Tax Glossary

Compound Interest

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Last updated on:
March 17, 2024

Compound interest is a fundamental concept in finance, often described as "interest on interest." It signifies the mechanism where the interest you earn on a sum of money subsequently earns interest itself. This principle can dramatically impact savings and investments over time, accelerating the growth of your initial amount.

The Basics of Compound Interest

When you deposit money into a savings account, you typically earn interest on your initial deposit. Compound interest takes this a step further by applying interest to both the initial principal and the accumulated interest from previous periods.

Example of Compound Interest in Action:

  • Initial Deposit: £1,000 in a savings account.
  • Annual Interest Rate: 1.5% AER/Gross.
  • First Year Interest: £15.10, making the total £1,015.10.
  • Second Year: Interest is calculated on £1,015.10, not just the original £1,000, leading to a slightly higher interest amount due to compound interest.

The Power of Compound Interest

The true value of compound interest becomes evident over time. Even without additional deposits, the amount in your savings account can grow significantly, especially over long periods. This exponential growth is why starting to save early can have such a profound effect on your financial future.

Comparative Scenarios:

  • James: Starts saving £100 monthly at age 30 until 60, with a 10% annual interest rate, ending with £217,132.11.
  • Alex: Begins saving £100 monthly at 20, stops at 30, but leaves the money to grow until 60, accumulating £367,090.06.

This illustrates how starting early and leveraging the power of compound interest can yield significantly more savings over time, even with fewer total contributions.

Compound Interest and Debt

On the flip side, compound interest also applies to lending and borrowing. Certain credit cards and loans calculate interest on previously accrued interest, potentially causing debt to grow rapidly if not managed. This dual nature of compound interest—as a boon for savings but a bane for debt—underscores the importance of prudent financial management.


Compound interest is a powerful financial principle that underscores the importance of early and consistent saving. By understanding and utilizing compound interest, you can significantly enhance your financial well-being over time. Conversely, being mindful of how compound interest affects debts is crucial to avoid financial pitfalls.

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