The guest count is the budget
Two figures come out of this page and they answer different questions. The average cost per guest is the total divided by the head count, which spreads the venue, the photographer and the rings across everyone. It is the number people quote to each other. The marginal cost of one more guest is the per head catering and bar figure with the service charge, the sales tax and the contingency stacked on top of it, and nothing else. It is the number that matters when you are looking at a list deciding who to cut.
The marginal figure is always smaller than the average, and it is always larger than the catering quote. A ninety-five dollar plate with a twenty-two percent service charge and eight percent tax is not ninety-five dollars; it is a little over a hundred and twenty-five before the contingency touches it. Multiply that by twenty people you invited out of obligation and the comparison against a flat cost like photography becomes concrete.
Service charge, gratuity and tax
These three are the most common source of a wedding invoice that does not match the wedding budget, because they are usually quoted separately and they multiply rather than add.
A service charge is a percentage the venue or caterer adds to food and beverage. It is a charge to you, not a tip to staff, and in most places it is legally the venue's revenue. Some contracts state that all or part is distributed to staff and some do not, which is why a contract can carry a service charge and still leave a gratuity expected. Read for both words, and if only one appears, ask directly whether the other will.
Sales tax then applies on top, and whether it applies to the service charge as well as the food varies by state. The checkbox on this page controls that, and the difference on a large catering bill is not small: on twenty thousand dollars of food with a twenty-two percent service charge, taxing the service charge at eight percent adds roughly three hundred and fifty dollars. If you do not know which rule applies, ask the caterer to show a line-itemised sample invoice rather than a package price.
What the flat costs really contain
Each flat row here is one number, and in a real contract each is several. Photography quoted for eight hours becomes overtime at hour nine. Flowers quoted as arrangements become arrangements plus delivery plus setup plus a strike fee for collecting the vases. Rentals quoted per chair become chairs plus delivery plus a labor charge if the delivery window does not match the venue's access hours. Music quoted for a reception becomes ceremony sound as a second line.
The contingency exists for exactly this, and it is a budget line rather than a prediction. Ten percent is a common working figure and it is not a researched one; it is simply large enough to absorb a few of these without a conversation about money in the last month. Vendor meals, marriage license fees, alterations after the second fitting, and the hotel night before are the items most often missing from a first draft.
Deliberately not modelled: gifts
This calculator does not net anything off the total for expected gifts. Cash gifts vary enormously by family, region and guest, they arrive after every deposit has already been paid, and treating them as an offset turns a budget into a bet on other people's generosity. A wedding budget is a spending plan; what arrives on the day is a separate and unpredictable thing, and the plan should stand without it.
How the arithmetic is done
Every input becomes whole cents. The per head figure is multiplied by the guest count first, then the service charge is calculated and rounded to the cent, then the tax on whichever base you selected, then the flat costs are added, then the contingency is calculated on that subtotal and rounded. Each displayed line is the rounded integer that went into the total, so the breakdown table adds exactly to the total shown. The comparison rows for ten and twenty guests either way are full recalculations rather than a multiplication of the marginal figure, so they include the way the contingency scales as well.
No typical or average wedding cost appears anywhere on this page, and none should. Published averages come from vendor and planning-site surveys with self-selected respondents, they vary by a factor of several between metro areas, and a national figure tells you nothing about what your venue will quote. The only useful numbers here are the ones from your own quotes.
Questions people ask
Should the venue minimum go in the venue field or the catering field?
Neither, directly. A food and beverage minimum is a floor on the per head spending rather than a separate charge, so the way to model it is to check whether the per head figure multiplied by the guest count clears the minimum. If it does not, you will be billed the minimum anyway, and the honest way to enter that is to raise the per head figures until the food and bar subtotal reaches it, since that is what you will end up receiving in food and drink.
How do I handle a plated dinner with different entree prices?
Use a weighted average as the per head catering figure. If sixty guests take the eighty-five dollar option and forty take the one hundred and ten, the average is ninety-five, and that is what belongs in the field. The service charge and tax apply to the total either way, so the loaded figure comes out correct even though no individual plate cost exactly the average.
Is ten percent the right contingency?
It is a working figure rather than a validated one. What determines the right size is how much of your budget is already contracted at a fixed price and how many decisions are still open. A wedding with every vendor signed and a firm guest count needs less slack than one six months out with a venue that charges for overtime. If you have never priced a wedding before, more is safer, and unspent contingency is not a loss.