Enter a starting amount, an optional monthly contribution, an interest rate, and a number of years, and you’ll see the future value, how much of it is your own money, and how much is interest — with a year-by-year table showing exactly when compounding takes over. The formula behind every number is published below, with sources.
How to use this calculator
- Starting amount — the lump sum you begin with (zero is fine if you’re starting from scratch).
- Monthly contribution — what you’ll add every month; even small amounts matter over long periods.
- Annual interest rate — your savings APY or an assumed investment return.
- Years — how long the money stays put.
The result updates as you type and the URL captures your scenario, so you can share a projection or save it for later. Interest is compounded monthly, with contributions added at each month’s end.
Time in the market: starting at 25 vs 35
Compound growth is brutally unfair to late starters, and the table makes it visible. Take $200 a month at a 7% annual return. Started at age 25 and left until 65, it grows to roughly $525,000 on about $96,000 of contributions. Started at 35, the same $200 a month reaches only about $244,000 on $72,000 contributed. The decade of delay cost $24,000 in skipped deposits but roughly $280,000 in final value — because the earliest deposits compound for the full 40 years, and those are precisely the ones the late starter never made. Scroll the year-by-year table and you’ll see the pattern: in the first years your contributions dwarf the interest, but somewhere in the second decade the interest column starts outrunning the money you add. A handy shortcut is the Rule of 72: divide 72 by the rate to estimate the years to double. At 7% that’s about a decade — a 25-year-old’s first deposits double four times before 65, while a 35-year-old’s manage only three, and that missing doubling is the whole gap. Waiting for a “better time to invest” usually costs more than any fee ever will.
Reading the year-by-year table
Each row splits the balance into contributed (your money) and interest (growth). Two things are worth watching. First, the crossover year — when cumulative interest exceeds cumulative contributions — is the point where your money is doing more work than you are; higher rates and longer horizons pull it earlier. Second, the last few rows usually add more dollars than the first ten combined, which is why raiding a long-term account early is so expensive. This same mechanism powers workplace retirement plans — our 401(k) calculator adds employer matching on top — and it’s the engine behind the 25× rule in our retirement calculator.
Limits of this projection
The model assumes one fixed rate for the whole period. Real savings rates change and real market returns arrive unevenly, so actual balances will wander around the smooth curve shown here. It also ignores taxes on interest or gains, investment fees, and inflation — $525,000 in 40 years will not buy what it does today. For account-specific quotes, the compounding convention matters too: daily compounding yields a whisker more than monthly at the same nominal rate. Use the output to compare scenarios and habits, not as a guaranteed future balance.
The math runs entirely in your browser — nothing you enter is sent anywhere. Browse all our calculators with published formulas, or try our file-based tools, which are just as private.