:4 |
| File Formats | JPG, PNG |
| Notes | Horizontal text logo shown in the publication header as an alternative to the square logo; transparent background recommended |
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 |