Slack emoji size and limits
Everything Slack checks when you upload a custom emoji, and how to get an image that passes the first time.
The short answer
| Size | Square, 128 × 128 px works best |
|---|---|
| File size | Under 128 KB |
| Formats | PNG, JPG or GIF |
| Animation | GIF, up to 50 frames |
| Background | Transparent is best |
Slack shows emoji at about 22 px in messages, a little larger when a message is only emoji, so fine detail disappears. Bold shapes, thick outlines and a subject that fills the square read far better than a photo with lots of background.
Why uploads get rejected
Almost every failed upload is a file over 128 KB. Still images rarely hit that at 128 px, but animated GIFs do, because every extra frame and color adds weight. The fixes, in the order that costs the least quality, are fewer colors, fewer frames, then a slightly smaller canvas. Moji Locker's export does exactly that and stops as soon as the file fits.
Other causes: a GIF with more than 50 frames, a file type Slack doesn't take (WEBP and HEIC photos from phones are common), or a name that's already used in the workspace.
Naming your emoji
Names use lowercase letters, numbers, hyphens and underscores, and each one must be unique in the workspace. You type it between colons, like :party_parrot:, so short and obvious names get used the most.
How to add it to Slack
- In any message field, open the emoji menu with the smiley face icon.
- Choose Add Emoji, then Upload Image, and pick your file.
- Give it a name and select Save.
Members can add emoji by default, but workspace owners can limit it to admins. If you don't see Add Emoji, ask an admin.