How to calculate investment
Monthly growth = (1 + annual rate / compound periods)^(compound periods / 12) − 1. Contributions are added at the end of each monthly or annual period. With inflation on, each year's balance and deposits are converted to today's dollars using the assumed price growth.
Worked example. In 20 years you'll have: $300,851.
Example inputs — Starting amount: $10,000; Contribution amount: $500; Contribute: Monthly; Years to grow: 20 years; Interest rate: 7%; Compound frequency: Monthly; Adjust for inflation?: No — show future dollars. Starting values are illustrative.
From George Guju, founder of Calculyn · Educational calculator guidance, not professional advice.
How it works
Your money grows from three sources shown in the chart: your starting amount, your regular contributions, and the interest earned on all of it. Interest compounds at the frequency you choose — the more often it compounds, the faster the balance grows. The year-by-year graph shows how growth starts slowly and then snowballs as interest earns its own interest.
Using the investment estimate
An investment projection asks what a starting balance and a repeatable saving habit could become under a chosen return assumption. It does not forecast the stock market. Begin with money you already have invested, add the contribution you plan to make, and choose how many years the money can remain invested. The result is most useful when you compare several realistic scenarios instead of choosing the largest possible return.
Step-by-step inputs
- Starting amount. Replace the starting value with your own, keeping the displayed unit and period. Starting example: $10,000 .
- Contribution amount. Replace the starting value with your own, keeping the displayed unit and period. Starting example: $500 .
- Contribute. Replace the starting value with your own, keeping the displayed unit and period. Starting example: Monthly .
- Years to grow. Replace the starting value with your own, keeping the displayed unit and period. Starting example: 20 years.
- Interest rate. Replace the starting value with your own, keeping the displayed unit and period. Starting example: 7% .
- Compound frequency. Replace the starting value with your own, keeping the displayed unit and period. Starting example: Monthly .
- Adjust for inflation?. Replace the starting value with your own, keeping the displayed unit and period. Starting example: No — show future dollars .
Worked example with the starting values
Using the starting inputs, the estimate is In 20 years you'll have: $300,851 .
- Starting amount
- $10,000
- Total contributions
- $120,000
- Total interest earned
- $170,851
- Growth rate (CAGR)
- 18.6%
Compare interest rate scenarios
All other inputs stay at their starting values. These are modeled comparisons, not quotes, predictions or individual recommendations.
| Interest rate | In 20 years you'll have |
|---|---|
| 3% | $182,359 |
| 5% | $232,643 |
| 7% | $300,851 |
Make a useful comparison
The contribution frequency changes the total deposited. A monthly contribution of $500 means $6,000 over a year, while an annual contribution of $500 means $500. Keep that distinction in mind when comparing two projections. Contributions in this model arrive at the end of their monthly or annual period, rather than all arriving on day one.
The interest-rate input is a nominal annual assumption. The compounding selection converts it into the modeled monthly growth. Daily compounding and monthly contributions are different ideas: one describes how growth accumulates, and the other describes when you add money. Neither setting means a real investment produces steady gains every day.
Switch inflation on to make the large headline a purchasing-power estimate in today's dollars. The starting amount, contribution breakdown and year-by-year graph follow the same view. Switch it off to see future dollar amounts. Compare like with like rather than subtracting a today's-dollar result from a future-dollar target.
Common mistakes to avoid
- A smooth upward graph is a mathematical scenario, not a promise. Actual investments can fall in value, and the timing of gains and losses matters when you add or withdraw money.
- This simplified calculator does not separately subtract management fees, investment taxes or withdrawals. If those matter, use a more conservative assumption or model them separately. Do not count a gross return as a guaranteed spendable balance.
- The interest portion is the projected ending balance minus the modeled starting money and contributions in the selected dollar view. It is not a distribution you can necessarily withdraw without taxes, nor is it a historical return measurement.
Assumptions & limitations
- A constant rate of return or inflation every year.
- Excludes fees and taxes unless entered.
- Contributions are made regularly and on time.
Formula and assumptions reviewed October 2026 by the Calculyn team. How we build our calculators
Frequently asked questions
What does compound frequency mean?
How often earned interest is added to your balance so it starts earning interest itself. Monthly compounding grows slightly faster than annual compounding at the same rate.
What return should I assume?
Use a range of illustrative returns appropriate to your scenario rather than treating the default as a forecast. Enter a nominal return if you also use the inflation adjustment, so inflation is not subtracted twice. Actual investment returns are uncertain.
Should I adjust for inflation?
Choose Yes to show the headline result, chart and breakdown in today's purchasing power. A dollar in 20 years buys less than a dollar today, so the inflation-adjusted figure is the more realistic one for planning.
What is CAGR?
Compound annual growth rate — the single steady yearly rate that would take your starting amount to your ending amount. It smooths out ups and downs so you can compare investments fairly.
What do the two investment graphs show?
The breakdown separates starting money, contributions and projected growth. The year-by-year graph compares the modeled balance with the money paid in, using the same inflation setting as the headline.
Should I choose the highest return to set my savings goal?
No. Try a range of assumptions and focus on the contributions you can sustain. A higher assumed return is not evidence that the result is achievable.