Compound Interest Calculator
See how your savings or investments grow over time with the power of compounding. Add monthly contributions to project realistic outcomes.
Reviewed by Hassan Ahmed, Founder and Software Developer. Not a licensed tax advisor.
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.