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Compound Interest Calculator — Watch Your Money Grow

$
$
%
years
Final Balance
$300,851
Total Contributions
$130,000
Total Interest
$170,851
Interest / Contribution
131%
ContributionsInterest Earned
$130,000$170,851

Growth Over Time

Year 1Year 20
Contributions Interest
The Power of Compounding: Your $130,000 in contributions grew to $300,851 — earning $170,851 in interest alone. The longer you invest, the more compound interest works in your favour.
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The Power of Compound Interest

Compound interest is the single most powerful force in wealth building. Unlike simple interest, which only grows linearly, compound interest accelerates over time as you earn returns on your returns. This calculator shows exactly how your money grows with regular contributions and compounding.

Compound Interest Formula

The formula for compound interest with regular contributions is: A = P(1 + r/n)^(nt) + PMT × [((1 + r/n)^(nt) - 1) / (r/n)], where P is the principal, r is the annual rate, n is the compounding frequency, t is years, and PMT is the periodic contribution.

The Rule of 72 — Quick Doubling Estimates

Annual ReturnYears to DoubleExample
4%18 yearsSavings account / bonds
6%12 yearsConservative portfolio
7%~10 yearsBalanced portfolio
8%9 yearsGrowth portfolio
10%~7 yearsAggressive / historical stock market

Why Starting Early Matters

The most important factor in compound interest is time. Consider two investors:

  • Early starter: Invests $300/month from age 25 to 65 (40 years) at 7% → $791,000
  • Late starter: Invests $600/month from age 35 to 65 (30 years) at 7% → $730,000

The early starter contributes half as much per month but ends up with more money — that's the power of 10 extra years of compounding.

Tips to Maximise Compound Growth

  • Start as early as possible — Time is more valuable than the amount you invest
  • Be consistent — Regular contributions matter more than timing the market
  • Reinvest dividends — Dividend reinvestment compounds your returns further
  • Minimise fees — A 1% management fee over 30 years can cost you 25% of your final balance
  • Stay the course — Avoid withdrawing during downturns; compounding requires patience

Frequently Asked Questions

What is compound interest?
Compound interest is interest calculated on both the initial principal and all previously accumulated interest. Unlike simple interest (calculated only on the principal), compound interest grows exponentially over time because you earn "interest on interest." This is why Albert Einstein reportedly called it "the eighth wonder of the world."
How does compounding frequency affect returns?
The more frequently interest compounds, the more you earn. Daily compounding earns slightly more than monthly, which earns more than quarterly or annually. However, the difference is relatively small — the bigger factors are your interest rate, contribution amount, and time horizon. For example, $10,000 at 7% for 20 years: annually = $38,697, monthly = $40,387, daily = $40,552.
What is the Rule of 72?
The Rule of 72 is a quick way to estimate how long it takes to double your money. Divide 72 by your annual interest rate: at 6%, your money doubles in approximately 12 years (72 ÷ 6 = 12). At 8%, it doubles in about 9 years. This rule is most accurate for rates between 4% and 12%.
What is a realistic return rate to use?
Historical averages: stock market (index funds) ~7-10% per year before inflation, bonds ~3-5%, savings accounts ~2-4%, term deposits ~3-5%. For conservative planning, use 5-7% for diversified investments. Always account for inflation (typically 2-3%) — a 7% nominal return is roughly 4-5% in real purchasing power.
How much should I save each month?
A common guideline is the 50/30/20 rule: 50% of income for needs, 30% for wants, and 20% for savings and debt repayment. However, the right amount depends on your goals, timeline, and circumstances. Even small regular contributions grow significantly over time thanks to compound interest.
When should I start investing?
As early as possible. Time is the most powerful factor in compound interest. Starting at 25 with $200/month at 7% gives you ~$525,000 by 65. Starting at 35 with the same amount gives only ~$243,000 — less than half, despite only 10 years difference. The cost of waiting is enormous.
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