How Many URLs Fit in Six Characters
Your URL shortener uses codes drawn from [a-z A-Z 0-9] — 62
characters.
How many unique URLs can a 6-character code hold, and at 1,000 new URLs per second, how long until the keyspace runs out?
Show the arithmetic, and say whether you'd launch with 6 or 7 characters.
C) ~57 billion — which sounds infinite and runs out in under two years.
62⁶ = 62 × 62 × … × 62 = 56.8 billion combinations.
At 1,000 new URLs/second:
56.8 × 10⁹ ÷ 1,000/s ≈ 5.7 × 10⁷ seconds ≈ 657 days ≈ 1.8 years.
And that's the theoretical ceiling — random generation needs the keyspace to stay mostly empty to avoid collision retries, so the practical limit arrives even sooner. One more character fixes it: 62⁷ ≈ 3.5 trillion, or ~110 years at the same rate. That's why real shorteners standardized on 7.
The rule of thumb to say out loud: each base62 character multiplies capacity by 62 — 6 chars ≈ 57B, 7 chars ≈ 3.5T — so size the code to write rate × lifetime, then add one.
Worth naming in an interview: the counter-vs-random tradeoff (a global counter uses the keyspace densely but leaks creation order; random codes don't leak but collide as density grows), and that checking "does this code exist" is why generation strategy and keyspace sizing are the same conversation.
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