Estimate supplies for a house party.
Disposable supplies are consumed at a rate that has nothing to do with appetite. Guests abandon cups — three per person is the working figure because people set one down, lose track of it and take another, and it rises with party length. Plates run at two a head where there is food to return for. Napkins go at four each and are the cheapest thing on the list, so over-buying costs nothing. Converting the counts into packs is the useful step, since cups come in fifties, plates in forties and napkins in hundreds. Add bin bags at roughly one per forty guest-hours, and put a bag out visibly early — it is the single most effective way to keep the cup count down.
House Party Supplies
Cups = guests x 3; plates = guests x 2; napkins = guests x 4; packs = each count divided by pack size (50 cups, 40 plates, 100 napkins), rounded up
About 75 — three per person. Writing names on cups with a marker cuts this by a third and is the cheapest waste reduction available.
Cups, then bin bags. Both are consumed faster than expected and both are cheap enough that buying an extra pack costs less than running out.