The service style you're running changes staffing needs more than the guest count does. A plated dinner for 80 needs meaningfully more hands than a buffet for the same 80, and a passed-hors-d'oeuvres cocktail hour has its own math entirely. These are industry rules of thumb, not guarantees - treat them as a starting point you adjust for your specific menu and venue, not a formula to quote blind.
Plan on roughly one server per 12-16 guests. Plated service is timing-dependent - courses need to land at tables close together - so understaffing here shows up immediately as cold food and uneven pacing.
Roughly one server per 20-25 guests, since guests are serving themselves for the meal itself. You still need enough hands for replenishing chafing dishes, bussing, and keeping the line moving, especially in the first 20 minutes when most of the room hits the buffet at once.
This one scales by tray count and room size, not guest count alone - roughly one passer per 25-30 guests for a room with reasonable circulation space, more if the venue has multiple rooms or an outdoor overflow area.
Sits between plated and buffet - plan for one server per 15-18 guests, since servers are actively refilling shared platters at the table rather than guests self-serving.
A single bartender can reasonably handle about 50-75 guests for a straightforward beer/wine/one-cocktail setup; a full bar with multiple cocktails needs a second bartender well before that guest count.
Once you know the service style, the staffing line on your quote stops being a guess - it's a fixed ratio you can defend if a client questions the number. Catering Calc's staffing ratio calculator does this math automatically once you've entered your service style and guest count, and saves it as part of your event template for the next job like it.
Plug in your guest count and get exact numbers in seconds.