Free tool
Pick the kind of file you're streaming, or type its size and length, then your download speed. You get the bitrate it needs, how much room you have, and whether the busy scenes will fit. It all happens in your browser, and nothing is sent anywhere.
These are typical averages, not promises: 720p web 4 Mbps, 1080p web-dl 8 Mbps, 1080p remux 25 Mbps, 4K web-dl 20 Mbps, 4K remux 60 Mbps. Your file can be well off them, so for a real one type its size and length instead. The average is the size in gigabytes times 8,000, divided by the length in seconds.
A film's bitrate isn't flat. Dark, quiet scenes use a trickle and fast, grainy ones spike. A web-dl stays within about 1.5 times its average, while a remux keeps the disc's video untouched and can reach twice that or more. A connection that covers the average but not the peak buffers in exactly the scenes you most want smooth. The comfortable figure is the peak plus a quarter, for Wi-Fi and the player's own overhead.
The speed that counts is between the server and the screen. With a debrid service, that's your own connection down from the debrid host; with a remote server it's also the server's upload. If a file is too heavy, pick a lighter release. Real-Debrid with Plex walks through the setup, and how StreamBridge works shows where the stream travels.
StreamBridge is a native Mac app that puts your debrid library in Plex, with no Docker and no server to look after.