20 years against 30
20 years gives $300,850.72. 30 years gives $691,150.47 — a difference of $390,299.75 more (+129.7%).
The last decade is where compounding does most of its work.
| Input | 20 years | 30 years |
|---|---|---|
| Starting balance(usd) | 10,000 | 10,000 |
| Monthly contribution(usd) | 500 | 500 |
| Annual return(percent) | 7 | 7 |
| Years(years) | 20 | 30 |
| Compounded | Monthly | Monthly |
| Inflation(percent) | 3 | 3 |
| Tax on the gain(percent) | 0 | 0 |
| Balance at the end | $300,850.72 | $691,150.47 |
Balance over time, side by side
Every figure, both ways
| Step | 20 years | 30 years | Difference |
|---|---|---|---|
| Effective annual yield(%) | 7.23 | 7.23 | 0 |
| Balance at the end | 300,850.72 | 691,150.47 | +390,299.75 |
| Total you contributed | 130,000 | 190,000 | +60,000 |
| Earned by compounding | 170,850.72 | 501,150.47 | +330,299.75 |
| Balance in today's money | 166,573.75 | 284,744.84 | +118,171.1 |
| Tax on the gain, if it is taxable | 0 | 0 | 0 |
| Balance after that tax | 300,850.72 | 691,150.47 | +390,299.75 |
| After tax, in today's money | 166,573.75 | 284,744.84 | +118,171.1 |
Questions people actually ask
- Does raising years change the answer in proportion?
- No, and that is the point of putting them side by side. Years changes by 50% while balance at the end changes by 130%. The step table below shows which line breaks the proportion.
- What is held constant between the two columns?
- Everything except years: starting balance, monthly contribution, annual return, compounded, inflation, tax on the gain. That is what makes the difference attributable — change two things at once and neither column tells you which one moved the answer.
- Can I compare my own two numbers?
- Yes, from the address bar: add ?a.years=… and &b.years=… to set each side, and a parameter without a prefix sets both at once. The page is computed per request, so any pair works — these two are only the ones worth a link.
Change either side by adding query parameters — ?a.years=… and b.years=…. A parameter without a prefix sets both sides at once. Source: SEC Investor.gov — compound interest, Federal Reserve — savings and investing data