Quick answer
This calculator scales compound growth: it shows how a starting amount plus monthly contributions multiplies over long horizons. $5,000 invested with $250 a month at a 7% annual return becomes about $150,425 in 20 years — $65,000 contributed and $85,425 of growth, more than everything paid in over the two decades.
How the Compound Interest Scaler works, step by step
Enter a starting amount, a monthly contribution, an annual return, a time horizon and a compounding frequency for the lump sum. Contributions are always modelled monthly, so each one starts earning from the month after it lands. The frequency choice changes the result by a little: at 7%, annual compounding turns a $5,000 lump sum into $19,348 over 20 years, while monthly compounding gives $20,194 — about $845 more from crediting interest twelve times a year instead of once.
The point of the tool is the scaling effect. In the first years, contributions do most of the work. Later, returns on the accumulated balance overtake contributions: in the example below, the first 10 years build about $53,320, and the second 10 add about $97,105, even though the deposits in each decade are identical at $30,000. The crossover arrives in the second decade: by year 17, growth of about $57,900 has passed total contributions of $56,000.
Test the horizon before you test the rate. Extending the example from 20 to 25 years adds about $80,720, taking the balance to roughly $231,145 — and the horizon is the one input you control completely. Raising the assumed return by a point looks similar on screen, but no investment guarantees the higher return.
Choose the return honestly. US stocks have returned roughly 10% a year before inflation over very long periods, but with deep falls along the way, and many planners model 6% to 8% for a diversified portfolio. This calculator assumes a smooth return; real returns arrive unevenly, and a bad sequence early on leaves a smaller base for later growth.
Remember the wrapper matters as much as the return. In a taxable account, dividends and realised gains are taxed as they occur, which drags the compounding rate down. Inside a 401(k) or IRA, the same return compounds untaxed until withdrawal — or never taxed, for a Roth. The calculator shows pre-tax growth either way.
The formula
Future value = P × (1 + r/n)^(n×t) + PMT × [((1 + r/12)^(12t) − 1) / (r/12)], with P the starting amount, PMT the monthly contribution, r the annual return, n the compounding periods per year for the lump sum and t the years.
Worked example — try it on the calculator above
Worked example: $5,000 and $250 a month at 7% for 20 years
Invest $5,000, add $250 a month, and earn a steady 7% a year for 20 years. Total contributions are $65,000. The final value is about $150,425, which means compounding added $85,425 — more than you put in. Stop after 10 years instead and the balance is only about $53,320, which is why the second decade matters so much: it contributes roughly $97,105 of growth on the same $30,000 of deposits. Extend the plan five more years, to 25, and the balance reaches about $231,145.
Frequently Asked Questions
What return should I assume for investments?
Use a cautious planning figure, not a best case. US stocks have returned roughly 10% a year before inflation over very long periods, but with deep falls along the way. Many planners model 6% to 8% for a diversified portfolio. This calculator assumes a smooth return; real returns arrive unevenly.
Why does the last decade add so much more than the first?
Because interest earns interest on a bigger base. In the worked example, the first 10 years build about $53,320; the next 10 add about $97,105, even though contributions are identical. The base, not the deposit, drives late growth — which is why starting early beats starting big.
Is compound interest taxed every year?
In a taxable brokerage account, dividends and realised gains are taxed as they occur. In a 401(k) or IRA, tax is deferred or, for a Roth account, not charged on qualified withdrawals — which is why the same return compounds faster inside a retirement account.
What is the difference between compounding monthly and annually?
Monthly compounding credits returns 12 times a year, so each credit starts earning sooner. At 7%, monthly compounding produces an effective annual yield of about 7.23%. The gap grows with the rate and the horizon, but it stays small next to the effect of the rate itself.
Estimates, not advice. This calculator provides educational estimates based on the inputs you enter and the assumptions shown on this page. It is not financial, tax or investment advice, and it cannot see your full situation. Verify important figures against official sources, and consider a qualified professional for decisions about your own money. See our Disclaimer and Methodology.