The formula
amount = portion per person × equivalent guests × factors
Where it comes from
The portions per person are worked out for an adult at a full meal. A child counts as half a person, drinks rise for longer events, and nibbles take around half the food of a sit-down meal.
How to work it out by hand
- Count children as half a person
- Multiply by the standard portion of each food
- Adjust the drinks for the length of the event
- Always round up
What is worth knowing
Two rules from experience that no calculation captures. The first: running short is far worse than over-buying, so always round up on what keeps — drinks, bread, nibbles — and adjust on the fresh things. The second: drinks go very unevenly depending on the hour and the weather; in summer heat, the water can double against the calculation.