How Compound Interest Works — and Why Starting Early Wins

Compound interest explained in plain language: interest on interest, the simple formula, why frequency and time matter more than the amount.

Compound interest is the reason small, steady saving turns into a large sum over time. It is interest earned not only on the money you put in, but also on the interest you have already earned — so the balance grows a little faster every period.

Interest on interest

Say you save 1,000 at 5% a year. After one year you have 1,050. In the second year you earn 5% on 1,050, not on 1,000, so you gain 52.50 instead of 50. That extra 2.50 is the compounding at work, and it snowballs: the longer you leave the money, the bigger each year's gain becomes.

The formula

For a lump sum, the balance is A = P(1 + r/n)^(nt), where P is the starting amount, r is the annual rate as a decimal, n is how many times a year interest is added, and t is the number of years. You rarely need to work this by hand — the savings calculator runs it for you and also handles regular monthly contributions.

Why regular contributions matter

Adding a fixed amount every month has an outsized effect, because each contribution has its own time to grow. A modest monthly deposit kept up for years often ends up adding more to the final balance than the starting amount did. The earlier you begin, the more each unit of money earns.

How often interest compounds

Interest can be added monthly, quarterly or yearly. The more often it compounds, the sooner earlier interest starts earning its own interest, so monthly compounding gives a slightly higher balance than yearly at the same headline rate. Over one year the gap is small; over decades it adds up.

A quick example

Start with 1,000, add 100 a month at 5% compounded monthly for 10 years. You contribute 13,000 in total, but the balance grows to about 17,240 — roughly 4,240 of it interest earned along the way.

The catch

Compounding works against you on debt in exactly the same way it works for you on savings. Money you owe on a credit card compounds too, which is why clearing high-interest debt is often the best "investment" you can make.