Estimate speech length from word count and pace.
Public speaking pace is markedly slower than conversation — 120 to 140 words a minute against 150 to 170 — because effective delivery uses pauses for emphasis and gives the audience time to absorb each point. Speakers who rehearse at conversational pace consistently overrun. Two additions then apply: slide transitions cost around eight seconds each as the audience reorients, and each expected laugh or round of applause costs roughly seven seconds you cannot talk through. Words per slide is a useful secondary check, since much above 150 usually means a slide is carrying text that should be spoken instead.
Podium time
Minutes = words / delivery pace + slides x 8 s + reaction pauses x 7 s
Total slot
Total = podium time + question and answer minutes
Because comprehension needs pauses. A speaker at conversational speed gives an audience no time to process a point before the next arrives, and the delivery loses all emphasis. Deliberate pacing at 120 to 140 words a minute is what makes a talk followable.
Work backwards. Subtract Q and A, then slide and reaction time, then multiply the remaining minutes by your pace for the word budget. A 20-minute slot with 10 minutes of Q and A and 12 slides leaves roughly 8 minutes of speech — about 1000 words.