How this calculator works
This calculator projects an investment or savings balance forward with a starting amount, optional monthly deposits, and your choice of compounding frequency. The year-by-year table shows exactly how much of the final balance is your own money and how much is interest.
Deposits are added at the end of each month, which is how most savings plans and brokerage auto-invest schedules work. The compounding setting controls how often earned interest starts earning interest itself — daily compounding at the same nominal rate always ends slightly higher than annual.
The formula
Future value of a lump sum: FV = P × (1 + r/n)^(n·t)With monthly deposits (monthly compounding): FV = P(1+i)^m + PMT × ((1+i)^m − 1) ÷ i, where i = r/12 and m = monthsP is the starting amount, r the annual rate as a decimal, n compounding periods per year, t years. When compounding is not monthly, this calculator simulates month by month with an equivalent monthly growth factor, so deposits and compounding combine correctly.
Worked example: $10,000 + $200/month at 7%
- Start with $10,000, add $200 at the end of every month, 7% annual rate compounded monthly, for 10 years.
- The lump sum grows to 10,000 × (1 + 0.07/12)^120 ≈ $20,097.
- The deposits grow to 200 × ((1 + 0.07/12)^120 − 1) ÷ (0.07/12) ≈ $34,617.
- Total ≈ $54,714 — of which $34,000 is money you put in and about $20,700 is interest.
Frequently asked questions
What compounding frequency should I choose?
Match your account. Most savings accounts compound daily or monthly; many bonds annually. If you are not sure, monthly is the common default and the difference from daily is small at typical rates.
Does it matter if deposits happen at the start or end of the month?
Slightly. Start-of-month deposits earn one extra month of interest each. This calculator uses end-of-month deposits, the standard assumption for salary-based saving.
What about inflation?
Results are in nominal dollars. To think in today's purchasing power, use a real rate: roughly your expected return minus expected inflation (7% return with 3% inflation ≈ 4% real).
Is the rule of 72 accurate?
It is a good quick check: money doubles in roughly 72 ÷ rate years. At 7% that is about 10.3 years; the exact answer with monthly compounding is 9.9 years.
Can I calculate a lump sum alone, with no monthly deposits?
Yes — enter $0 for the monthly contribution and the calculator uses only the lump-sum formula, FV = P × (1 + r/n)^(n·t). In the worked example, that formula alone grows $10,000 at 7% compounded monthly to about $20,097 over 10 years, without the additional roughly $34,617 that comes from also adding $200 every month.