Compound Interest Calculator – Savings Growth Over Time
See how your money grows over time with compound interest. Enter your starting amount, monthly contributions, and expected interest rate to visualise your savings journey.
Compound interest is one of the most powerful forces in personal finance. Unlike simple interest—which only pays returns on your original deposit—compound interest earns returns on your returns. Over time, this creates an exponential growth curve that can turn modest regular savings into a significant sum.
In the UK, compound interest applies to most savings accounts, Cash ISAs, and investment returns (through reinvested dividends and capital growth). The key variables are your starting amount, how much you add each month, the interest rate, and—crucially—how long you leave your money to grow. Even a small difference in rate or time horizon can produce dramatically different outcomes.
This calculator lets you experiment with different scenarios so you can make informed decisions about where to save, how much to contribute, and how long you'll need to reach your financial goals. Whether you're saving for a house deposit, building a retirement pot, or simply growing your wealth, understanding compound interest is essential.
Compound Interest Calculator
Why this final figure looks bigger than expected
Most of the growth shown here doesn't come from your contributions — it comes from interest earning interest. In a typical 30-year scenario, contributions might make up only 30–40% of the final pot; the rest is pure compounding. That's why starting just 5 years earlier can be worth more than doubling your monthly contribution later.
The big risks to this projection are inflation (which eats into the real value of the final figure) and taxation on growth. Sheltering this money inside an ISA or pension protects every penny of compounded growth from HMRC — a difference that's often worth six figures over a working life.
Why Compound Interest Matters
Albert Einstein allegedly called compound interest "the eighth wonder of the world." Whether or not he actually said it, the principle is genuinely powerful. The longer your money stays invested, the more dramatic the effect becomes—because each year's interest earns its own interest the following year.
Consider this: £10,000 invested at 5% for 10 years grows to about £16,470. But over 30 years, that same £10,000 becomes £43,219—even without adding a single penny more. The growth in the final 10 years alone (£26,749) dwarfs the growth in the first 10 years. That's the compounding effect in action.
This is why financial advisers constantly stress the importance of starting early. Someone who invests £200/month from age 25 to 35 (10 years, then stops) can end up with more at retirement than someone who starts at 35 and invests £200/month for 30 years—despite contributing three times less money.
Start Early
Time is the most powerful factor. Even small amounts invested early can outperform larger amounts invested later.
Stay Consistent
Regular monthly contributions, even modest ones, create a powerful compounding engine through pound-cost averaging.
Reinvest Returns
Don't withdraw interest or dividends. Reinvesting them is what creates the exponential growth curve.
Savings vs Investing
In the UK, "saving" and "investing" mean different things, and each benefits from compound growth in its own way. Cash savings accounts offer guaranteed returns with FSCS protection up to £85,000—but rates tend to barely keep pace with inflation over the long term.
Investing in stocks, funds, or ETFs offers higher potential returns (historically 7-8% per year for global equities) but comes with volatility. Over shorter periods, you could lose money. Over longer periods (10+ years), the probability of positive returns increases significantly—making investing more suitable for long-term goals like retirement.
A balanced approach often works best: keep an emergency fund in cash savings (3-6 months' expenses), then invest the rest in tax-efficient wrappers like ISAs or pensions. Both benefit from compound growth, but investing gives your money the best chance of beating inflation over time.
How Small Contributions Add Up
Many people delay saving because they feel they can't afford to put away enough to make a difference. But even £50 per month at 5% grows to over £41,000 in 30 years—despite only contributing £18,000. The remaining £23,000+ comes entirely from compound interest.
Increasing your contribution by just £25/month can add tens of thousands over a lifetime. If you're paid a salary, consider setting up a standing order on payday so the money goes into savings before you have a chance to spend it. Many people find they don't even miss it after the first month.
For workplace pensions, salary sacrifice can be particularly effective. Your employer contributes before tax and NI are deducted, meaning you get more invested for less net cost. Use our Salary Sacrifice Calculator to see the real impact on your take-home pay.
Frequently Asked Questions
What's Next?
Not sure which calculators you need?
Answer 7 quick questions and we'll show you exactly which UK costs apply to your situation. Takes under 60 seconds.