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  1. 022Estimate how many people work in a 30-storey office tower in Mumbai.Estimation and market sizingWarm upHSBCCentral · 2026

    Try it first

    Which order of magnitude feels right?

    Show the worked solution

    About 5,100 people, as a range of roughly 3,400 to 5,100. Take a floor plate of 25,000 square feet. After lifts, stairs and services about 80% is usable, 20,000 square feet. At 100 square feet a seat that is 200 seats, and at 85% occupancy about 170 people a floor. Thirty floors gives about 5,100. Roomier offices at 150 square feet a seat bring it down to about 3,400.

    Why start from one floor?

    Think of estimating the people in a cinema: you count the seats in one row, then the rows. A tower is the same: thirty copies of one floor, so estimate one floor carefully and multiply, because every assumption is easier to defend on a single floor you can picture. Define the answer too: people who work in the building, meaning assigned staff, not visitors and not the count present on any one day.

    What goes into one floor?

    Four assumptions, each said out loud. A floor plate of about 25,000 square feet, typical for a large commercial tower. About 20% of that is the coreThe central block of a floor holding lifts, stairs, toilets, shafts and service rooms, which cannot be used for desks., leaving 20,000 usable. At 100 square feet a seat, including aisles and meeting rooms, the floor holds 200 seats, and with 85% of seats assigned that is about 170 people. The rest are vacant desks, new hires not yet in, or space held for growth.

    The relationship
    N=25,000×0.80100×0.85×30=200×0.85×30≈5,100N = \frac{25{,}000 \times 0.80}{100} \times 0.85 \times 30 = 200 \times 0.85 \times 30 \approx 5,100
    25,000floor plate, square feet
    0.80usable share after the core
    100square feet per seat, aisles and meeting rooms included
    0.85share of seats occupied
    30floors
    What it says in wordsSeats on one floor, times the share filled, times the number of floors.
    Build one floor, then multiply by 3030 floorsone floor shownCore: lifts, stairs,services, 20%One floor: 25,000 sq ft plateUsable, 80%20,000 sq ftAt 100 sq ft a seat200 seats85% occupied170 peopleoccupied seat170 people a floor x 30 floors = about 5,100 peoplePale squares are empty seats: leave, travel, vacancies. Floor plate and seat size are assumptions to state.
    One 25,000 square foot floor, 80% usable at 100 square feet a seat, holds 200 seats, of which 170 are occupied. Thirty such floors put about 5,100 people in the tower.

    Which assumption would you flex, and how would you check it?

    Seat density moves the answer most. A dense back office runs near 100 square feet a seat; a bank's front office with larger desks and more meeting rooms nearer 150. At 150 square feet a seat the tower holds about 3,400 people, so give the answer as about 3,400 to 5,100. Then trim for floors that hold no desks: if the lobby and a plant floor take two of the thirty, the dense case falls to about 4,760. A check from another side: assume a dozen lifts, each carrying about 15 people a trip on a round trip of about 3 minutes. That moves about 12 x 15 x 20, or 3,600 people an hour, which fits a building of a few thousand people arriving over a morning, not tens of thousands.

    Where candidates lose it

    The common loss is guessing the whole building in one leap, which gives answers anywhere from 1,000 to 50,000 with nothing to defend. One floor, four assumptions, then multiply.

    The second loss is forgetting the core and the empty desks, which overstates the answer by a third. Saying both out loud is what the interviewer marks.

    What the interviewer asks next

    • How many lifts does the tower need to get everyone in between 8.30 and 9.30?
    • With hybrid work, how many people are in the building on a typical Friday?
    • What would you charge a coffee chain for the ground floor, and how would you justify it?

    Asked at HSBC, Sales and Trading, Central, 2026 (Wall Street Oasis): How many employees in London hsbc building How many taxis are in HK central