Compound Interest Calculator

This compound interest calculator estimates how a single starting principal could grow when interest is added back into the balance and starts earning interest of its own. It is useful for clean, focused questions such as what a lump sum might become over time, or how monthly compounding compares with annual compounding.

Enter your initial investment, annual interest rate, time period, and compounding frequency. The calculator then shows future value, total interest earned, a simple-interest comparison, extra growth from compounding, and a year-by-year balance table.

It does not include regular contributions, withdrawals, tax, fees, or inflation. That is intentional: keeping this page to one starting amount makes the compounding effect easier to see. Use the investment calculator when you need contribution-based projections, and use the savings calculator when you are planning a cash savings target.

Suppose you invest GBP 5,000 at an annual return of 6% and leave it for 20 years. With annual compounding and no further deposits, the balance grows to about GBP 16,036.

The striking part is that the growth accelerates. In the first year, 6% earns £300. In year 20, the same 6% applies to a much larger balance, so the interest added that year is far higher. That is the compounding effect: previous growth starts creating more growth.

That example is deliberately a lump-sum calculation. If you want to include regular monthly or annual deposits, use the investment calculator instead. This compound interest page keeps the calculation to one starting amount so the effect of compounding frequency is easier to compare.

Compound interest rewards time because the biggest gains often arrive late in the journey. A 30-year calculation is not simply a 10-year calculation repeated three times. The later years start from a larger balance, so each percentage point is worth more in pounds.

This is why starting earlier can beat contributing more later. A smaller amount invested for longer may grow more than a larger amount invested for a shorter period. The calculator is useful for testing that trade-off before setting a savings or investment target.

The first mistake is assuming a return is guaranteed. Bank savings rates may be predictable for a period, but investment returns move up and down. A 6% example is an assumption, not a promise.

The second mistake is ignoring inflation, fees, and tax. A balance can grow in nominal terms while buying power grows more slowly. Platform fees, fund charges, account fees, and tax can all reduce the effective return.

The third mistake is withdrawing too early. Taking money out interrupts the compounding chain because the withdrawn amount no longer earns future interest. That does not mean withdrawals are always wrong, but they should be part of the plan.

The calculator assumes one starting balance and a fixed annual rate. It does not add regular deposits or withdrawals during the period, which keeps the result focused on the compounding mechanism itself.

Compounding frequency changes how often interest is added back into the balance. Monthly or daily compounding can produce a higher future value than annual compounding at the same headline rate, although the difference may be modest over short periods.

The simple-interest comparison helps show how much extra growth comes from interest earning interest. That contrast is often more useful than the final balance alone because it shows what compounding added on top of the basic rate-and-time calculation.

Fees, tax, inflation, variable rates, and investment volatility are not included, so treat the result as a clean maths estimate rather than a full account forecast.

Entering a rate that already includes fees or inflation without keeping that assumption consistent.

Using this calculator for regular monthly contributions; the investment calculator is a better match for that because it has contribution fields.

Treating a fixed annual rate as guaranteed when actual savings rates or investment returns may change.

Comparing two results without checking whether they use the same compounding frequency.

Use investment when you want to add monthly, quarterly, or annual contributions.

Use savings when the goal is a cash savings target rather than a single compound-interest example.

Use retirement when the question includes retirement age, ongoing saving, or income needs.

Rerun the calculation when the rate, time period, principal, or compounding frequency changes.

For forward-looking assumptions, test lower and higher rates rather than relying on one neat result.

Keep the compounding frequency visible when comparing accounts or projections because the same annual rate can produce different balances.