Calculators: budget, headcount and break-even

Three calculators: what a contingency line leaves you to plan with and the cost per guest; the planning number from your replies and your own allowances; and how many tickets cover a fundraiser's costs. All arithmetic; every assumption is listed.

1. Contingency and cost per guest

Answers: how much a contingency line takes from your ceiling, what is left to plan with, and what both come to per guest. Used in The budget.

Assumptions behind the numbers

• The contingency is one row of the budget, sized by you, rather than a cushion added to every line. The University of Houston's student-organization budgeting guide warns against overestimating each item (University of Houston's budgeting guide).
• The 10 percent default is this manual's starting convention. Arts and Culture Exeter suggests 10 percent (Exeter's budgeting page); the City of Boroondara's outdoor event guide gives 10 to 15 percent for outdoor events (Boroondara's event budget guide); the University of Birmingham's conference office suggests 15 to 20 percent (Birmingham's event budget article). We found no US public body that gives a figure.
• Per guest is the total divided by the guest count you enter. It is an average, not a price.

2. Break-even for a fundraiser

Answers: what the event costs and brings in at your expected count, and how many tickets cover the costs. Used in The budget. Fixed costs are the ones that do not change with the count (the room, a permit, decorations); cost per guest is what each extra person adds (food, plates).

Assumptions behind the numbers

• Break-even count = (fixed costs minus donations and sponsors) divided by (ticket price minus cost per guest), rounded up. When the ticket price does not exceed the cost per guest, no count breaks even. The method is the plain arithmetic behind the income and expense worksheets in the University of New Mexico's event planning guide (UNM Student Activities' planning guide) and the Community Tool Box's fundraiser tools (Community Tool Box).
• One ticket price. If adults and children pay different prices, enter the average you expect, or run it twice.
• The $75 note is IRS Publication 1771's threshold for a written disclosure on a quid pro quo contribution (IRS Publication 1771). Whether your group must give one is a question for its treasurer or tax adviser.

3. Headcount: the planning number

Answers: the number of guests to plan food, seats and a final count around, from your replies and two allowances you set. Used in Guests and invitations. Count people, not households: the party size total on the guest list is your invited count.

Assumptions behind the numbers

• Planning number = yes × (1 minus your no-show share) + no reply × your come share, rounded up. Both shares default to 0: no source we found measures replies or no-shows at private, invitation-only parties, so this manual sets no rate.
• Published no-show figures describe registered and ticketed events. One events-software company measured median no-show rates of about 28 percent for free events and 17 percent for paid ones (PheedLoop's Event Data Lab report). They are not a birthday party; Guests and invitations explains when they might inform your allowance.
• Caterers bill on the count you guarantee. Carnegie Mellon's catering terms bill the guaranteed count or the attendance, whichever is greater (Carnegie Mellon Dining's catering policies); Puget Sound's bill the guarantee and charge extra for guests served above it (Puget Sound's booking terms). Your caterer's contract is the one that applies.