Slack Emoji Maker - 128x128 PNG under 128 KB

Slack runs the tightest custom-emoji spec of the messaging apps: 128 x 128 pixels and a 128 KB file cap - half of Discord's allowance. This tool exports a 128 x 128 PNG with transparency that sits comfortably inside that limit. Slack shows emojis at about 22 px in messages and 64 px in the picker, so a simple, legible mark beats a detailed image that disappears at that size.

To add it, open your workspace menu → Tools & settings → Customize workspace → Emoji (or go straight to yoursubdomain.slack.com/customize/emoji), click Add Custom Emoji, upload the PNG, and give it a name like :coffee-cat: - it's available workspace-wide immediately. Non-square sources are padded with transparency rather than cropped; crop to 1:1 first if you want a tight square. Because 128 KB also clears Discord's 256 KB bar, one export at this spec uploads to both platforms.

The sizing and PNG export run locally on the Canvas API, and iPhone .heic files decode on-device through a WebAssembly module before resizing. Nothing is uploaded - confirm in the Network tab or by going offline once the page loads. Free, no signup, no watermark; the tool flags any result that creeps over the 128 KB cap.