Use this free compound interest and growth calculator to see how a starting amount plus regular contributions can grow over time — and how much of the result is growth versus what you put in.
How the compound growth calculator works
Enter a starting amount, a regular contribution, an expected annual return, and a time horizon. The tool compounds your balance each period and adds your contributions to project a future value.
The compound growth formula
Future value = P × (1 + i)ⁿ + C × [((1 + i)ⁿ − 1) ÷ i], where P is your starting amount, C is your contribution per period, i is the annual return divided by periods per year, and n is the number of periods. Results are nominal and before inflation, fees, and taxes.
A worked example
Start with $5,000 and add $200 every month for 20 years, at an assumed 7% annual return. The starting amount alone grows to roughly $19,300 over 20 years at 7%. The monthly contributions add up to $48,000 in deposits, but because each one starts compounding as soon as it lands, they grow to roughly $104,600 by the end. Total ending balance: about $123,900 — against $53,000 actually put in. That gap, roughly $70,900, is what compounding contributed, not new money out of pocket.
How to read the results
The calculator splits the ending balance into two parts: total contributed (starting amount plus every deposit added along the way) and total growth (everything above that, generated by returns compounding on themselves). Watch how the growth share changes with time horizon — in the first few years contributions dominate the balance, but the growth share accelerates the longer money stays invested, because later years are compounding on a much larger base than early ones.
Why the assumed return rate matters more than it looks
Small changes to the annual return assumption compound into large differences over long horizons. Moving the assumption from 7% to 9% on a 30-year projection doesn’t add roughly 2 years of extra growth — it can add tens of thousands of dollars to the ending balance, because the higher rate compounds on itself every single year. This cuts both ways: an optimistic assumption can make a plan look far more comfortable than it will actually be. Use a conservative, long-run average rather than a recent strong year, and re-run the numbers at a lower rate to see how sensitive the plan is.
Common mistakes with compound growth projections
The first is treating the output as a forecast rather than a projection. This tool applies a fixed annual rate every year; real markets deliver a sequence of very different up and down years that average out to something similar only over long periods — see drawdown explained for what that path can actually look like along the way.
The second is ignoring inflation and fees, both of which quietly erode the number shown here — see the FAQ below for how to account for them. The third is stopping contributions the moment growth “looks good enough,” which throws away exactly the years where compounding is doing the most work, since the largest dollar gains happen on the largest account balances, which only exist late in the timeline.
Frequently asked questions
Is the return guaranteed?
No. A constant annual return is modeled; real returns vary and can be negative.
Does it account for inflation or fees?
No — the figure is nominal and before fees and taxes, which reduce real growth.
How often does it compound?
Monthly or annually, matching the contribution frequency you choose.
What contribution frequency should I use?
Monthly contributions timed to a paycheck are the most common and easiest to sustain, but the calculator works the same way for weekly, quarterly, or annual deposits — what matters for the total is the total amount contributed per year, not how finely it is split. Splitting a fixed annual amount into smaller, more frequent deposits gives a very small edge, since each dollar starts compounding slightly sooner on average, but the difference is minor compared to simply contributing consistently and not skipping periods.
Educational tool, not financial advice. Projections are illustrative and not a forecast.
Where to invest: brokers with fractional shares
To act on what this tool shows you, you’ll need a brokerage account. These are established brokers we use that support fractional shares — buying a slice of a share by dollar amount, which makes dollar-cost averaging and diversifying with small amounts easy:
- Robinhood — commission-free investing with fractional shares from $1, and the only major broker offering a daily recurring-investment schedule. Recommended for most beginner-to-intermediate investors who want the simplest way to automate investing — see how to automate your investing for the full setup.
- Charles Schwab — fractional shares of most U.S.-listed stocks and ETFs from $1, commission-free online.
- Fidelity — “Stocks by the Slice” fractional investing from $1, with strong research and low-cost index funds.
Referral disclosure: the broker links above are referral links. If you open and fund an account through them we may receive a referral reward at no cost to you; it never affects our tools or conclusions. This is educational information, not investment advice — choose the broker that best fits your needs.