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Investing basics2026-06-21

Compound Interest and Long-Term Investing: How Time Grows Money

How compound differs from simple interest, the Rule of 72, how time, return and reinvestment shape your wealth, and what erodes compounding (fees, taxes, inflation).

The most powerful force in investing isn't flashy stock picking — it's compound interest, and the time for it to work. Over long horizons, compounding defies intuition.

What compounding is

  • Simple interest accrues only on the principal. $1,000 at 10% a year earns $100 every year.
  • Compound interest earns "interest on interest." Year one: $100. Year two: 10% of $1,100 = $110… and the pace keeps accelerating.

The key is reinvesting your returns to grow the base. Compounding works when you roll dividends and interest back in rather than spending them.

The Rule of 72

The time for money to double is roughly 72 ÷ annual return (%).

  • At 6%, about 12 years; at 8%, about 9 years; at 10%, about 7.2 years.

A slightly higher return, given enough time, opens an exponentially wider gap.

Time is the most powerful variable

In compounding, time often matters more than the rate of return. With the same amount and the same return, starting 10 years earlier changes the outcome dramatically. "Early and long" is the heart of compounding.

What erodes compounding

Compounding cuts both ways. These eat into it:

  • Fees: annual expense ratios and trading costs compound as losses (why low costs matter).
  • Taxes: depending on when you realize gains and the account type, they shrink the effect.
  • Inflation: subtract inflation from nominal returns to get real compounding.
  • Early withdrawals: pulling out principal shrinks the snowball.

How to use it

  1. Start early and stay long. Time is your biggest weapon.
  2. Reinvest. Roll dividends and interest back in.
  3. Keep costs low. Fees and taxes erode compounding the most quietly.
  4. Diversify to ride out volatility. Hold a portfolio you can stick with so you don't bail mid-way.

Indicators worth watching alongside

To gauge the real effect of compounding, watch inflation and Treasury yields (the risk-free return). On the Global Market Dashboard, check the inflation and rate gauges to see how far your expected return runs ahead of inflation.

This article is for informational purposes only and is not investment advice.

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