How to use the guest attendance forecast calculator
- List each kind of invitation: how many invitations, how many people are on each (a household counts as one invitation) and the chance an invitation comes, as a percent. The examples show the layout.
- Set the coverage level, the share of likely outcomes that your planning headcount should cover. The 95% shown is an example.
- Optionally enter your capacity, such as the number of chairs, to see the chance that more people come than you can seat.
- Read the expected guests, the planning headcount, the likely range and the table of chances for each headcount.
Formula
expected guests = sum of (invitations x people on each x chance it comes)
variance = sum of (invitations x people on each^2 x chance x (1 - chance))
chance of exactly k guests = built by adding one invited person at a time
planning headcount = smallest k whose chance of k or fewer reaches your coverage level
The distribution is exact when each invitation comes or not independently of the others. Adding an invitation with chance p moves a share p of the probability up by the number of people on it.
Ninety-six people on fifty-four invitations
Invite 8 families of 3 who are 95% likely to come, 20 couples of friends at 75%, 20 coworkers alone at 55% and 6 neighbor couples at 40%. The expected number of guests is 22.8 + 30 + 11 + 4.8 = 68.6. Because a household arrives together, the standard deviation is 5.40, noticeably wider than the 4.06 you would get by treating all 96 people as separate.
To be covered 95% of the time, plan for 77 guests, and to be covered 90% of the time, plan for 75. The middle 80% of outcomes is 62 to 75 guests. With 80 seats there is a 1.02% chance of more coming, and with 70 seats there is a 37.05% chance.
Planning with a range
A single expected number hides the risk. If you plan for the average, about half the time more people come. A planning headcount that covers 95% of outcomes gives you a number for food and seating that you will rarely exceed. The width of the range comes from the number of people and how uncertain each one is, so large groups of nearly certain guests are more predictable than a smaller group of maybes.
To check seats against a room, use the Room Capacity Calculator, and for the budget, the Wedding Budget Calculator.
What the forecast ignores
Members of one household come or stay home together, and the tool models that by treating a household as one invitation. Different invitations are still treated as independent, so carpools, group plans, plus-ones, last minute changes and weather can make the real range wider. No attendance rates are built in: every chance is your own estimate, so the forecast is only as good as they are.
More planning tools
For other schedules and plans, try the Shift Coverage Calculator, Time Range Calculator and Backward Schedule Generator.
Assumptions and limits
- Each invitation comes or does not come independently of the other invitations, and everyone on it comes together.
- The chance for a row applies equally to each invitation in it.
- Up to 3,000 people can be invited in all, with up to 50 people on one invitation.
- The chances you enter are estimates, not facts.
Frequently asked questions
How many guests will actually show up to my event?
Multiply the people invited in each group by the chance they come and add up the results for the expected number. The tool also shows the likely range around that number, treating a household as one invitation.
What headcount should I plan food for?
Choose a coverage level that fits the cost of running short. Planning for 95% coverage means you would run short about one time in twenty. The tool gives the exact number for your groups.
Why is the real range often wider than the forecast?
Even with households grouped, invitations are treated as independent, and guests who share rides or plans can still move together, so outcomes can swing more than the forecast shows.
What is the chance of exceeding my capacity?
Enter your seat count and the page adds up the chances of every larger headcount, giving the chance that more people come than you can seat.