How many windows are there in New York City?
Estimate about 28 million windows, within 25 to 35 million, one per frame, excluding vehicles. Two million apartments at 6 and 1 million houses at 12 give 24 million, plus 4 million for workplaces at one per worker. A building count gives 26 million.
Updated
How a 90-point answer starts
First, what counts? I'll count windows in buildings, one per frame, and leave out cars, buses and subway trains.
See the full answer, the 50 and 70 versions, and what moves each score up.
Practice this question freeThe numbers, step by step
The numbers assume about 3 million households, split into apartments and houses, at least 4 million people working in the city, and roughly a million buildings for the check.
- Apartment windows2 million apartments × 6
- 12 million
- House windows1 million houses × 12
- 12 million
- Windows at home12 million + 12 million
- 24 million
- Workplaces and other buildings4 million workers × 1
- 4 million
- Total windows24 million + 4 million
- 28 million
- Check: small buildings900,000 buildings × 12 windows
- about 11 million
- Check: windows on a six-story building6 floors × 24 windows (12 front, 12 back)
- 144
- Check: bigger buildings100,000 buildings × about 150 windows
- 15 million
- Check: all buildings11 million + 15 million
- 26 million
What the interviewer listens for
- Do you say what counts as a window before you count?
- Do you split buildings into types and not use one average?
- Can you justify windows per home by walking through the rooms?
- Do you remember buildings that aren't homes?
- Do you check the total from a second direction and name the assumption that could move it most?
Common mistakes
- Picturing skyscrapers.Ordinary apartments and houses, home to 8 million people, hold most of the windows.
- Using one figure for every home.A row house with a front and back on three floors has about twice the windows of an apartment with one outside wall.
- Offices, shops, schools, hospitals and hotels are easy to forget, and leaving them out passes off the homes figure as the whole city.
- Is a glass curtain wall one window or hundreds of panes?Settle it first, because it moves the workplace number more than any other input.
Likely follow-ups
- Suppose apartments average 5 windows. What's the new total?About 26 million. Apartments drop from 12 million to 10 million, and houses and workplaces stay at 16 million.
- Every pane on a glass tower counts as a window. Now what?The workplace line could double to 8 million or more, putting the total near 32 million, still inside my 25 to 35 million range.
- Should cars, buses and subway cars count?Only if the interviewer wants them. About 2 million cars at six windows each would add 12 million, so it's worth settling before counting anything.
“Market sizing got quicker once I stopped debating every assumption. The Soreno drills helped me pick reasonable numbers and get on with the estimate.”
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