Contingency is the most misunderstood line in an event budget. It is not, contrary to popular belief, a buffer for sloppy planning. It is a small amount of optionality you buy at the start, in exchange for being able to say yes — quickly — when something on the day goes wrong.
Most contingency lines are 5%. That’s the floor. The ceiling depends on how exposed the event is: outdoor, multi-day, international, anything involving travel. The bigger the variables, the bigger the line.
What contingency is not for: paying for the things you forgot to budget. If you’re using it to cover known costs, you’ve moved the line from “insurance” to “avoidance” and the next surprise will hurt.