A trip has two halves. Flights and accommodation are decided and paid before you go, and everything else is spent while you are there: food, local transport, entries and small purchases. Treating the first half as the price of the holiday is how a budget ends up half as much again as planned.
The daily allowance is the input worth getting right, because it is the one you can still adjust during the trip. Look up what a meal and a local journey cost in the specific place you are going rather than using a general impression: the same allowance can be generous in one country and inadequate in the next. The Travel Budget Calculator turns a daily figure into the whole trip, splits it per person and gives you the amount each traveller can actually spend per day once the fixed costs are paid.
Add the costs that are easy to forget: visas, travel insurance, airport parking, pet care, and the airport transfer at both ends. These are usually paid once for the whole party rather than per person, which is why they belong in their own line.
Keep a contingency. Ten per cent is not pessimism, it is the difference between a plan and a surprise, and a missed connection or a medical cost is exactly what it exists for. The contingency is not a budget for an extra dinner.
One rule sits above the arithmetic. A holiday funded from the emergency fund is not a holiday you could afford, it is a loan you have not priced yet. Check the fund is intact afterwards, not just before.