Whenever I have to book a flight, I usually approach it the same way: check Kayak, check Google, check different airline websites, maybe check Kayak again, and inevitably get increasingly stressed and cranky. 

Tickets are almost always more than I expect them to be — especially these days — and I’m always wondering if I’ll find something cheaper if I wait.

“The vast majority of the time, that isn’t going to happen,” said Mike Arnot, an airline industry commentator and spokesman at the airline analytics company Cirium. “If it’s within three months, chances are you’re getting the lowest price that’s actually available to book. So you can book with some confidence.”

Most people book flights less than 90 days before they want to travel. And airlines know this, so that’s also when ticket prices tend to start going up.

The best time to buy does vary by route and airline and whether you’re flying economy, business or first-class; airlines have years of data they use to constantly adjust fares and maximize revenue for different routes. 

But since they aren’t sharing it, Arnot said, he usually goes by this general rule of thumb: “I would say three to four months out, you’re likely to get those lowest fares that are available. And they’re not likely to magically reappear.”

That, and check the online travel search engines — “the Kayaks and the Hoppers of the world, and the big behemoth, which is Google Flights,” and see what they recommend. Because, he said, these sites also have years of data on bookings and flight prices — more, actually, than any one individual airline.

“We have been archiving global prices of airfare for more than 10 years now,” said Hayley Berg, lead economist at the travel app Hopper. “Our archive is up to, I think, at last measurement, maybe 80 or 85 trillion priced itineraries.”

Yep, trillion with a T. “And that algorithm looks at, all the way down to an origin, destination and departure return dates, what the price of that trip is today and makes a forecast of what the future price is,” Berg said. “And we take all of that data and a very complex price prediction, and boil it down into something very simple and user-friendly, which is a recommendation.”

A recommendation of whether to book now or wait. And she added that those recommendations are accurate about 95% of the time.

Still, for me, somehow, seeing those recommendations has never significantly lessened my anxiety around booking flights and whether I’m doing it at the right time.

Jade Kessler, a product manager at Google Flights, said she hears that from people a lot — “that they really want to know if they’re getting a good deal or not on their airfare, and if they should book now or wait a little bit longer for a better deal to come along.”

That’s one reason Google is now experimenting with a new feature — a price drop guarantee. It’s a pilot right now, only available on certain flights, with certain airlines. 

But if you find one of those flights, and you buy it on Google, “then we monitor the price every day until takeoff, and we automatically pay you back the difference if there’s a price drop,” Kessler said.

Other sites do different versions of this. At Hopper and certain airlines, for example, you can freeze the price of a flight for a little while, but you have to pay for it — kind of like buying insurance. Google’s is free. 

“For Google, I think it’s very low-risk, because they have a huge amount of bookings and passenger data,” said Mike Arnot at Cirium. “And they make their money not from Google Flights … they make their money when you do a search and you click on an ad. And they make so much money that the risk to Google of having to pay out up to $500 per year to a passenger is really a drop in the bucket.”

In a way, it wouldn’t be for other companies like Hopper, Kayak or Expedia, that make money by taking a cut of bookings people make through their sites.

Plus, right now, Google is only offering the guarantee on a limited basis. So limited, in fact, that I found it hard to find a flight that had it at all. I did, eventually, on some Spirit flights from New York to Miami that were already so cheap I wouldn’t need a guarantee to buy.