Flight booking: Rule of 21 and Rule of 34

Traveling is one of my great hobbies. Each time I go somewhere far away from home, I get new inspirations and ideas, such as opening my first credit card and starting this blog. And I love air travels; the Wright brothers are among my favorite heroes, and it so happens that Seattle is the host of the famed Museum of Flight. It also helps that I have friends on the other side of the continent, and also on the other side of the Pacific Ocean. I don’t need excuses to travel, really. And I hope that you share with me my passion for flying.

Because if you do, you will find the information I present in this post very helpful. I am making this post the shortest I’ve ever written since I want to focus your attention on the 2 simple rules of flight booking: the Rule of 21, and the Rule of 34

Book a domestic flight 21 days in advance, or an international flight 34 days in advance.

The reason: these time windows offer the best prices for your trips. Why, you ask?

For each flight, the airlines set a goal of selling a set number of tickets at regular prices, and put the rest on sale. Throughout the time window prior to the flight, the airline company periodically releases discount tickets, and the sweet spot is right at 21 days before departure, according to a survey by Kayak.  For international flights, the sweet spot is 34 days prior to departure.


You won’t believe this. I accidentally tested the rules on myself earlier this week, when I was randomly checking the price of a ticket that I had already booked 10 days prior. To my amazement (and dismay), the price had dropped by 20%. Twenty-per-cent. On the exact same flights. How could this happen to me? Seriously, how could this happen to me?? The answer, which I regretfully did not know at the time, is the Rule of 21 in force.

21Today, I checked again, and the price has now gone up from the price I had paid. This makes me feel less guilty, and makes me more highly regard the power of the Rule of 21. I have not tested the Rule of 34, but I intend to follow this rule when I book my next international flight with cash.

Obviously, the Rule of 21 and the Rule of 34 are generalizations, but they give you the best chance of success. As an actuary, I believe in numbers and statistics. But this perspective earned me a pair of tickets to “The Book of Mormon” in Seattle. And I have witnessed the Rule of 21 with my own eyes. When you book your travels, just keep these 2 numbers in mind, and I wish you success.

Another trivia for you: the best time of the week to book a flight is 3pm on a Tuesday, according to a study by FareCompare. Feel free to test this out!

8 thoughts on “Flight booking: Rule of 21 and Rule of 34

  1. Booking a flight 21 or 34 days in advance may save money on the airfare component of a trip, but it seems to me that focusing on only a cheap flight would compromise other parts of a trip. Is there data out there that looks at how hotel prices or rental car rates fluctuate? Who is to say that hotel rates don’t bottom out at a time and that 21 is not the best time to book for overall cheapest trip? What about the availability of theater or concert tickets or tables at a popular restaurant? While the rule of 21 and the rule of 34 may be the best on airfare, it seems the algorithm of when to book the perfect vacation is far more complicated. Personally, I’d rather plan far in advance so that I can do that activities that are important to me and make sure I am staying in a location I find desirable, rather than book three weeks ahead and leave the details to chance.

  2. Hi Howard. I actually have never booked hotels or rented a car on my own; I have had good friends at travel destinations hosting me for just about every long-distance trip I’ve made. But I speculate that car rental and hotel booking do not follow a more or less uniform pattern like flight booking. As a result of aggressive consolidation in the airline industry in the past several years, there are only a few big players left, and they are highly susceptible to herd behavior, e.g. they follow more or less the same business strategies. When tickets from one airline go one sale, other airlines almost immediately follow suit. When an airline offers a holiday sale, competing airlines very soon start matching the offers on the same route. This is why the Rule of 21 and the Rule of 34 are so universal.
    On the other hand, the lodging and car rental industries are very diverse with most businesses being relatively small-sized, quite often family-owned. These small businesses have a very wide spectrum of strategies that depend on individual management styles, and therefore aren’t as predictable as the airline giants.
    The Rule of 21 and the Rule of 34 are about finding the best deal for a fixed itinerary. When you already have a particular schedule and location in mind for your travel, it is then time to find the best value, which can take a while to find. Some people prefer planning everything way ahead of time for peace of mind, and then there are others that are willing to spend the time to try to obtain the best bang for the buck. The mushrooming of travel booking websites is evidence for the fast growth of the latter class of travelers. But even if you do not belong to this class, there may be situations in which you need a quick turnaround, and for that, it may be in your best interest to time your fare search around the time when flights go for cheaper than usual. For example, if you know you’ll be flying to Seattle 22 days from today, you can look up the fare today, and then again tomorrow when tickets are likely to go down in price, to find the best deal.
    The Rule of 21 and the Rule of 34 should be simple enough to remember to apply when the situation arise. Even if you only take advantage of these rules occasionally, saving something is still better than saving nothing, right?

  3. Hi Richard,

    I recently search an airfare that use Rule 21, but it does not work. I searched an airfare on May 27 , 22 days from today, but the price is high. Does the Rule work on weekend too?

    Thank you

  4. Tu: glad you found the post helpful. Keep up the good work on Facebook!
    Tusen: that means the cheap tickets probably have not been released yet. I would check back on kayak everyday till you notice a price drop. Also a good idea to sign-up for kayak’s fare alert so you get alerted when the system finds you a better price. And keep in mind that 21 days is the average figure. Last week I found the best deal 19 days prior to departure.

  5. Hi anh Richard,

    I’m sorry to tell that the rule 21 was not helpful at all, since my last flight’s price dropped at 13 or 11 days before departure 🙂

  6. Hi Tusen,
    Thanks for reporting. Remember, the Rule of 21 and the Rule of 34 are general rules, not absolutely-true-in-all-circumstances rules. Occasionally airlines issue unsold tickets closer to departure dates if there are many of them available. Good job buying on the dip!

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