Bandwidth is what most consumers conflate with "speed". Fast connections have high bandwidth (lots of bits per second move through). Slow connections have low bandwidth. The number is independent of latency (how long any single byte takes to arrive) — a satellite connection can have high bandwidth but terrible latency.
For YouTube specifically: the player measures real-time bandwidth and uses it to pick stream quality. 1080p at 5 Mbps fits comfortably in a 25 Mbps connection but borderline on a 6 Mbps one (any dip and you buffer). 4K's 25 Mbps doesn't fit cleanly in most consumer home connections — 4K viewing is bandwidth-tight even when the marketing speed number is high.
Connections marketed as "100 Mbps fiber" deliver bandwidth that fluctuates significantly within the day. Late evening peak hours commonly drop to 30-50% of the marketed number. Speed test sites (fast.com, speedtest.net) show current bandwidth at the moment of testing.
Common questions
How much bandwidth does YouTube actually need?
Related terms
Bitrate
Bitrate is the amount of data a video or audio stream carries per second, measured in bits per second (bps) or kilobits (kbps) and megabits (Mbps).
Playback buffer
A playback buffer is data downloaded ahead of the playhead, ready to play if the network slows down.
Adaptive streaming (ABR)
Adaptive streaming (Adaptive Bitrate, ABR) is a protocol where the player switches between multiple quality variants of the same video during playback based on real-time network conditions.
VidPickr is a free, browser-based YouTube downloader. Every term in this glossary either describes how YouTube delivers video or why your downloads behave the way they do. Try the downloader →