Compound Interest Guide

Quick answer

Compound interest is growth on growth. Instead of earning interest only on the original amount, you also earn interest on previous interest. The longer the money remains invested or saved, the more powerful the effect becomes. Time is often more important than a perfect starting amount.

The compound interest formula

A = P(1 + r/n)^nt, where A is the future value, P is the starting amount, r is the annual rate, n is the number of compounding periods per year, and t is time in years.

For regular monthly investing, the calculation is more complex because every contribution has a different amount of time to grow. This is why a calculator is useful: it can combine starting balance, monthly deposits, rate, and time into one projection.

Simple worked example

If you start with £1,000 and earn 5% per year for 10 years, the money grows to about £1,629 if compounded annually. The first year earns £50. Later years earn more because interest is being calculated on a larger balance.

Monthly contribution examples

The real power appears when regular contributions are added. A person saving £100 per month for 30 years contributes £36,000 before growth. If growth averages 5% per year, the final value can be much higher because each contribution has time to compound.

The table is intentionally directional because real returns vary. The lesson is that time and consistency do a large part of the work.

Fees, inflation, and tax

A 6% return does not mean you become 6% richer in real terms. Fees reduce growth. Inflation reduces purchasing power. Tax can affect interest, dividends, or gains depending on account type and jurisdiction. A good projection should consider the difference between nominal returns and real after-cost results.

Common mistakes

• Starting late because the first amount feels too small.
• Assuming high returns are guaranteed.
• Ignoring fees.
• Stopping contributions during normal market volatility without a plan.
• Comparing short-term results to long-term projections.
• Forgetting inflation.
• Using compound growth for investments that do not actually compound.

Practical takeaway

Compounding rewards patience more than perfection. Starting with £50 per month is better than waiting years to start with £500. Increase contributions when income rises, keep costs low, and avoid judging a long-term plan by a few bad months.