Most over-ordering happens on three lines and most shortfalls on two. Here are the counts we give when someone asks, with the reasoning behind each one.
Glassware is where the numbers get away from people. A hundred guests do not need a hundred glasses; they need closer to two hundred and fifty across the evening, because nobody carries the same glass from the reception to the toast. Plan for two and a half per head and you will not be washing up mid-service.
Napkins and cutlery run the other way. The instinct is to add a generous margin, but a formal sit-down loses far fewer than a standing reception does, and the surplus goes back in the box unused. Order to the covers plus five per cent and keep the spare boxes sealed so they are still returnable.
The one line worth over-ordering is candles. They are cheap, they burn at different rates depending on where the room draws air, and running out halfway through the evening changes the light in a way photographs pick up immediately.