3 times you should book your flight with cash (not points)

Racking up airline miles with each purchase is one of the best things about using a travel rewards credit card. But a new NerdWallet study finds that you may want use another way to pay for your summer flight and save the credit card rewards points you've earned for later.

NerdWallet booked sample flights on the 20 most popular routes from the U.S. and the four largest domestic airlines to see whether consumers would be better off using airline points or cash – including a debit card or credit card – to book their trips for summer vacation. Airline frequent flyer points are worth an average of about 1 cent each. It's financially savvy to use points only for trips where the average point value exceeds 1 cent, with a couple of exceptions discussed later. For trips where the value is below 1 cent, aim to pay with cash and use your points when you can get more out of them.

[See: 12 Frugal Ways to Save on Vacation.]

1. Domestic business-class flights. If you're staying in the U.S. this summer and plan on flying in style, you should probably use cash. For domestic, round-trip travel in business class or first class, airline point values average 0.83 to 0.86 of a cent each, significantly lower than the 1 cent guideline. To compare, domestic flights in economy have airline point values averaging 1.03 to 1.08 cents.

How far the points go: On average, 50,000 airline points would be worth $415 to $430 in business class, as opposed to $515 to $540 for flights in economy.

2. Short business-class trips. If you're going to be using points to fly business class, at least save them for flights longer than 1,000 miles. Point values average 1.13 cents for those trips. On trips shorter than 1,000 miles, they're worth only 0.72 cent apiece, on average. That's 36 percent less value for your points.

How far the points go: On average, 50,000 airline points would be worth $360 for short flights and $565 for long flights.

[See: 12 Habits of Phenomenally Frugal Families.]

3. Round-trip flights. When compared with one-way flights, point values for round-trip flights are significantly lower. In fact, around two-thirds (62 percent) of the one-way flights NerdWallet analyzed had higher airline point values than round-trip flights on the same travel routes. One-way airline travel averaged 1.15 to 1.21 cents per point. Round trips averaged 0.89 to 0.99 of a cent.

How far the points go: On average, 50,000 airline points would be worth $445 to $495 for round-trip flights and $575 to $605 for one-way flights.

See a ranking of the U.S. airlines:

When to use points (even when the values are low). Even if your airline points are worth less than 1 cent each for a particular trip, it may make sense to use them instead of cash if they're about to expire or if you aren't sure about your itinerary.

Some airline programs have points that never expire, but others expire after a few years. Know the policy of your preferred airline's frequent flyer program and keep track of the expiration dates of your points, if applicable. If your points are set to expire before your next trip, use them now, regardless of value.

If you think you might have to cancel or change the dates of your trip, it's also often cheaper to use points, even with low values. The average change fees NerdWallet looked at were $358 for tickets bought with cash and $129 for tickets bought with points.

[See: 8 Ways to Maximize Your Credit Card Rewards.]

How to calculate the value of points on your summer trip. When it comes to air travel, the guidelines above are rules of thumb that apply in most cases, but not all. You can calculate point values for a flight yourself. Here's what you do: Take the dollar cost of a flight and divide it by the point cost of the same flight. That will give you the dollar value per point.

For even greater accuracy, take into account the special tax on airline tickets often referred to as the "9/11 security fee." These fees are rolled into cash prices, but when you use points, you have to pay them separately.

Let's say your desired flight is either $450 in cash, or 50,000 airline points plus an $11.20 security fee. The point value would be 0.88 cent, as demonstrated below.

$450 - $11.20 = $438.80

$438.80 / 50,000 = $0.008776

Based on the average point value of 1 cent, it would likely be advisable to use cash for this trip. But in cases where that value is higher than a cent, put your points to good use and book your summer vacation with travel credit card rewards points.

Copyright 2017 U.S. News & World Report