Calculate interest earned or owed using either the simple interest formula or compound interest formula. Toggle between modes. Simple interest is used for short-term loans and some savings accounts. Compound interest is used for most investments, savings accounts, mortgages, and credit cards — and is the basis for long-term wealth building.
Where P = principal, r = annual rate (decimal), t = time in years, n = compounding periods per year. Interest earned = A − P.
| Rate | Rule of 72 Doubling | Simple vs Compound (10yr, monthly) |
|---|---|---|
| 2% | 36.0 years | $200 vs $221 on $1,000 |
| 5% | 14.4 years | $500 vs $647 on $1,000 |
| 7% | 10.3 years | $700 vs $1,007 on $1,000 |
| 10% | 7.2 years | $1,000 vs $1,708 on $1,000 |
| 12% | 6.0 years | $1,200 vs $2,308 on $1,000 |
Investment accounts grow through compound interest — your returns earn returns. Unlike simple interest (where you only earn on the principal), compounding means that every dollar of gain becomes part of the base that earns the next round of gains. The longer the time horizon, the more dramatic the effect.
A = P(1 + r/n)^(nt) + PMT × [((1 + r/n)^(nt) − 1) / (r/n)]
Investing $300/month starting at age 25 at 8% annual return gives you ~$1,006,000 by age 65.
Starting at 35 with the same $300/month: ~$440,000 — less than half, even though you only missed 10 years.
Those 10 years cost you $566,000 in future value, even though the missed contributions were only $36,000. That gap is the compounding effect in action.
Investment returns are not guaranteed. Stock market returns vary year to year — 8% is a long-term historical average for diversified index funds, not a promise. Tax-advantaged accounts (401k, IRA, Roth IRA) shelter gains from annual taxation, which can significantly increase real returns compared to a taxable brokerage account.