If you are trying to grow your savings in the UK, a Cash ISA can be a smart home for your money. Use this free calculator to estimate how much interest you could earn over time based on your starting balance, monthly top-ups, rate, and time horizon.
Cash ISA Interest Calculator
This calculator provides estimates only and is not financial advice. Rates on Cash ISAs can change, and terms vary by provider.
What is a Cash ISA?
A Cash ISA (Individual Savings Account) is a UK savings product where interest is paid tax-free. For many savers, that means all interest stays in your account and can continue compounding year after year.
The value of the tax shelter can be meaningful over long periods, especially if rates stay elevated or your balances become large.
How this Cash ISA calculator works
This tool estimates your future balance by combining:
- Your initial deposit
- Regular monthly contributions
- Your annual interest rate (converted to a monthly rate)
- Your chosen number of years
It also shows an estimated “tax saved” figure by comparing potential interest tax you might owe in a taxable account (based on your entered tax rate).
Formula concept
At a high level, each month your balance grows by interest, then your new monthly contribution is added. Repeating that process over many months creates compound growth.
Why compounding matters
Compound interest means you earn interest not just on your contributions, but also on interest earned in prior periods. The longer your timeline, the more this snowball effect can dominate results.
Even modest monthly deposits can build significant balances when paired with consistency and time.
Cash ISA planning tips
- Watch the annual ISA allowance: The current overall ISA allowance is typically £20,000 per tax year.
- Compare variable vs fixed rates: Fixed products may offer certainty; variable products may rise or fall.
- Check withdrawal rules: Some accounts are flexible; others apply penalties for early access.
- Review rates regularly: Moving to a better rate can significantly improve long-term outcomes.
- Automate contributions: Standing orders help maintain saving discipline.
Example use case
Suppose you start with £1,000, add £200 per month, earn 4.5% AER, and keep going for 10 years. The calculator will break down:
- Total amount you contributed
- Total interest earned
- Projected final balance
- Estimated tax avoided by using an ISA wrapper
Important assumptions and limitations
Rates can change
Many Cash ISA products are variable-rate, so actual returns may differ from this estimate.
Inflation is not included
The calculator shows nominal growth, not inflation-adjusted purchasing power.
Personal Savings Allowance varies
Your real-world tax position can differ based on your Personal Savings Allowance, income bracket, and other factors. Treat the tax estimate as directional only.
Final thought
A cash ISA interest calculator is a practical way to turn vague savings goals into a concrete plan. Try a few scenarios, test different monthly contributions and rates, and pick a strategy you can sustain consistently.