Party Turnout Calculator
Inviting eighty people at a seventy per cent acceptance rate does not mean fifty-six will come — it means fifty-six on average, with a real chance of anything from forty-five to sixty-five. Catering for the average leaves you short about half the time. This works out the whole distribution, the head count that covers ninety-five per cent of outcomes, and the probability of more people arriving than the room holds.
What this generator does
Treats each acceptance as an independent coin flip and computes the binomial distribution over the whole invitation list, then reads off the expected turnout, the spread, the head count covering ninety-five per cent of outcomes, and the tail beyond your capacity.
How to use this tool
- Enter how many invitations went out.
- Set the acceptance rate you expect.
- Enter how many places the room or the catering holds.
- Read the expected turnout, the number to plan for, and the overflow risk.
Understanding the controls
- People invited
- How many invitations went out, counting each person rather than each household. A household that always comes or stays away together should be counted as one.
- Acceptance rate
- The share you expect to accept. Past events are a far better guide than optimism — for a work party it is often nearer half than three-quarters.
- Places available
- What the room, the table or the catering actually holds. This is the number the overflow risk is measured against.
Common use cases
- Deciding how much food and drink to buy for a party
- Working out whether a room is big enough for the list
- Seeing why catering for the average is a coin flip
- Setting an invitation list to fill a fixed number of places
- Explaining to someone why the average turnout is not the number that matters
How this generator works
If each invitee accepts independently with the same probability, the number who come follows a binomial distribution. The tool computes the probability of every possible turnout using logarithms of factorials, so a list of four hundred does not overflow, and accumulates them into a cumulative column. The number to plan for is the first turnout whose cumulative probability reaches ninety-five per cent, and the overflow risk is whatever is left beyond your capacity. Before display the probabilities are summed and must come to one, the mean and spread are re-derived from the distribution itself rather than the formula, and both the plan-for figure and the overflow risk are found again by scanning the finished table.
Randomness and fairness
Nothing here is random. An invitation list and an acceptance rate give exactly one distribution — this computes what would happen on average rather than simulating one party.
For how randomness is produced across the whole site, see how Generate Random works.
Limitations and good to know
- Acceptances are assumed independent, and they are not. Couples, lifts and work teams travel together, which makes the real spread wider than this shows.
- The acceptance rate is your estimate, and the answer is only as good as it. A rate that is ten points out matters far more than the arithmetic.
- People who accept and then do not come are not modelled separately; fold them into the acceptance rate instead.
- This gives the head count, not the catering. How much food that turns into is the party quantity planner's question.
Privacy and your data
The calculation runs in your browser. No guest names are entered, and the numbers you type are never transmitted, stored or included in analytics.
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