Intermittent Fasting Calculator
- Free Calculator Site
- $400 monthly ad revenue
A solo founder in the US built a tool that does one thing… tell you when to eat.
You enter your last meal time, pick a fasting method (16:8, OMAD, whatever), and the calculator instantly shows your fasting window and next meal time. No signup, no app to download.
Earnings via Google ads… apparently $400/month, mostly passive 
Costs are basically a domain and cheap hosting, so it’s nearly all profit.
The clever part isn’t the calculator. It’s the cluster. The site has quietly stacked ~10 related tools – BMR, body fat, a fasting timer, printable templates – each catching its own slice of search traffic 
It’s a proven play: build one genuinely useful tool around something people Google, slap display ads on it, then add more tools off the same audience.
But now you can vibe-code something like this in a weekend. Pick any number people search for – mortgage payments, due dates, sleep cycles, freelance rates – and build the calculator for it 