See How Your Money Grows: Compound Interest Calculator

Simple vs Compound Interest: The Difference That Matters

Simple interest is calculated only on the principal amount. Compound interest is calculated on the principal plus accumulated interest, creating an exponential growth effect. Over long periods, the difference becomes dramatic.

Growth Comparison Over 30 Years

Principal: $10,000 | Rate: 8% annually

Year  | Simple Interest | Compound Interest (annual)
------|-----------------|--------------------------
  1   |    $10,800      |    $10,800
  5   |    $14,000      |    $14,693
 10   |    $18,000      |    $21,589
 20   |    $26,000      |    $46,610
 30   |    $34,000      |   $100,627

After 30 years, compound interest earns nearly 3× more than simple interest.

Always check whether your investment uses simple or compound interest. The compounding frequency (daily, monthly, annual) also matters: more frequent compounding means faster growth.

Monthly Contribution Scenarios: How $100/month Grows Over Time

Regular monthly contributions supercharge the power of compound interest. Even a modest $100 per month, invested consistently, can grow into a substantial nest egg over decades. The key is time — the longer your money compounds, the more dramatic the growth becomes. The table below shows how different monthly contribution amounts grow over 10 to 40 years at a 7% annual return.

Assumes 7% annual return compounded monthly. Values rounded.

Catch-Up Scenarios: Starting Late vs Starting Early

Starting earlier is more powerful than investing more. The 25-year-old investing $300/month beats the 45-year-old investing $1,200/month.

The Rule of 72: Estimating Doubling Time in Your Head

The Rule of 72 is a mental-math shortcut that answers a question compound interest calculators get asked constantly: how long until my money doubles? Divide 72 by your annual return rate and you get the approximate number of years to double. At 8%, money doubles roughly every 9 years; at 12%, every 6. The rule works because ln(2) ≈ 0.693, and 69.3 rounded up to 72 divides cleanly by the numbers people actually use — 2, 3, 4, 6, 8, 9, 12. It is an approximation, most accurate for rates between 6% and 10%, and it only holds when interest compounds annually at a steady rate. For monthly compounding the doubling happens slightly faster.

You can also run the rule backwards: if a sales brochure promises your money doubles in 6 years, the implied return is 72 ÷ 6 = 12% per year — a useful reality check. And doubling twice is quadrupling, so $25,000 at 8% becomes roughly $100,000 in about 18 years. Use the calculator above for exact figures with monthly contributions or non-annual compounding.

What $10,000 Becomes: Worked Examples at 6%, 8%, and 10%

Abstract formulas are hard to feel, so here are concrete numbers. Each figure below is a one-time $10,000 deposit with no additional contributions, compounded annually. Notice how the columns spread apart over time: the gap between 6% and 10% is only $8,000 after 10 years, but nearly $120,000 after 30. That widening gap is compounding doing its work — the return rate matters far more in year 30 than in year 5.

Scaling is linear: if $10,000 becomes $100,627 at 8% over 30 years, then $100,000 becomes about $1,006,270 and $750 becomes about $7,547 over the same period. Two cautions: these figures assume a constant return, which no real investment delivers — a 30-year journey will include drawdowns; and taxes and fees are not included. An 8% gross return in a taxable account behaves more like 6% after a 1.5% fee and dividend taxes. Run your own amount through the calculator with monthly contributions to see your specific path.

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