The moment your money out-earns you
If you deposit the same amount every month into an account growing at a steady rate, the monthly interest starts tiny and keeps growing. Eventually one month's interest is bigger than your deposit. At 7% a year (7% ÷ 12 each month), that happens in month 121, a little over 10 years in, whether you deposit $50 or $500.
Why the deposit size cancels
Double the deposit and you double the balance at every point, which doubles the interest too. The ratio between interest and deposit stays the same, so the crossover month doesn't move. Only the rate moves it: at 5% the crossover comes in month 168, at 10% in month 85.
A shortcut for the crossover balance
Interest beats the deposit once balance × (rate ÷ 12) is bigger than the deposit. Rearranged, the balance you need is 12 × deposit ÷ rate: a year of deposits divided by the rate. At $50 a month and 7%, that's $600 ÷ 0.07 ≈ $8,571. The simulated balance at the start of month 121 is $8,654, just past the line.
It's one turn of the doubling clock
Here's why. After n months of deposits, the balance is deposit × ((1 + i)^n − 1) ÷ i, where i is the monthly rate. That month's interest is the balance × i, which equals deposit × ((1 + i)^n − 1). It passes the deposit exactly when (1 + i)^n passes 2, which is the definition of one doubling time. At 7% compounded monthly, money doubles in about 119.2 months, so the first full month past that line is month 121.
What moves the month in real life
Real returns aren't steady, so the actual crossover month would shift around. Fees and taxes slow it; raising your deposit over time doesn't change the steady-rate crossover month but does raise every dollar figure. The model here starts at $0, adds the deposit at the end of each month and ignores inflation.
Try it
Move the rate slider and watch the crossover marker slide. Then change the deposit and notice the marker stays put while the dollar amounts scale.
Sources
Last reviewed: Sep 28, 2026