The rule of thumb most companies repeat — one unit per fifty people for four hours — is roughly right for a dry, daytime event with a moderate guest list. It falls apart fast under real conditions.
Add alcohol, and usage roughly doubles. People drink, then they go. Add food, and you're adding another thirty percent. Add an evening shift past dark, and usage clusters dramatically as guests get more comfortable and stop holding out. Add hot weather, and hydration rates push everything higher.
The cleaner formula: figure out your peak hour, not your headcount. A wedding reception of eighty people that runs from 5 PM to 11 PM has a peak hour around 8 PM, when dinner is wrapping and the bar is busy. That's when you need capacity. If your peak hour can't be served in under ten minutes of wait time, you're under-rented.
For weekends, we generally recommend:
One standard unit per 40 guests for events with alcohol
One standard unit per 50 guests for dry events
Plus one ADA unit for any public event regardless of size
Erring slightly high costs less than the one complaint that ruins an otherwise good event