What it means
WPM is a word count divided by elapsed minutes. As an illustrative calculation, a 1,200-word rehearsal that takes eight minutes has a measured rate of 1,200 ÷ 8 = 150 WPM for that rehearsal.
Planning is the reverse calculation
Given a script and an assumed rate, estimated spoken minutes are words ÷ WPM. Two rates give a planning range. Neither rate measures the future performance; pauses, pronunciation and delivery can change the result. A synthesis service can also expose its own speaking-rate setting, as Google Cloud's configuration documents illustrate.
Count the relevant units
The words-to-minutes tool separates non-Han word tokens from Han characters and uses independent rates for them. Its counting method is not a language-aware tokenizer for every writing system. Use a short read-through to calibrate your own material.
Do not equate speech time with runtime
A video can include silent visual holds, demonstrations, transitions and other non-spoken sections. Budget those separately. For a worked plan, see planning a 15-minute script.