Answer & explanation
The official answer, with Fermi’s reasoning step by step.
Answer & explanation
Official answer
10,400
keys
How to estimate it
Fermi’s official “Napkin math”
Tower Bridge crosses the Thames river and is famously photographed, so hopefully you have some visual reference. The river might be two to two and a half football fields/soccer pitches, maybe 750 feet. A piano key is around an inch (I played the piano and can cover an octave with my thumb to pinky so I can use that as an additional reference). Now we just do some division.
- River width
- ~750 ft
- In inches
- 750 × 12 = 9,000
- About 1 key per inch
- ≈ 9,000
Tower Bridge crosses the Thames river and is famously photographed, so hopefully you have some visual reference. The river might be two to two and a half soccer pitches, maybe 250 meters. A piano key is around 2.5 cm (I played the piano and can cover an octave with my thumb to pinky so I can use that as an additional reference). Now we just do some division.
- River width
- ~250 m
- Keys per meter
- ~40
- 250 × 40
- ≈ 10,000
Rounded assumptions may give a slightly different estimate. The official answer above is the game’s reference value.
Source calculation & reference
- Tower Bridge, end to end
- = 244 m (800 ft)
- One white key (165 mm octave ÷ 7)
- ≈ 23.6 mm
- 244,000 ÷ 23.6
- ≈ 10,400
Reference cited by Fermi: PBS Building Big · standard piano octave
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