How do you know if you have found the cheapest air fare on a given route?

You can find out the current best price for a route by using multiple search engines like Google Flights, Skyscanner, Kayak and also check booking sites such as Expedia and Booking.com.

If you’re on short-haul, also check airlines like Ryanair who only sell through their own sites.

It is pretty unlikely that you’ll find a much cheaper price elsewhere. Also, prices will go up with time, so waiting doesn’t really help. When prices do go down, it will be a special offer by some airline, and there is no real way to predict those.

The “normal” prices will go up the closer you get to the departure date. So you can either wait for a special offer and if none comes buy the ticket at a higher rate – or you buy now, and risk missing out on an offer that would’ve started tomorrow. Some sites even let you set a “price alert” to automate the wait.

Better ways to save money usually include:

• Be flexible with your dates. Low-season times are much cheaper.
• Be flexible with your route. Sometimes changing you itinary can make a big difference.
• Also search for flight+hotel on long-haul flights. Sometimes a package with a hotel booking is cheaper than the flight alone.

Airline ticket prices vary depending on the day of the week, time of year, and upcoming holidays such as Christmas, New Year’s Eve, Thanksgiving, or the Fourth of July. Overseas, August is a big month for traveling around Europe as many Europeans go on vacation then, so airline tickets tend to be higher.

In my view it’s always cheaper to fly during the middle of the week than on a weekend, because most people travel on the weekends and airlines hike their prices then. Prices are also a bit cheaper if you fly after a major holiday.

If you search on ITA Matrix and under advanced options uncheck Only show flights and prices with available seats, you will get a good idea of the cheapest possible fare.

It will show all possible tickets even if they haven’t been released yet or have all been bought. Unfortunately though, it does not include some budget airlines, e.g. Ryanair.

And define “cheapest”. What exactly is “cheapest” for you? If I can get a flight on a route where the average ticket takes 1 hour but the cheapest is $10 lower and takes 3 hours, do you consider that cheaper?
I wouldn’t, I value my time at more than $5 an hour. Someone else might.
And if I’m flying to London, I’m factoring in the transit fees from the airport to the city proper. For Heathrow that’s about 12 pounds (return). For Gatwick, that’s about 60 pounds (return). If your flight through Gatwick is 10 pounds cheaper than through Heathrow, that transit fee makes the overall package more expensive yet those price comparison websites never mention that.
If I fly to Birmingham with KLM my luggage is included in the price. If I fly with FlyBe, it’s extra, making KLM cheaper for the complete package price. Yet again those comparison websites don’t take that into account.
Years ago I flew to Curacao. Could have taken an indirect route through Lisbon and Caracas with TAP, save me 100 Euro over the direct route with KLM. But it would have taken me almost twice as long. Not worth it for me.
Mind those examples were from a few years ago, specific situation might be different there, principle holds.

Same with many other considerations. If I’m looking to rent a car at my destination, and the “cheapest” ticket brings me to an airport where I can’t get a rental from the agency with which I get a steep discount because I’m a repeat customer, overall package price will be higher, maybe much higher.

I don’t think you can know the cheapest price for a given trip unless you know how airfare works. You need to know that in advance to come up with the average cheapest possible price then compare it with the listed prices in various sites. This knowledge is not as easy as it sounds, there are dozens of things to learn.

Anyway, even if you have this knowledge, you will come up with the cheapest possible normal price which will be the same for all IATA members, special prices are tricky and you will need to use common sense or experience, but at least after getting the cheapest normal price possible you will know for sure that any special price should be less than that. Here is a PDF file about fares and ticketing that can you help you a bit in understanding how air fares are calculated and types of them and so on.

You can’t. At any point a new deal could come out. And these aren’t entirely predictable either – demand to some extent is predictable, but all you need is a ‘random’ event – a sudden football final in an unexpected location, for example, to drive air traffic and crowds.

Personal source: I run a flight deals website – Beat That Flight, and even when I’ve just published a deal, I can sometimes have someone complain within minutes that the prices have changed as well!

Kayak has recently tried to predict, and they have a ‘prediction’ algorithm, that’s quite indepth (article about how it works and what it looks like.

However, it’s not a certain thing, as I mentioned. That’s why many, many articles are written about the ‘best time’ to get a good price. And there are guidelines. ‘Generally’ cheaper flights aren’t during school holidays. Generally, they’re not at Christmas. But that’s the ‘cheapest’. What about the best at Christmas? We have an epic answer by Kate on this very topic. Still not perfect though.

However, even when I’ve seen kayak’s algorithm at nearly 100% certain you should buy, and they have a price – that makes you think ‘oh – it’s the cheapest it’s going to get’. Then some flight hacker on Flightfox or Darjeelin comes up with an even lower bet, through some clever trick hopping through South Neverheardofit and a 45 min window in the airport. So sometimes you may have the cheapest price, but at a cost. And you have to decide if that cost is worth it to you.

I’ve often heard that the best time to buy a ticket is when you’re ‘ok’ with the price. At that point, that is the right price. Buy it and don’t look back.

Kayak provides a little “price trend” graph that includes advice on whether to buy or wait, and their confidence. This applies to both domestic US and international flights, but not all flights. This is the closest thing I’ve found to knowing the rock bottom real price.