Compound Interest Calculator
Compounding is the engine behind long-term wealth building: you earn returns not just on your deposits, but on every dollar those deposits have already earned. This calculator projects your future balance from an initial deposit plus optional monthly contributions.
The year-by-year table makes it obvious when growth starts outpacing your own contributions — the moment compound interest starts working harder than you do.
Future balance
$300,850.72
- Total contributed
- $130,000.00
- Interest earned
- $170,850.72
- Rule of 72 doubling time
- 10.3 years
Compounding adds 131% on top of everything you deposit.
| Year | Contributions to date | Interest earned | Balance |
|---|---|---|---|
| 1 | $16,000.00 | $919.19 | $16,919.19 |
| 2 | $22,000.00 | $2,338.58 | $24,338.58 |
| 3 | $28,000.00 | $4,294.31 | $32,294.31 |
| 4 | $34,000.00 | $6,825.16 | $40,825.16 |
| 5 | $40,000.00 | $9,972.70 | $49,972.70 |
| 6 | $46,000.00 | $13,781.53 | $59,781.53 |
| 7 | $52,000.00 | $18,299.43 | $70,299.43 |
| 8 | $58,000.00 | $23,577.68 | $81,577.68 |
| 9 | $64,000.00 | $29,671.22 | $93,671.22 |
| 10 | $70,000.00 | $36,639.02 | $106,639.02 |
| 11 | $76,000.00 | $44,544.25 | $120,544.25 |
| 12 | $82,000.00 | $53,454.70 | $135,454.70 |
| 13 | $88,000.00 | $63,443.02 | $151,443.02 |
| 14 | $94,000.00 | $74,587.14 | $168,587.14 |
| 15 | $100,000.00 | $86,970.62 | $186,970.62 |
| 16 | $106,000.00 | $100,683.03 | $206,683.03 |
| 17 | $112,000.00 | $115,820.45 | $227,820.45 |
| 18 | $118,000.00 | $132,485.91 | $250,485.91 |
| 19 | $124,000.00 | $150,789.85 | $274,789.85 |
| 20 | $130,000.00 | $170,850.72 | $300,850.72 |
How the Compound Interest Calculator works
The calculation follows the standard method used across the US — no shortcuts, no hidden assumptions. Here is exactly what happens behind the scenes:
Formula
FV = PV(1 + r)ⁿ + PMT × (((1 + r)ⁿ − 1) / r)
- PV is your starting balance.
- PMT is the regular contribution added at the end of each period.
- r is the periodic rate of return; n is the number of periods.
Frequently Asked Questions
How does the Rule of 72 work?
Divide 72 by your annual return to estimate the years needed to double your money. At an 8% average return, money doubles roughly every 9 years — try it against the table above.
What is a realistic long-term return to assume?
The S&P 500 has averaged about 10% nominal (roughly 7% after inflation) per year over multi-decade periods. For conservative planning, many advisors model 5–7% for stock-heavy portfolios.
Why is starting early so powerful?
Because early dollars compound through the most doubling cycles. A 25-year-old investing $300/month at 7% can end up with roughly twice what a 35-year-old investing the same amount accumulates by 65.
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