For Kathy Lopez, it’s all about the spreadsheet. When she started planning a future trip to Europe with her friends, she put every expense into Microsoft Excel to determine how much the monthlong adventure would cost.

“We’re going to save a lot of money and planning time on the road,” says Lopez, a retired city manager from Prescott, Arizona.

If your eyes glazed over when you saw the word “spreadsheet,” please keep reading. Travel companies don’t want you to make a budget. They’d prefer that you mindlessly swipe your credit card at the ticket counter, the front desk, the restaurant. You’ll spend more, and you could end up in debt.

They win, you lose.

“The key to budgeting for vacation is planning,” says Rosalyn Glenn, a financial planner with Prudential Advisors in Columbia, South Carolina. “The vacation budget should be considered in the annual budgeting process, and the amount for the budget should be determined by the resources you have available to allocate toward that line item.”

Here are a few general rules for budgeting a vacation:

Save for your vacation: Set an amount in your household budget. “See how much each month you can save for your vacation between now and your planned dates,” says Tanya Peterson, a vice president at Freedom Financial Network, a debt-solutions company.

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Don’t go into debt: Experts agree, it’s just not worth it. “Nothing ruins a vacation like the stress of having to pay it off when you get back,” says Dan Simon, a retirement-planning adviser at Daniel A. White & Associates in Middletown, Delaware.

Know your percentages: The typical consumer spends $2,400 on travel annually, according to Bank of America. That’s about 5% of average annual income.

But how exactly is someone like Lopez going to save money? Her trip from Arizona to Europe’s microstates, including Liechtenstein, Monaco and Vatican City, could be a budgetary black hole. She listed every destination, every hotel and every activity on the spreadsheet and shared it with her travel companions.

“By doing this, we determined that a Eurail Pass would be a good idea, so we bought them on sale,” she says. “We were able to book flights on days that were cheaper. We got good prices on hotels. And we made the best use of our reward points.”

How much should you set aside for travel? About a quarter of Americans save 5 to 10 % of their monthly income for vacations, according to a Harris Poll on vacation spending conducted for CIT Bank last year. One-third set aside less than 5 % of their income for vacations, and more than one-third don’t save at all.

Lopez describes the process — particularly the negotiations with her friends — as “excruciating,” but well worth it. They had to agree on destinations, hotels, fares and how much they wanted to spend.

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Of course, there’s a shortcut for planning a vacation. You can hire a travel adviser. Agents are trained to help you with the budgeting process. And they also know a thing or two about mediating disputes between travel companions over how much to spend — a skill that has probably saved a marriage or two.

One other rule seems to hold true: The money goes fast. Chuck Czajka, a financial adviser in Florida, says you’re probably not saving enough. He says you should be allocating 15% of your gross income for “vacations and fun spending.”

“While that may be hard in the beginning, like a diet, you have to try to get as close as possible to this goal,” he says. Otherwise, your next vacation could place a financial strain on you — and possibly leave you in debt.

Perhaps a more important question: What should you spend the money on? Ask leisure travelers, and they’ll give you answers that are all over the map. But it turns out that business travelers have some useful benchmarks.

Ride-hailing is the single largest business travel expense, according to Certify, a developer of expense-management software. More than 17% of the average trip budget goes to Uber, Lyft or taxis. Meals are the second-highest expense, at just under 17%. That’s followed by airfare (13%), fuel (11%) and hotel (11%).

Ride-hailing has been a rising expense for several years. For leisure travelers, it means that ground transportation expenses — whether it’s a rental car or a ride service like Uber — could be one of their biggest line items.

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Whatever you do, don’t leave anything to chance. Angela Rice, an accountant and travel adviser from Paradise Valley, Arizona, says that’s when things go off the rails.

“We require that clients be open to discussing their travel budget,” she says. “How much are you able to spend on travel? That will play a big role in determining where you will go, how long you will stay, when you will go, and what you can do when you get there.”

Rice is right. As painful — and, yes, boring — as creating a vacation budget may be, it’s essential. When people go on vacation, they tend to leave common sense at home. I’ve watched my parents, kids and close friends pay for something they couldn’t afford while they were on the road. I think travel companies know our defenses are down and take advantage of it.

The fix is deceptively simple: Create a budget, save for your vacation, plan carefully and then stick to the plan. Use a spreadsheet if you must. Consider enlisting the help of a trusted travel agent. But whatever you do, think about how much you’re going to spend before you go. Otherwise, you could end up spending money you don’t have.