Use the result as a scenario, not a forecast. A steady annual return makes the effect of time easier to compare, but real returns usually vary from year to year.
What compound interest means
Compound interest is growth earned on both the original balance and earlier growth. As the balance becomes larger, the same percentage rate applies to a larger base. That is why time can have a strong effect on long-term projections.
Recurring contributions add a second engine: money is added throughout the period, and each contribution has a different amount of time to compound.
How to read your result
The future-value figure combines three parts: the amount you started with, all monthly contributions, and estimated growth. The contribution-versus-growth bar makes that split visible.
If you are comparing scenarios, change one input at a time. That makes it easier to see whether saving more, waiting longer, or using a different return assumption has the greatest effect.
- Use a conservative return assumption for planning.
- Compare the estimate with a 0% return to isolate the effect of saving.
- Remember that inflation affects what the future balance can buy.
Important limits
The calculator assumes a constant annual rate and monthly compounding. Markets and many savings products do not produce a perfectly steady return. The estimate also excludes fees, taxes, inflation, and withdrawals.
A higher entered rate will always produce a larger mathematical result; it does not make that rate more achievable. Use the tool to compare assumptions, not to select an investment.
How this estimate is calculated
FV = P(1 + r/12)^(12t) + PMT × [((1 + r/12)^(12t) − 1) ÷ (r/12)]FV is future value, P is the starting amount, r is the annual decimal return, t is years, and PMT is the end-of-month contribution. When the rate is 0%, the calculator simply adds all contributions.
Example: a ten-year savings plan
Start with $10,000, add $250 at the end of every month, and assume a 6% annual return compounded monthly. Over 10 years, you contribute $40,000 in total. The calculator estimates a future value of about $59,164—roughly $19,164 of estimated growth. Fees, tax, inflation, and fluctuating returns would change the real outcome.
Sources behind this calculation
These primary sources support the formula and help you verify rules or terms that may change.
Investor.gov — Compound Interest Calculator↗Investor.gov — What is compound interest?↗