I plugged in a scenario when I was unemployed last year, so had little money, and was making over 150K this year while self-employed. Its recommended payment was about 10% of what it really should have been, presumably because it was reporting the lowest amount I could get away with due to safe harbor rules, not the actual estimated amount for the quarter. If I actually paid what this site says, I'd end up with a massive tax bill at EOY.