The most powerful number in personal finance isn’t a stock tip. It’s $27.40.

That’s what you get when you divide $10,000 by 365. One lunch out and a coffee. A couple of streaming subscriptions and a delivery fee. The kind of money that leaks out of a normal day without anyone noticing it’s gone.

A few months ago I built a little compound interest calculator to teach my 20-year-old sons about compound interest. I wanted them to see the curve instead of just hearing me talk about it. It worked well enough that I wanted to share it further. The thing that keeps grabbing me is how absurd the gap is between how small the daily number feels and how big the 20-year number gets.