About this calculator
This calculator is for education only and does not constitute investment advice. It shows what happens when a starting amount plus a fixed monthly contribution grows at one assumed annual return, compounded monthly — before taxes and fees, which real accounts also charge.
The yearly table separates what you put in from what compounding added, so the curve is honest: early years look slow, later years do most of the work. The inflation-adjusted line answers the other question that matters — what the final balance buys in today's money.
How to use it
- Enter your starting amount (0 if you are starting from scratch).
- Enter the monthly contribution you can sustain, and the annual return you want to assume.
- Set the years and an inflation rate for the purchasing-power line.
- Read the final balance, then check the table: contributions vs growth shows how much of the result is compounding.
The formula
Each month the balance earns r = annual return / 12, then the monthly contribution is added at month end. The yearly table groups twelve such months: growth in a year is the end balance minus the start balance minus that year's contributions. The inflation-adjusted value divides the final balance by (1 + inflation rate) raised to the number of years — a constant-rate illustration, not a prediction of future inflation. Taxes, fees and return variability are not modeled.
Worked example
Start with $10,000, add $500 a month, assume 7% a year for 10 years at 3% inflation: after year 1 the balance is $16,919.19; after 10 years it is $106,639.02. You put in $70,000 ($10,000 start + $60,000 contributions), so the gain is $36,639.02 — worth $79,349.44 in today's purchasing power.
Sources: Standard future-value formula with monthly compounding; contributions assumed at each month end. Inflation adjustment uses a constant assumed annual rate. Past returns do not guarantee future results. Last updated: October 2026.
Frequently asked questions
Why does growth look slow at first?
Compounding is back-loaded: in year 1 the 7% applies to about $13,000 on average, but by year 10 it applies to nearly $100,000. Same rate, much bigger base — that is why the growth column rises every year even though the contribution never changes.
Monthly or annual compounding — does it matter?
A little. Monthly compounding credits growth twelve times a year instead of once, so the same 7% headline ends slightly higher than annual compounding. The difference over 10 years on this example is a few hundred dollars, not a different answer.
Does this include taxes or fees?
No. Real accounts charge fees and taxable accounts owe tax on gains and distributions, both of which reduce the result. Treat the output as a pre-tax, pre-fee illustration.
Is the inflation-adjusted value a prediction?
No. It only re-expresses the final balance at one constant assumed inflation rate, so you can compare it with today's prices. Actual inflation will differ year to year.
What formula does this compound interest calculator use?
Balance after n months = P × (1 + r)ⁿ + C × (((1 + r)ⁿ − 1) / r), with r = annual return / 12; real value = balance / (1 + inflation)ʸᵉᵃʳˢ Each month the balance earns r = annual return / 12, then the monthly contribution is added at month end. The yearly table groups twelve such months: growth in a year is the end balance minus the start balance minus that year's contributions. The inflation-adjusted value divides the final balance by (1 + inflation rate) raised to the number of years — a constant-rate illustration, not a prediction of future inflation. Taxes, fees and return variability are not modeled. The worked example on this page applies the formula step by step with real numbers, so you can verify the result by hand.