What this calculator does
Compound interest is the engine of long-term investing: returns earn returns, and the effect accelerates the longer you stay invested. A single extra percentage point of return can change the outcome by tens of thousands over decades.
This calculator projects a starting balance plus monthly contributions, with an optional inflation-adjusted view and tax drag. Use it to compare savings accounts, index funds and retirement accounts on the same basis.
Future value with contributions
FV = P × (1 + r/n)^(nt) + PMT × [((1 + r/n)^(nt) − 1) ÷ (r/n)] P is the starting balance, PMT the monthly contribution, r the annual return and n the compounding frequency. The calculator compounds monthly and adds contributions at the end of each month.
How this calculator works
- 01
Set a starting balance
Enter what you already have invested, or zero if you are starting fresh.
- 02
Add monthly contributions
Regular contributions matter more than timing. The calculator adds them every month and compounds the balance.
- 03
Compare inflation and taxes
Toggle inflation to see purchasing power, and add a tax rate to estimate the after-tax result.
Practical tips
- Increase contributions by 1% each year to accelerate growth without a noticeable budget hit.
- Keep fees low: a 1% annual fee can consume a quarter of your lifetime returns.
Frequently asked questions
What return should I assume?
A diversified global equity portfolio has historically returned roughly 6–8% nominal per year over long periods, but with large swings. Many planners use 5–7% for projections. Past returns do not guarantee future results.
How does inflation affect the result?
Inflation reduces purchasing power. A nominal balance of $500,000 in 30 years may buy far less than today. The inflation-adjusted view discounts the future balance by the assumed inflation rate.
Should I include taxes?
Tax treatment varies by account and country. If your gains are taxed annually, include an effective tax rate. Retirement accounts may defer or eliminate tax, which changes the comparison significantly.
Data basis: 2026 · Annual constants live in structured JSON files and are updated once a year.
Results are unverified model calculations and do not replace tax, legal or financial advice.