Finance
6 min read
March 1, 2025

How Compound Interest Works: Why Starting Early Changes Everything

The math of compound interest proves why starting 10 years early beats investing 2.5x more money.

How Compound Interest Works: Why Starting Early Changes Everything

Rs. 100,000 at 10% for 5 years:

  • Simple interest: Rs. 150,000
  • Compound interest: Rs. 161,051 (+Rs. 11,051)
  • Over 30 years: Rs. 1,744,940 compound vs Rs. 400,000 simple. That's 4.4× more.

    The Rule of 72

    Divide 72 by annual rate to estimate years to double:

  • 10% → 7.2 years
  • 15% → 4.8 years
  • Pakistan NSS (15–17%) → ~4–5 years
  • Starting Early vs Investing More

    Ali saves Rs. 5,000/month ages 25–35 (Rs. 600K total). Sara saves Rs. 5,000/month ages 35–60 (Rs. 1.5M total). At 10% return, Ali ends up with nearly as much as Sara despite investing 2.5× less — because of the 10-year head start.

    Monthly Contributions Multiplier

    Rs. 100,000 at 10% over 30 years:

  • No contributions: Rs. 1,744,940
  • +Rs. 5,000/month: Rs. 11,498,000
  • Every contributed rupee generates ~5.4× in final value. Calculate with our Savings Calculator.

    Written by the GMC Tools team