Compound Interest: How Time Can Grow Your Money

Compounding adds growth to earlier growth

With simple interest, the calculation is based only on the original amount. With compound interest, interest already credited becomes part of the balance used for a later calculation. The effect can begin quietly and become more noticeable over longer periods.

The same principle can work against you when interest is charged on borrowing. That is why the rate, payment pattern and time matter on both sides of your financial plan.

Your contributions usually do most of the early work

In the first years of a savings plan, most of the balance will often come from the money you put in. Interest then has a larger base on which to build. A regular amount you can sustain is more useful than an ambitious amount that repeatedly has to be cancelled.

The timing of contributions also affects an illustration. Money added earlier has longer to earn interest, while irregular deposits will create a different result from equal monthly deposits.

Compare rates using the right information

AER, or Annual Equivalent Rate, helps compare savings accounts by showing an annualised rate that assumes interest is added to the account. Check whether the rate is fixed, variable, introductory or dependent on conditions such as limited withdrawals.

The headline rate is only one part of the choice. Access, penalties, minimum balances and the date a bonus ends can matter more if the money has a near-term purpose.

A projection is an assumption, not a promise

A savings calculator needs an opening balance, regular contribution, assumed rate and time period. Write those assumptions beside the result. If the rate changes, the final balance will change too.

Investment growth is less predictable than a fixed savings illustration. Values can fall as well as rise, returns are not steady and charges can reduce the amount retained. Never describe a smooth illustration as an expected outcome.

Inflation and tax affect what growth can buy

A larger number in the future does not automatically mean greater spending power. Inflation can reduce what each pound buys, so a useful review considers both the balance and the goal it is meant to fund.

Savings interest may be taxable outside an ISA when it exceeds the allowances available to you. Tax treatment depends on your income and circumstances, and the rules can change. Use current HMRC guidance when the amount becomes material.

Use time as a planning tool

Run more than one scenario. A cautious rate, a shorter period and a lower contribution can show whether the plan still has value when life does not follow the best case.

Review the projection when the rate, contribution, goal or deadline changes. The purpose is to support a decision today, not to predict an exact future balance.

Frequently asked questions