Let’s do some beach house math. Three sisters and their families are paying $3,000 to rent a four-bedroom house in Isle of Palms, S.C., for a week. How should they split the costs?

Easy: Divide $3,000 by three, so they each pay $1,000.

But wait: One sister has three kids, who are staying in the fourth bedroom. One sister is single and has none. And the third sister, along with her husband and baby, is only staying five days. Help!

The worst part of a multi-family vacation is often sharing the expenses. If one person feels they’re paying too much, that can sour the entire trip for everyone. So what’s the fairest way to split vacation rental costs? And how can travel insurance help?

Why travel insurance is crucial for multi-family vacations

If you’re planning a domestic vacation (like a beach house rental) with multiple family members and/or friends, you need a travel insurance plan with two key benefits: trip cancellation and trip interruption.

These benefits can save your budget (and your family harmony) in case you must cancel the trip or cut it short due to a covered reason. Here’s one example: A few days before the trip, what if your daughter becomes seriously ill and is hospitalized? Of course, you decide to stay home and care for her — but your other relatives are still going on vacation, and you’ll be on the hook for your share of the rental costs. If you have trip cancellation benefits, you can be reimbursed for your pre-paid, nonrefundable trip costs when you must cancel for a covered reason.

Trip cancellation insurance doesn’t have to be expensive. If you’re traveling within the U.S., and your main concern is protecting your financial investment, consider the affordable OneTrip Cancellation Plus plan. Get a quote to see which plan best fits your vacation budget .

Now, what’s the best way to split those vacation costs?

Option 1: Splitting vacation rental expenses by room

The simplest way to split a three-family vacation rental is in thirds, of course, even if each family is a different size. This works if everyone is pretty relaxed about money. You can offer other compensation to make things fair, such as giving the single traveler/smallest family the master bedroom.

And if they’re not so relaxed? One method is to treat the beach house like a hotel and have everyone pay a nightly rate per room. So, in the example above, the nightly room rate would be $107. Sister A, with the three kids, would pay around $1500 for using two rooms. Sister B, with no kids, would pay $750. And sister C, who’s staying for five days, would pay $535. Easy, right?

But who’s going to pay the remaining $215 for the two nights sister C’s bedroom is empty? And will sister A agree to pay fully half the cost of the beach house just because her family is the largest? This is harder than it looks.

Option 2: Splitting vacation rental expenses by person

Another method for splitting vacation expenses is to treat it like an all-inclusive resort: There’s a set rate per adult, with kids half-price and babies free. So, with 5 adults and 3 kids sharing a $3,000 beach house… whoa, now we’re getting into algebra. If 5x + 3(.5x) = 3,000, each adult must pay $461.50 for the week, plus about $231 for each kid. Fair, right?

Well, now the family of five is paying even more than half of the house costs: a total of $1615. And that presents its own problem. Is it fair for the largest family to essentially subsidize the cost of a vacation for everyone else? The single sister gets all the benefits of a big, beautiful beach house for a super-low weekly rate, after all.

And what about income? Let’s say sister C, the one with a spouse and baby, is a corporate lawyer and makes twice as much money as either of her siblings. Should she offer to pay more? The answer: Most people agree that income should not matter when splitting vacation expenses, and that it’s not fair to expect more from a wealthier sibling/parent.

Read more: Family Travel Insurance: When You’re Paying, What’s Covered?

Option 3: Splitting vacation rental expenses by person per night 

The best way to split vacation rental costs among families, according to Splitwise, is that “each person should pay proportionally to the number of nights they were there. This system is robust, because anytime someone stays in the shared accommodations for an additional night, the per-night price goes down for everyone.”

Confused? Use Splitwise’s vacation expenses calculator. If we calculate rates based only on the adults in our hypothetical group, then Sister A’s family pays $1355; Sister B pays $677; and Sister C (whose family is staying for 5 nights) pays $968. That seems pretty fair to us.

How to split food costs and other expenses on a multi-family vacation

After all this vacation-rental math stress, you’re going to need a cold drink on the deck. But who pays for that? Sharing drink and food costs on vacation is a whole other challenge.

Some families take a buy-your-own approach, which makes sense. Pay for what you eat, and eat only what you’ve paid for. For communal dinners, each family can take a turn buying the ingredients and preparing a meal.

But for big family vacations — the kind with 20 people all sharing a house — it gets tricky to keep track of who bought the blueberry bagels. Math for Grownups has a sensible solution: Use the concept of shares, where each adult has one share of food expenses and kids have a half-share. Buy communal groceries at the start of the trip and divide the cost accordingly. Alcohol costs should be split only by those adults who drink.

Got all that? Just don’t forget to buy travel insurance as soon as you book your vacation! Compare travel insurance plans from Allianz Global Assistance and get a free quote today.

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