What’s compound interest?

Understand what compound interest means and how it's calculated

When you take out a loan, you usually have to pay off the sum you borrowed - the principal - and a percentage of that amount as interest. This is normally done via one or several payments made over a pre-set timescale.

Compound interest works slightly differently. Instead of paying the same amount of money at regular intervals, you’ll have to pay an ever-increasing amount of money on each payment date.

The reason for this is that your interest payment will be based on the principal plus the prior interest payment you made. This is easy to understand when you see it in an example.

Let’s say someone borrowed £10,000 that had to be paid back at the end of ten years, with an annual interest rate of 2 per cent per.

With a normal loan that would mean the borrower would have to pay interest of £200 for each of those ten years. Add that to the principal and, at the end of ten year period, the borrower would have to pay back £12,000.

If someone took the same loan but had to pay compound interest, things would look slightly different:

Calculating compound interest on borrowing £10K for 10 years with 2% annual rate

As you can see, the borrower will end up having to pay £12,189.94, slightly more than the regular loan which would have ended up costing £12,000.

A difference of £189.94 isn’t too bad but if a loan has a higher and more frequent interest rate, it can make borrowing very, very expensive.

Learn more:

Investing 101

How to invest in stocks and shares

More terms

Inflation

The increase in the prices of goods and services over time, and the process by which money loses its value.
Read more

Equity

The amount of money a company would be left with by subtracting its liabilities from the value of its assets.
Read more

Clean price

The quoted price of a gilt, which excludes accrued interest
Read more

Dirty price

The total price payable on the purchase of a gilt. It’s calculated as the clean price plus accrued interest.
Read more

Free Trade

The other free trade. International trade in which countries allow goods to flow across their borders without imposing import or export taxes.
Read more

Geometric Mean Return

A way of calculating compound returns on an investment or savings over a set period of time.
Read more

Equity ETF

An exchange-traded fund that is comprised of a set of stocks.
Read more

Nominal amount

The face value of a gilt. It represents the amount that will be repaid to the holder at maturity and is also used to calculate the dividend or coupon payment.
Read more

Xetra

A trading venue operated by the Frankfurt Stock Exchange.
Read more

You’re just minutes away from commission-free investing

When you invest, your capital is at risk