See how your money grows over time with compound interest and monthly contributions. Year-by-year breakdown, rate comparison, and visual chart included.
| Year | Starting Balance | Contributions | Interest Earned | Cumulative Interest | End Balance |
|---|
Based on your starting amount and monthly contribution. Your current rate is highlighted.
Compound interest means you earn interest not just on your original principal, but also on the interest you've already accumulated. For example, if you invest $10,000 at 7% annually, you earn $700 in year one. In year two you earn 7% on $10,700, which is $749 — $49 more than year one. Over decades this 'interest on interest' effect becomes dramatic. After 30 years at 7%, $10,000 grows to about $76,123 without any additional contributions — the original $10,000 grew 7.6x entirely through compounding.
APR (Annual Percentage Rate) is the stated nominal interest rate without accounting for compounding within the year. APY (Annual Percentage Yield) is the effective rate after accounting for within-year compounding. A savings account with 7% APR compounded monthly has an APY of 7.229% — you earn slightly more than 7% annually because each month's interest itself earns interest for the rest of the year. The difference grows with compounding frequency: daily compounding at 7% APR produces a 7.250% APY. When comparing savings accounts or investments, always compare APY for an accurate apples-to-apples comparison.
Monthly contributions have an enormous impact because they also compound over time. With a $10,000 starting balance at 7% compounded monthly over 20 years: with $0/month contributions the final balance is about $40,387. Add $200/month and it reaches $104,568 — an increase of $64,181. Add $500/month and it reaches $213,958. The extra $200/month contributed over 20 years totals $48,000 in actual cash contributed, but generates $64,181 more in final balance because all those contributions compound over time. Starting early magnifies this further: the same $200/month over 30 years at 7% produces $227,977 vs $75,977 without contributions.
More frequent compounding always earns slightly more interest. On a $10,000 investment at 7% over 10 years: annual compounding produces $19,671, quarterly compounding produces $19,898, monthly compounding produces $20,097, and daily compounding produces $20,137. The difference between monthly and daily is only $40 on $10,000 over 10 years — practically negligible. The difference between annual and monthly is more meaningful at $426, but still small compared to the impact of the rate itself. For most purposes, monthly compounding (used by most savings accounts and investment vehicles) is the most practically relevant choice.
The Rule of 72 is a mental math shortcut: divide 72 by the annual interest rate to estimate how many years it takes to double your money. At 6%, your money doubles in about 72/6 = 12 years. At 8%, it doubles in 9 years. At 4%, in 18 years. The Rule of 72 is accurate to within about 0.5 years for rates between 2% and 20%. The exact doubling time uses the formula ln(2)/ln(1+r). At 7%, the exact answer is 10.24 years versus the Rule of 72 estimate of 10.29 years — a difference of just 3 days.
The amount you need to save monthly depends heavily on your time horizon and expected return. Assuming 7% average annual return (roughly the historical inflation-adjusted S&P 500 average): starting at age 25 you need to save about $381/month to reach $1 million by 65. Starting at age 30, that rises to $555/month. Starting at age 35, you need $820/month. Starting at age 40, you need $1,234/month. Starting at age 45, you need $1,920/month. This demonstrates why starting early is so powerful: waiting 10 years (from 25 to 35) more than doubles your required monthly contribution to hit the same goal.
For planning purposes: a US total stock market index fund (like VTSAX) has returned approximately 10% nominal annually since 1926, or about 7% after inflation. A 60/40 stock/bond portfolio has returned approximately 8-9% nominal, or 5-6% real. High-yield savings accounts and CDs are currently offering 4-5% APY. For conservative planning, many financial advisors use 6-7% real return for diversified equity portfolios. For nominal projections (not inflation-adjusted), 8-10% is commonly used for stock-heavy portfolios. Always use your expected real (inflation-adjusted) return when planning purchasing-power goals.