Product Management interview question [Guesstimate]: Estimate the number of people passing through security scans daily in an airport of your choice.

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I have a few clarifying question before I work on the solution -

1. By people do you mean just the passengers or any people — passengers, airport staff, airport shopkeepers, security staff, on-flight staff etc? — Just the passengers
2. What is the exact definition of security scan here? — Because one scan takes place the moment you enter the Airport and another one happens after the check-in. Do we count the initial one here too? — Just the main security scan after the check-in

I’ll start with the top-down approach -

I’ll take an example of Indira Gandhi International Airport in Delhi, India because as an Indian I relate to it and have visited it multiple times. The airport has an average daily traffic of 900 aeroplanes. Considering that out of these 900 aeroplanes, 10% are freight aircrafts i.e used only for goods and services. Considering another 2% of aeroplanes as private aeroplanes. Another consideration that we need to make is the aeroplanes that might be damaged or need some repairing or parts replacement, this leaves around -

900 — (0.1 * 900 + 0.02 * 900 + 0.08 * 900) = 720

So there are around 720 running aeroplanes for the public.

These 720 aeroplanes can be further divided into Domestic and International flights -

Considering 70–30 split in Domestic and International flight respectively -

Domestic — 720 * 0.7 = 504

International — 720 * 0.3 = 216

So we have established that there are 504 Domestic flights, and 216 International flights taking off everyday from IGI Airport Delhi.

These domestic flights can be categorised into 3 categories -

1. Jumbo Jets: For example Airbus A380 has a maximum capacity of around 800 passengers. Some smaller Jumbo jets carry around 400 passengers. Let’s take the capacity to be around 600 passengers.
2. Mid-size passenger Jet: These jets carry around 350 passengers on an average.
3. Light passenger Jet: These jets carry around 100 passengers on an average.

As there are very few Jumbo jets flying domestically and a large number of Midsize flights and small size flights I consider the split of 10–35–55 between Jumbo Jets, Mid-size passenger Jets, and Light passenger jets. Also, keeping into account the vacant seats on the flights. Let’s keep this around 10%. This makes the number of passengers in domestic flights taking security scans everyday to -

Jumbo Jets — 504 * 0.10 * 600 * 0.90 = 27216

Mid-size passenger Jet — 504 * 0.35 * 350 * 0.90 = 55566

Light-weight passenger Jet — 504 * 0.55 * 100 * 0.90 = 24948

Total domestic attendance i.e the number of passengers passing the security check = 107730

Now coming to International flights -

International flights can also be categorised into 2 categories -

1. Jumbo Jets: For example Airbus A380 has a maximum capacity of around 800 passengers. Some smaller Jumbo jets carry around 400 passengers. Let’s take the capacity to be around 600 passengers.
2. Mid-size passenger Jet: These jets carry around 350 passengers on an average.

Mid-size passenger flights are usually used when there is a small distance to travel internationally, say Maldives, Sri Lanka, Malaysia, Singapore, Thailand, Dubai, etc.

Jumbo aircrafts are used to travel to the far east or far west, say Europe, USA, Canada, South America, Japan, Australia, etc. Considering this, let’s consider the split to be around 70–30 between Jumbo jets and Mid-size passenger jets. Also, keeping into account the vacant seats on the flights. Let’s keep this around 50% as there are not many travellers to each and every destination. For example, say the flight to Europe might be 90%-100% but the flight to Sri Lanka can be 20% occupied. This makes the number of passengers in international flights taking security scans everyday to -

Jumbo Jets — 216 * 0.70 * 600 * 0.50 = 45360

Mid-size passenger Jet — 216 * 0.30 * 350 * 0.50 = 11340

Total international attendance i.e the number of passengers passing the security check = 56700

So the total attendance at IGI Airport Delhi on a daily basis is 164430

So the total number of people going through airport security check at IGI Airport Delhi on a daily basis is 164430

Sanity check — It was reported that IGI Airport handled 69 million passengers in 2020 which roughly translates to 189041 passengers every day, so the estimate doesn’t seem too far off from the actual number.