Compound Interest Explained: Why Starting Early Matters

Compound interest is often called the most powerful force in personal finance. It is the reason someone who starts saving small amounts early can end up with more money than someone who saves larger amounts later. It also explains why expensive debt grows so fast when it is not paid off. This guide explains compound interest in plain language, shows how it works with examples in different currencies, and explains why starting early matters so much.

What is compound interest?

With simple interest, you earn interest only on the money you originally saved. With compound interest, you also earn interest on the interest you have already earned. Each year, your interest is added to your balance, and the next year's interest is calculated on that larger amount. Over time, the growth speeds up, like a snowball rolling downhill and picking up more snow.

A simple example

Suppose you save 100,000 at 10% a year. It could be shillings, naira, rand or dollars; the maths is the same. With simple interest, you earn 10,000 every year, so after 10 years you have 200,000. With compound interest, you earn 10,000 in the first year, then 11,000 in the second because your balance is now 110,000, and so on. After 10 years you have about 259,000, and after 20 years about 673,000, compared with 300,000 under simple interest. The longer the time, the bigger the difference.

Why starting early matters

Time is the most important ingredient in compound interest. Imagine two savers who each earn 10% a year. Amina saves 5,000 a month from age 25 to 35 and then stops, leaving the money to grow. Brian starts at 35 and saves 5,000 a month until 60. Amina contributes for only 10 years and Brian for 25, yet at 60 Amina's balance is larger, because her money had an extra decade to compound. Starting early with small amounts often beats starting late with large ones.

The rule of 72

A quick way to estimate compound growth is the rule of 72. Divide 72 by the yearly interest rate to find roughly how many years it takes for your money to double. At 6%, money doubles in about 12 years; at 9%, in about 8 years; at 12%, in about 6 years. The same rule shows how quickly debt can double if you do not pay it: a debt growing at 24% a year doubles in about three years.

Compound interest works against you in debt

Credit cards, overdrafts and many short-term loans also compound, which is why unpaid balances can grow alarmingly quickly. Interest is added to what you owe, and you then pay interest on that interest. This is why paying off high-interest debt is usually one of the best "investments" you can make. Our guide on getting out of debt on a low income shows how to start.

How to make compound interest work for you

Start now, even with a little. Small regular amounts given decades to grow can become large sums. Reinvest your returns. Leave interest and dividends in the account rather than withdrawing them, so they keep compounding. Be consistent. Regular monthly contributions add fuel to the snowball. Watch fees and inflation. High fees reduce the rate at which your money compounds, and inflation reduces what your money can buy, so look for returns that beat inflation after costs.

Money market funds, savings accounts, government securities, pension funds and diversified investments all use compound growth, at different levels of risk. Try different amounts, rates and timeframes in our savings calculator to see the effect for yourself.

Frequently asked questions

What is compound interest in simple terms?

It is interest earned on both your original savings and the interest already added, so your money grows faster over time.

How long does it take money to double?

Divide 72 by the annual interest rate. At 8%, money doubles in about nine years.

Why is starting early so important?

Because compound growth depends on time; money saved earlier has more years to grow on its own earnings.

Compound interest explained: why starting early matters