Compound Interest Calculator (with Sensitivity)
Project the future value of an initial amount plus monthly contributions at a given return, with a sensitivity table showing how the rate changes the outcome. Runs in your browser.
Sensitivity to return rate
| Return | Ending value |
|---|---|
| 4% | $284,202 |
| 6% | $391,147 |
| 7% | $462,290 |
| 8% | $548,915 |
| 10% | $783,986 |
Future value with monthly compounding. The sensitivity table shows how much the outcome swings with the return assumption — a reminder that projections are estimates. Returns are not guaranteed. Informational, not financial advice.
About this tool
Compound growth is the engine of long-term investing, and this calculator shows where a plan of regular contributions could end up. It compounds an initial lump sum and a stream of monthly contributions monthly at the return you assume, and splits the result into what you put in versus what growth added. Its distinctive feature is the sensitivity table: because the return is an assumption, not a fact, the tool recomputes the ending value across a range of rates around your estimate so you can see how much the outcome depends on it — often a difference of hundreds of thousands of dollars over decades. That is the honest way to read any compound projection: as a range of plausible outcomes, not a single guaranteed number. Use it to set savings targets, compare contribution levels, and stress-test optimistic return assumptions. Markets are volatile and returns are never guaranteed. It is informational, not financial advice. Everything runs in your browser.
How to use it
- Enter an initial amount and your monthly contribution.
- Set an expected annual return and the number of years.
- Read the ending value and the contributed-vs-growth split.
- Check the sensitivity table to see how much the return assumption matters.
Frequently asked questions
- How is the future value calculated?
- The initial amount compounds as principal × (1 + monthly rate)^months, and the monthly contributions compound as an annuity. Their sum is the ending value. Monthly compounding closely matches how most investment accounts grow.
- Why include a sensitivity table?
- Because the return rate is the biggest unknown and small changes compound into large differences. Seeing the ending value at, say, 4% vs 7% vs 10% reframes the projection as a realistic range rather than a single false-precise figure.
- What return rate should I assume?
- It is your call. A diversified stock-heavy portfolio has historically averaged roughly 7% real (≈10% nominal) over long horizons, with big year-to-year swings. Use a conservative estimate and lean on the sensitivity table for the optimistic and pessimistic ends.
- Does this account for inflation or taxes?
- No — it projects nominal pre-tax dollars. For purchasing power, see the inflation-adjusted compound calculator; for taxes, the account type (Roth, traditional, taxable) matters and is not modeled here.
- Why does growth eventually dwarf contributions?
- Compounding: returns earn returns. Early on most of the balance is your contributions; over decades, accumulated growth can far exceed everything you put in. Starting earlier dramatically increases the growth share.
- Is this financial advice?
- No. It is an informational projection. For personalized planning, consult a financial advisor.