Compound Interest Visualizer
Initial balance, monthly contributions, and return rate, visualized year by year — with the split between what you put in and what compounding earned you.
Your plan
Balance vs. contributions, year by year
The gap between the two lines is compounding at work — notice how it stays small for years, then explodes late. Time in the market matters more than timing it.
Year-by-year ledger
| Year | Starting | Contributed | Growth | Ending |
|---|---|---|---|---|
| 1 | $5,000 | $6,000 | $558 | $11,558 |
| 2 | $11,558 | $6,000 | $1,032 | $18,590 |
| 3 | $18,590 | $6,000 | $1,540 | $26,130 |
| 4 | $26,130 | $6,000 | $2,085 | $34,215 |
| 5 | $34,215 | $6,000 | $2,670 | $42,885 |
| 6 | $42,885 | $6,000 | $3,296 | $52,181 |
| 7 | $52,181 | $6,000 | $3,968 | $62,149 |
| 8 | $62,149 | $6,000 | $4,689 | $72,839 |
| 9 | $72,839 | $6,000 | $5,462 | $84,300 |
| 10 | $84,300 | $6,000 | $6,290 | $96,591 |
| 11 | $96,591 | $6,000 | $7,179 | $109,770 |
| 12 | $109,770 | $6,000 | $8,132 | $123,901 |
| 13 | $123,901 | $6,000 | $9,153 | $139,054 |
| 14 | $139,054 | $6,000 | $10,249 | $155,303 |
| 15 | $155,303 | $6,000 | $11,423 | $172,726 |
| 16 | $172,726 | $6,000 | $12,683 | $191,409 |
| 17 | $191,409 | $6,000 | $14,033 | $211,442 |
| 18 | $211,442 | $6,000 | $15,481 | $232,923 |
| 19 | $232,923 | $6,000 | $17,034 | $255,958 |
| 20 | $255,958 | $6,000 | $18,699 | $280,657 |
| 21 | $280,657 | $6,000 | $20,485 | $307,142 |
| 22 | $307,142 | $6,000 | $22,400 | $335,542 |
| 23 | $335,542 | $6,000 | $24,453 | $365,994 |
| 24 | $365,994 | $6,000 | $26,654 | $398,648 |
| 25 | $398,648 | $6,000 | $29,015 | $433,663 |
| 26 | $433,663 | $6,000 | $31,546 | $471,209 |
| 27 | $471,209 | $6,000 | $34,260 | $511,469 |
| 28 | $511,469 | $6,000 | $37,170 | $554,639 |
| 29 | $554,639 | $6,000 | $40,291 | $600,930 |
| 30 | $600,930 | $6,000 | $43,638 | $650,568 |
A model, not a prediction — pre-tax, no fees, future dollars, and the return is assumed constant every year. Real markets zigzag; the average is what compounds.
How the math works
Each month the balance grows by annual return ÷ 12, then the monthly contribution is added (end-of-month compounding). Year by year, the chart separates contributions (every dollar you put in) from growth (balance minus contributions).
The closed-form equivalent is the future value of an ordinary annuity plus the compounded lump sum: FV = P·(1+r)n + PMT·(((1+r)n − 1) / r), where r is the monthly rate and n is the number of months. The simulation matches this formula exactly.
This is a nominal model — it doesn't subtract inflation. At 3% inflation, a 7% nominal return is roughly 4% in today's purchasing power. And it assumes a smooth fixed return every year; real markets are volatile, which changes the path but not the long-run math.
Common questions
Why does the growth line stay flat for so long and then shoot up?+
Compounding is exponential but starts from a small base — early on, your contributions dominate. Around the crossover point (often year 15–20 at 7%), the balance is large enough that its own growth exceeds what you're adding. That's why starting early beats contributing more later.
Is 7% annual return realistic?+
The US stock market has returned roughly 10% nominal / 7% inflation-adjusted over long periods — but that's an average with brutal down years mixed in. This model assumes the smooth average every year, which overstates the predictability. Use 7% for planning, not as a promise.
Should I use nominal or real (inflation-adjusted) returns?+
For 'what will this buy me' questions, real returns are more honest: subtract ~2–3% inflation from the nominal rate. A 7% nominal return at 3% inflation is about 4% real — try both settings and compare the final balances to feel the difference inflation makes.
Does it matter whether I contribute at the start or end of the month?+
Barely — beginning-of-month contributions earn one extra month of growth each, worth roughly half a percent over decades. Consistency and amount swamp timing details. Automate the contribution and forget about it.