Compound Interest Calculator
See how your savings or investments grow over time with the power of compounding. Add monthly contributions to project realistic outcomes.
Future balance after 20 years
$300,851
Total contributed
$130,000
Interest earned
$170,851
Year-by-year growth
| Year | Contributed | Interest | Balance |
|---|---|---|---|
| 1 | $16,000 | $919 | $16,919 |
| 2 | $22,000 | $2,339 | $24,339 |
| 3 | $28,000 | $4,294 | $32,294 |
| 4 | $34,000 | $6,825 | $40,825 |
| 5 | $40,000 | $9,973 | $49,973 |
| 6 | $46,000 | $13,782 | $59,782 |
| 7 | $52,000 | $18,299 | $70,299 |
| 8 | $58,000 | $23,578 | $81,578 |
| 9 | $64,000 | $29,671 | $93,671 |
| 10 | $70,000 | $36,639 | $106,639 |
| 11 | $76,000 | $44,544 | $120,544 |
| 12 | $82,000 | $53,455 | $135,455 |
| 13 | $88,000 | $63,443 | $151,443 |
| 14 | $94,000 | $74,587 | $168,587 |
| 15 | $100,000 | $86,971 | $186,971 |
| 16 | $106,000 | $100,683 | $206,683 |
| 17 | $112,000 | $115,820 | $227,820 |
| 18 | $118,000 | $132,486 | $250,486 |
| 19 | $124,000 | $150,790 | $274,790 |
| 20 | $130,000 | $170,851 | $300,851 |
The math behind compounding
Compound interest is what Albert Einstein allegedly called "the eighth wonder of the world." It's interest that earns interest — and the longer it runs, the more dramatic the effect.
Example
Start with $10,000, contribute $500/month, earn 7% annually, for 20 years. You contribute $120,000 of your own money. The interest adds another $200,000+. Your final balance is over $320,000 — most of it from growth you didn't have to work for.
What rate should I use?
Be realistic. The S&P 500's long-term average is about 10% nominal (7% real, after inflation). High-yield savings accounts pay 4–5%. Bonds historically return 4–6%. A diversified 60/40 portfolio averages 5–8% real. Plug in different rates to see best/worst case scenarios.