Compound Interest Calculator
The Compound Interest Calculator projects how savings grow over time, factoring in compound interest and any regular contributions you plan to make, so you can see the actual trajectory of a savings goal instead of estimating it by rough mental math that tends to badly underestimate how compounding works.
Compound interest is one of those concepts everyone’s heard is powerful but few people have actually seen graphed against their own numbers. The gap between simple and compound growth widens dramatically over long time horizons — a savings account or investment growing at the same rate looks nearly identical to simple interest over one year and radically different over twenty. Seeing your specific principal, rate, and contribution schedule projected out removes the guesswork and shows the real number a goal will land on.
To use it, enter your starting balance, expected annual interest or return rate, how often it compounds, and any regular contributions you plan to add (monthly, for instance). The tool projects the balance forward over your chosen time horizon, showing the growth curve and the split between what you contributed versus what interest actually earned. Adjust any input to immediately see how it changes the projection — a higher contribution, a longer horizon, or a different rate.
This works for modeling a straightforward savings account just as well as a retirement account or investment projection at a given assumed return rate — the math is the same regardless of what you’re actually saving toward. If retirement specifically is what you’re projecting, the Retirement Savings Projector adds age and withdrawal-phase modeling on top of this same compounding math.
FAQ
What return rate should I use if I’m not sure what to expect?
That depends heavily on what you’re actually invested in — a high-yield savings account, a bond fund, and a stock index fund have very different typical rates. Use a conservative estimate for the account type you’re modeling rather than an optimistic best-case number, since overestimating the rate is the easiest way to end up with an inflated projection.
Does this account for taxes on interest or investment gains?
No — this projects gross growth based on the rate and contributions you enter, without factoring in taxes, which vary by account type (a taxable account versus a tax-advantaged one) and jurisdiction. Keep that in mind when comparing the projection to what you’d actually keep.
How does contribution frequency affect the projection?
More frequent contributions (monthly versus annually, for instance) compound slightly more effectively over time since money is earning interest sooner — the calculator reflects this difference if you adjust contribution frequency.
Is this a guarantee of what my savings will actually grow to?
No — this is a projection based on the rate you enter, which is an assumption, not a guaranteed return. Actual investment returns vary and aren’t guaranteed, especially for anything invested in markets rather than a fixed-rate account.