Investment Calculator With Compound Interest
Estimate how an investment could grow when you combine a starting balance, regular contributions, an annual return assumption and compounding frequency. Optionally increase the monthly contribution each year and compare the result with keeping deposits level.
Fixed-contribution comparison: $—. Extra money deposited with annual increases: $—. Additional modelled growth beyond those deposits: $—.
| Year | Balance | Contributed | Growth |
|---|
When to use this investment calculator
Use this page for broad investing scenarios where contributions happen over time. If you only want to model a single deposit, use the lump sum investment calculator. If you need to hit a target, use the Monthly amount to reach a goal.
Formula logic
Where the money in a 20-year projection comes from
Using this page's inputs — $10,000 to start, $300 a month, 7% compounded monthly, 20 years — the projection ends at about $196,665. It is worth breaking that into its three sources, because they behave completely differently.
| Source | Amount | Share of final balance |
|---|---|---|
| Your starting balance | $10,000 | 5% |
| Your monthly contributions | $72,000 | 37% |
| Investment growth | $114,665 | 58% |
| Total | $196,665 | 100% |
Growth supplies about 58% of the outcome over twenty years. Extend the same plan to thirty and growth's share rises to roughly 74%. The longer the horizon, the less of the result you have to fund yourself — which is the entire reason timelines matter more than deposit sizes.
The $10,000 you begin with is 12% of everything you contribute, but it grows to $40,387 — 21% of the final balance. Every dollar of it is invested for the full twenty years, while the average monthly deposit is invested for about ten.
What this calculator deliberately does not model
- Variable returns. A constant 7% is a planning convenience. The order of returns matters when money is added or withdrawn. Constant-return projections do not capture that risk.
- Contribution changes beyond a fixed annual percentage. This page models an optional annual increase. Irregular bonuses, pauses and affordability changes are not included.
- Fees. This projection excludes charges. Use the fee comparison for an explicit monthly fee model; subtracting a percentage from the return is only an approximation.
- Tax. Depends on account type and country, and can be the largest single adjustment of all.
- Inflation. $196,665 in twenty years buys roughly what $120,019 buys today at 2.5%.
None of these make a projection useless. They make it a scenario rather than a forecast, which is the correct way to read any of them.
Questions about investment projections
How much will my investment be worth?
With $10,000 invested, $300 a month and a 7% return compounded monthly, about $196,665 after 20 years and $447,156 after thirty.
What return should I use for an investment calculator?
Something you would still be comfortable with in a poor decade. 7% before inflation is a common long-run equity assumption; 5% is the conservative version worth testing alongside it.
Should I include my pension or 401(k)?
Include everything genuinely invested for the same goal, including employer contributions. Employer matching is a return on your deposit before any market growth, and leaving it out understates the projection.
Why does this differ from my broker's projection?
Usually fees, contribution timing or compounding frequency. Providers often project net of their own charges and may compound annually. Neither convention is wrong, but they are not comparable without checking.
Compare the cost of investing
The investment fee calculator separates direct charges from the growth difference and compares two fee levels on the same monthly saving plan.
Deposit schedule
The monthly budget is grouped into equal deposits per selected compounding period. Choose monthly compounding for a monthly deposit schedule. A final partial period earns growth but receives no deposit. See the exact model and verification method.
How annual contribution increases work
The first year uses your entered monthly amount. At the start of year two it rises by the selected percentage, then rises again at each anniversary. A 5% increase turns 300 per month into 315 in year two and 330.75 in year three. The increased balance includes extra money you deposited; the comparison separates that money from additional growth.
The general calculator uses a nominal annual return and groups the monthly budget into deposits at each selected compounding boundary. Monthly compounding models monthly deposits; other frequencies use the grouped-budget approximation. A final partial period earns growth without a deposit. Annual contribution increases are supported for timelines up to 100 years. Tax, fees, inflation and variable market returns are excluded.
To compare costs, use the investment fee calculator. To measure an annualised rate between two values without intervening deposits, use the CAGR calculator.
Contribution comparison added September 27, 2026.
Measure a past return rather than assume a future rate
If you already know your ending balance and deposited money during the period, use the investment return calculator with contributions. The CAGR versus XIRR comparison explains why account growth can include new savings.