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:

Table showing compound interest growth over 10 years with year, year interest, total interest, and total money due columns.
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

Direct listing

Going public without the traditional IPO. Existing shares, straight onto the exchange, no new money raised.
Read more

Yield

Income from an investment as a percentage of its current price.
Read more

Forward pricing

Mutual funds are traded on a forward pricing basis, meaning the price you see will be different to the price you may trade at.
Read more

Interest Rate

The amount a lender charges for lending your money, or a borrower pays you for borrowing your money.
Read more

Bookbuilding

How banks work out what investors will pay for shares before an IPO, one order at a time.
Read more

UK Treasury bill

A debt instrument issued by the UK government with a maturity of less than one year.
Read more

After-hours trading

Trading outside of a stock exchange's opening hours.
Read more

Gross Margin

The difference between a company's revenue and the cost to produce its goods/services, divided by revenue.
Read more

index-linked gilts

Gilts where the dividends and principal repayments are related to movements in the Retail Prices Index (RPI). This is as opposed to a conventional gilt, where the dividends and principal repayments are fixed in nominal terms.
Read more

You're just minutes away from commission-free investing

Capital at risk. Other charges may apply.