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Estimate the number of planes that are in the air in an hour?

estimation guess-estimates Guesstimate Market sizing
New answer on Jun 29, 2020
6 Answers
19.4 k Views
PK asked on Apr 02, 2020
Director of Product at PayPal. Based in SF. Worked at Twitter, Yelp, Salesforce, Houzz in the past.

Clarifying questions:

  1. Planes can be commercial or military. Assuming, in this case, we are referring to commercial passenger flights.

  2. We are referring to planes flying over the US area only.


  • The number of planes in the air per hour = # of planes taking off per hour from airports across the US

  • # of planes taking off per hour from airports across the US = Sum of # of planes taking off split by large, mid, and small airports.

  • Planes taking off an airport = # of runways x (average planes taking off per hour)

  • Number of airports in the US: Large, Medium, Small

    • The US has 50 states. It is reasonable to assume there are 2 large/major international airports per state, 15 medium-sized airports, and 10 small airports.

    • Large airports have 2 runways, medium have 1 runway, small also have 1 runway

    • The average gap between flight take offs in:

      • a large airport is 5 min during, in medium 10 mins, and 30 mins in smaller

    • # of planes in 1 hour at:

      • Large Aiport = (2 runway x (60 mins/5 min) → 24 planes/hour

      • Medium = (1 runway x(60 mins/10)) → 6 planes /hours

      • Small = (1 runway x (60 mins/30) → 2 planes/hour

    • Number of airports

      • 50 x 2 large airports → 100 large

      • 50 x 15 medium → 750 medium

      • 50 x 10 small → 500 small

    • The number of planes taking off per hour:

      • Large - (100 x 24) + Medium (750x6) + Small (500x2)

      • ~7000 planes taking off per hour and will be in the air per hour

Is my breakdown reasonable?

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Content Creator
replied on Apr 03, 2020
McKinsey | Awarded professor at Master in Management @ IE | MBA at MIT |+180 students coached | Integrated FIT Guide aut


Your breakdown indeed is good, however, I think you are leaving out another branch of the tree.

  • Basically, you 1st break down into commercial and military, and then discard military.
  • Then, in the commercial bucket, you would need to make the difference between passenger and cargo. There, you chose passenger
  • Finally, the split I believe you are missing is that the prompt is "Estimate the number of planes that are in the air in an hour, in the US". Hence, you need to consider:
    • Planes taking off
    • Planes going over the US "aerial space", whose destination/origin is not the US. THis is the part you are missing and you would need to include

Hope it helps!



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Content Creator
replied on Apr 02, 2020
Bain Consultant | Interviewer for 3 years at Bain |Passionate about coaching |I will make you a case interview Rockstar

From my perspective, there are a number of reasons why your approach would end up not being 100% accurate given how many uncertain assumptions are included. I think the main logical fallacy here is that it is not the number of airports that drive how many planes are in the air at a given point in time but rather the demand for traveling. Some issues that I can think of:

  • The approach is driven by supply of airports rather than demand for flights
  • You are not considering differences in peak vs. off-peak traffic (at large airports there is a heavy concentration to certain "banks")
  • Most US states would not have a major airport with 2 runways, some have multiple
  • The gaps between take-offs do not really make sense for a large airport unless averaged out over 24 hours. In real life, the gap would be much smaller.
  • Large airports typically would use one runway for take-offs the other for landings to optimize utilization

If we are to take a demand-driven approach you could take an approach that looks roughly as follows:

  • Number of passengers flying at a given point in time/average airplane size/load factor = number of airplanes in the Air
  • Number of passengers at a given point in time
    • Assumption: average American flies x times per year (consider mix between business travelers and leisure)
    • US Population: 330M
    • Assumption: all flights are equally distributed over the year and time of day
  • Average airplane size:
    • Assumption: ~150 passengers to account for mix between short-haul and long-haul
    • Accounts for the fact that the same passenger volume can be served by larger or smaller airplanes. The mix will impact planes in the air.
  • Load factor:
    • Depends on the time of year/day but assume 70 or 80%

To make the approach more sophisticated you may want to consider international passengers flying to the US on long-haul flights. This is a bit more complicated and is likely a relatively small portion of overall flights.

Hope that helps!

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Content Creator
replied on Apr 02, 2020
McKinsey / ex-Interviewer at McKinsey / I will coach you to rock those interviews

Hi Piyush, you are getting better at this, good job :)

just out of curiosity: what company are you preparing for? I'm asking because at MBBs estimations are not that frequent (at McKinsey they can sometimes come in the second round, but also rarely) and it's better to focus your time and efforts on practicing the standard cases (structuring, analyzing exhibits and doing case math).


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Anonymous A on Jul 21, 2020

Not true. I have a good number of friends who had market sizing questions in their MBB interviews

replied on Apr 02, 2020
McKinsey / Accenture Alum / Got all BIG3 offers / Harvard Business School


It will depend on whether its a peak / off-peak hour. So I would clarify first what is exactly the hour we are talking of. Average here will not be helpful


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PK on Apr 02, 2020

If I divided the flight volume by peak and non peak flights, would that be reasonable?

Anonymous replied on Jun 29, 2020


Your approach is indeed a good one.

If you want to master your market sizing skills I suggest that you sholud focus on segmentations patterns. You can use the following segmentation for market sizes:

-Demographics (Age, education, income, family size, race, gender, occupation, nationality)
-Behavioral (Purchasing behavior, customer journey stage, occasion & timing,
customer loyalty & interest, risk tolerance, user status)
-Psychographic (Lifestyle, personality traits, values, opinions, interests of consumers)
-Geographic (Geographical boundaries)

-Company characteristics (Industry, company size, number of employees)
-Geography (Geographical boundaries)
-Purchasing Approach (Occasion & timing, customer capabilities, nature of existing relationship)
-Personal Characteristics (Loyalty, risk attitude, user status)

-Demographics (Type of agency, size of budget, the amount of autonomy)
-Geographic (Geographical boundaries)
-Government Tier (Federal , State, Local, Quasi-governmental, International)
-Bid type (Closed, Open)

But sometimes you don’t need to segmentation. Here is an example of case that could be solved with high level top down approach - estimate the size of credit card market in the US:

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Content Creator
replied on Apr 30, 2020
McKinsey | NASA | top 10 FT MBA professor for consulting interviews | 6+ years of coaching

Hi, I think it is a good approach. Always try to stay in 20 minutes and - if needed - accelerate some estimations by asking to the interviewer


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Clara gave the best answer


Content Creator
McKinsey | Awarded professor at Master in Management @ IE | MBA at MIT |+180 students coached | Integrated FIT Guide aut
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