A day has 86,400 seconds. Pretend it has 100,000.

transcript

A day has 86,400 seconds. In an interview, pretend it has 100,000. That one lie does most of your estimating for you.

A day: round 86,400 up to 100,000. A month: about 2 point 5 million seconds. A year: about 30 million.

Now the trick. Anything per day, divided by 100,000, is per second. 100 million a day? 1,000 a second. A billion a day? 10,000 a second. 50 million a day? About 500. The exact answer is about 580. Close enough.

Try it live. 200 million daily users, each viewing 20 tweets. 4 billion reads a day. Drop five zeros: 40,000 a second. With the exact day, about 46,000. Add writes, double it for the peak. Design for about 100,000 a second.

But here's the catch. The trick only works if you count zeros right. 50 million a day and you got 5,000? You divided by 10,000. Ten times off.

Never say 46,296 in an interview. Say about 46,000. Then move on. Round the day. Keep the zeros.

Full breakdown in the lecture.