The power of compounding, and why starting early matters

If you invest 10,000 and it earns 7% in a year, you have 10,700. In the second year you earn 7% on 10,700, not on the original 10,000. That is compounding: growth builds on previous growth, so your money earns returns on its own returns. It is often called the most powerful idea in personal finance, and it explains why time is such an important advantage for an investor.

This article shows the effect with real numbers, introduces the rule of 72, compares an early and a late investor, and explains what follows for the way you invest.

Simple versus compound growth

With simple interest, you earn the same amount every year on your original sum. With compound growth, you earn on a bigger base each year. Starting with 10,000 at 7% a year:

• Simple growth: 700 a year. After 30 years you have earned 21,000 and hold 31,000.
• Compound growth: after 30 years you hold about 76,100.

The gap is more than double, and it comes entirely from earning returns on earlier returns.

What time does

At an assumed 7% a year, 10,000 grows like this:

• After 5 years: about 14,000.
• After 10 years: about 19,700.
• After 20 years: about 38,700.
• After 30 years: about 76,100.
• After 40 years: about 149,700.

Notice that each decade adds far more than the one before. The first ten years add about 9,700, while the decade from 30 to 40 years adds about 73,600. These figures are illustrations at a constant rate. Real returns change from year to year, and some years are negative.

The rule of 72

A quick way to estimate how long money takes to double is to divide 72 by the annual return.

• At 3%, about 24 years.
• At 7%, about 10 years.
• At 10%, about 7 years.

It also works for costs and inflation. At 3% inflation, prices double in about 24 years. The rule is a mental shortcut, not an exact figure, but it makes the power of small differences in rate easy to see.

The early investor and the late investor

Here is a famous kind of comparison. Assume both earn 7% a year and invest at the end of each year.

• Early Anna invests 5,000 a year for 10 years, from age 25 to 35, then stops. She contributes 50,000 in total.
• Late Ben waits until 35, then invests 5,000 every year until 65, which is 30 years. He contributes 150,000 in total.

At 65, Anna has about 526,000 and Ben has about 472,000. Anna put in a third as much money, yet ended up with more, because her money had a ten-year head start to compound. The point is not that Ben should give up. If he continues to invest, he will also do well. It is that every year of delay has a real cost, and starting early, even small, is worth more than most people realise.

What follows from this

• Start as early as you can. A small amount now often beats a larger amount later.
• Invest regularly and stay invested. Contributions made steadily and left alone allow compounding to work.
• Avoid interrupting growth. Cashing out early, or moving in and out of the market, resets the process. Time in the market matters more than timing the market.
• Keep costs low. Fees compound against you in the same way. A small annual fee takes a growing share of a growing balance.
• Reinvest dividends and interest where it suits your plan, so they can earn returns too.
• Be patient. Most of the benefit arrives late in the journey, so quitting early forfeits the largest gains.

Compounding works against you too

The same maths applies to debt. A credit card balance compounding at a high rate grows quickly if you do not repay it. This is a reason to pay off expensive debt before investing heavily: it is a guaranteed compounding return in your favour.

Common misunderstandings

• "Returns are steady." They are not. A 7% average can be made up of strong years and losing years. Compounding still works, but the path is bumpy.
• "I need a large amount to start." You do not. Regular small contributions grow over time.
• "It is too late for me." Compounding still helps at any age, although the effect is smaller with fewer years. It is a reason to begin now, not to give up.
• "Higher returns are all that matters." Time, contribution amount and costs matter just as much.

A short checklist

• I invest regularly, ideally automatically.
• I keep costs low.
• I avoid cashing out during market falls.
• I reinvest returns where appropriate.
• I have paid down high-interest debt.

Compounding rewards patience and consistency. It is a reason to begin now, not a promise of any particular outcome, since actual returns will differ from the figures used here.