Math for MoneyCompound interest examples
Find the row for your amount and the column for your time period. Each cell shows the final balance at a hypothetical 7% annual return and links to a full breakdown: results for every rate from 1% to 12%, what happens if you invest longer or start later, and a calculator you can adjust.
Investing a fixed amount every month
For example, $500 a month for 20 years adds up to $120,000 in contributions and would grow to $253,768 at 7%. Keep going for 40 years and you pay in twice as much, $240,000, but end with $1,235,771, about 4.9 times the 20-year balance.
Balance at 7% a year, with no starting balance and contributions at the end of each month. On a small screen, scroll the table sideways to see every period.
| Monthly amount | 5 years | 10 years | 15 years | 20 years | 25 years | 30 years | 35 years | 40 years |
|---|---|---|---|---|---|---|---|---|
| $50 a month | $3,560 | $8,553 | $15.6K | $25.4K | $39.2K | $58.5K | $85.6K | $123.6K |
| $100 a month | $7,120 | $17.1K | $31.1K | $50.8K | $78.3K | $116.9K | $171.1K | $247.2K |
| $200 a month | $14.2K | $34.2K | $62.2K | $101.5K | $156.6K | $233.9K | $342.3K | $494.3K |
| $250 a month | $17.8K | $42.8K | $77.8K | $126.9K | $195.8K | $292.4K | $427.9K | $617.9K |
| $300 a month | $21.4K | $51.3K | $93.3K | $152.3K | $234.9K | $350.8K | $513.4K | $741.5K |
| $400 a month | $28.5K | $68.4K | $124.4K | $203K | $313.2K | $467.8K | $684.6K | $988.6K |
| $500 a month | $35.6K | $85.5K | $155.6K | $253.8K | $391.5K | $584.7K | $855.7K | $1.24M |
| $750 a month | $53.4K | $128.3K | $233.3K | $380.7K | $587.3K | $877.1K | $1.28M | $1.85M |
| $1,000 a month | $71.2K | $171.1K | $311.1K | $507.5K | $783K | $1.17M | $1.71M | $2.47M |
| $1,500 a month | $106.8K | $256.6K | $466.7K | $761.3K | $1.17M | $1.75M | $2.57M | $3.71M |
| $2,000 a month | $142.4K | $342.1K | $622.2K | $1.02M | $1.57M | $2.34M | $3.42M | $4.94M |
Investing a lump sum once
For example, $10,000 invested for 30 years with nothing added would grow to $76,123 at 7%. Of that, $66,123 is growth.
Balance at 7% a year, with nothing added or withdrawn. On a small screen, scroll the table sideways to see every period.
| Amount invested | 5 years | 10 years | 15 years | 20 years | 25 years | 30 years | 40 years |
|---|---|---|---|---|---|---|---|
| $1,000 | $1,403 | $1,967 | $2,759 | $3,870 | $5,427 | $7,612 | $15K |
| $5,000 | $7,013 | $9,836 | $13.8K | $19.3K | $27.1K | $38.1K | $74.9K |
| $10,000 | $14K | $19.7K | $27.6K | $38.7K | $54.3K | $76.1K | $149.7K |
| $20,000 | $28.1K | $39.3K | $55.2K | $77.4K | $108.5K | $152.2K | $299.5K |
| $25,000 | $35.1K | $49.2K | $69K | $96.7K | $135.7K | $190.3K | $374.4K |
| $50,000 | $70.1K | $98.4K | $138K | $193.5K | $271.4K | $380.6K | $748.7K |
| $100,000 | $140.3K | $196.7K | $275.9K | $387K | $542.7K | $761.2K | $1.5M |
How to read these numbers
Both tables use one example rate, 7%, so that the cells can be compared with each other. It is not a forecast. Each linked page shows the same scenario at 1% to 12%, with 4%, 7% and 10% called out as a cautious, a middle and an optimistic case.
Every figure assumes a constant rate compounded once a year. Real returns go up and down, some years are negative, and fees, taxes and inflation all reduce what you keep. Use the examples to see how the amount, the time period and the rate each change the outcome.
If your situation is not in the tables, for instance a starting balance plus monthly contributions, use the compound interest calculator and enter your own numbers.