Compound Interest Calculator

Compound interest formula

A = P × (1 + r/n)^(n×t)

• A = Final amount
• P = Principal (starting amount)
• r = Annual interest rate (decimal)
• n = Compounding frequency per year
• t = Time in years

Why compound interest is so powerful

With compound interest, you earn interest on your interest. £10,000 at 8% for 30 years grows to over £100,000 without adding a single extra pound. Einstein reportedly called it "the eighth wonder of the world."

The important point is not the quote, but the shape of the growth. In the early years, most of the final balance may come from your own contributions. Later, investment returns can become a larger share of the total because each year starts from a bigger base. That is why time in the market, regular contributions, and low fees matter so much in long-term planning.

Compounding frequency comparison

For a 10,000 investment at 10% for 10 years: annual compounding = 25,937 / monthly = 27,070 / daily = 27,179. The difference between monthly and daily is small; what matters most is the rate and duration.

Regular contributions change the story

The basic formula is useful for a single starting amount, but many real plans include monthly deposits. Contributions can matter more than the compounding frequency, especially in the first decade. Someone who starts with a modest balance but adds consistently can overtake someone who invests once and then stops. When comparing scenarios, look at starting amount, contribution size, time horizon, return assumption, and fees together.

Nominal return vs real return

A calculator result is usually a nominal future value unless inflation is built into the assumptions. If your investment grows by 6% per year while prices rise by 3% per year, your purchasing-power growth is closer to 3% before taxes and fees. For long horizons, inflation can make a large future number feel less impressive in today's money. Use the result as a planning estimate, then sense-check it against inflation and the actual costs of the account or fund.

Common mistakes with compound interest projections

• Using a guaranteed-looking rate: market returns vary, and a straight-line annual return is only a modelling assumption.
• Ignoring fees: a 1% fee can remove a meaningful share of the final balance over decades.
• Forgetting tax wrappers: ISA, pension, and taxable accounts can produce different after-tax outcomes.
• Comparing different timeframes: a high total return over many years may be weaker than it first appears when annualised.

How to use the output

Treat the output as a scenario, not a promise. Try a conservative return, a middle case, and an optimistic case. Then compare how much of the ending balance comes from contributions versus growth. If the plan only works with an aggressive return assumption, the savings rate, timeframe, or target may need adjustment.

Worked scenario: starting early vs catching up later

Compound interest is easiest to understand by comparing two savers. One starts with a smaller monthly contribution in their twenties and gives the money decades to grow. Another waits until later and contributes more aggressively. The late starter may still build a strong balance, but they usually need higher contributions because they have fewer compounding years. This is why the calculator is useful for testing not just "what return do I need?" but also "what happens if I start now?"

What to compare between scenarios

• Ending balance: the headline result, but not the only one that matters.
• Total contributions: how much of the final value came from your own deposits.
• Investment growth: the amount created by returns and compounding.
• Time horizon: the variable that is hardest to replace once it is gone.
• Assumed return: the input most likely to be wrong if it is too optimistic.

Before using the result in a plan

Check whether your return assumption is nominal or after inflation, whether fees are included, and whether contributions are realistic during weaker months. A plan that depends on perfect consistency may look good in a calculator but fail in real life. Build in room for tax, emergencies, and periods when contributions pause.

Related guides and tools

• Compound Interest Examples UK — Real examples covering fees, pensions, delayed starts, and ISAs
• Investment Growth Calculator — Model longer-term portfolio growth with additional assumptions
• The 5% Rule Rent vs Buy Guide — Useful when comparing investing a deposit versus putting it into property