Want to know how your savings could grow over time?
Our free Compound Interest Calculator UK helps you visualise the future of your savings, ISAs, and investments.
Whether you’re saving in a Cash ISA, investing through a Stocks & Shares ISA, or simply building an emergency fund, this tool lets you see how regular saving and compounding can transform your balance over the years.
Enter your current savings, monthly contributions, and expected return rate — and you’ll instantly see how your money could grow over 5, 10, or 20 years.
What Is Compound Interest?
Compound interest is the process of earning interest on both your original money and the interest it’s already earned. Over time, that creates an accelerating effect — your savings start to grow faster each year.
It’s one of the most powerful forces in personal finance because it rewards time and consistency, not just high returns.
Example:
If you save £200 a month at 5% annual growth, after:
- 1 year you’ll have around £2,460
- 10 years you’ll have around £30,000
- 20 years you’ll have around £79,000
The difference isn’t just the amount you’ve saved — it’s the power of your money earning interest on itself.
How to Use the Calculator
1. Enter your current savings
Add what you already have saved or invested.
Example: £10,000 in a Cash ISA or £20,000 in a Stocks & Shares ISA.
2. Add your monthly contributions
Enter how much you plan to save or invest each month. Even £50–£100 per month can make a big difference when compounded over years.
3. Adjust your expected annual return
Here are typical long-term averages in the UK:
- Cash savings: 2–3% (depends on interest rates)
- Stocks & Shares ISAs: 5–7% (historical average with balanced index funds, but returns vary)
You can adjust these figures to match your own expectations or risk level.
4. View your projections
You’ll see clear, visual projections showing your estimated balances after 5, 10, and 20 years — with pie charts comparing where your money is coming from (initial savings, monthly contributions, and growth).
5. Experiment with scenarios
Try changing your monthly savings or return rate to see how small adjustments can create big long-term differences.
📈 Why Compound Interest Matters
Compound interest is the foundation of wealth-building for UK savers and investors.
Here’s why it matters:
- It rewards time, not timing. You don’t need to predict markets — you just need to start early and stay consistent.
- It protects your future buying power. With UK inflation averaging around 2–3%, your money needs to grow just to keep its real value.
- It works in ISAs. Using your annual £20,000 ISA allowance means your compounded growth is completely tax-free — no income tax, no capital gains tax.
If you haven’t yet set up an ISA, check out:
Example Projection: Long-Term ISA Growth
Let’s compare saving £300 a month for 20 years across different types of accounts:
| Account Type | Annual Return | 20-Year Total (approx.) |
|---|---|---|
| Cash ISA (3%) | ~£3,000 growth | £87,000 |
| Stocks & Shares ISA (6%) | ~£27,000 growth | £115,000 |
| Stocks & Shares Lifetime ISA (6% + bonus) | ~£9,000 government bonus + growth | £125,000+ |
(Assumes consistent saving and reinvestment of growth. Figures are estimates.)
How to Make the Most of Compounding
- Start early – The sooner you begin, the more years your savings have to grow.
- Automate contributions – Set up a standing order each payday to “pay yourself first.”
- Reinvest interest or dividends – Let your returns keep earning more.
- Use your ISA allowance – Keep your growth tax-free.
- Review yearly – Adjust contributions if your income changes.
For more on automation and smart saving habits, see Pay Yourself First: The Easiest Way to Build Wealth in the UK.
FAQs About the Compound Interest Calculator
Q1: Is this calculator accurate for UK ISAs and savings accounts?
Yes — the calculator uses the same compound interest formulas that banks and investment platforms apply. It’s a projection tool, not a guarantee, but it’s designed for UK savers and ISA holders.
Q2: What’s a good annual return to use?
For realistic results:
- Use 2–3% for Cash ISAs or savings accounts.
- Use 5–7% for long-term stock market investments (balanced portfolios).
Q3: Should I focus on saving more or chasing higher returns?
Both help, but consistent saving matters more early on. Compounding needs time to work, so even modest returns on regular savings can lead to significant growth.
Explore More Tools
- Budget Checker – See where your money goes
- Inflation Calculator – Measure real returns
- Premium Bonds Calculator – Estimate your prize odds
- Retirement Calculator – Plan your long-term income
Or visit the full Investment Tools Hub to explore everything in one place.
Disclaimer
The Compound Interest Calculator UK is for educational and illustrative purposes only. It uses standard compound interest formulas but cannot predict actual investment returns.
Your results may vary depending on market performance, interest rate changes, inflation, and personal circumstances.
Always do your own research or seek financial advice before making major investment decisions.