Cumulative Interest Calculator
Calculate your savings growth with compound interest for UK accounts
Investment Details
Your Investment Growth
Enter your details and click “Calculate Returns” to see your projected savings growth.
Understanding Cumulative Interest
Cumulative interest, also known as compound interest, is the interest calculated on both your initial principal and the accumulated interest from previous periods. This creates a snowball effect where your money grows at an accelerating rate over time.
How It Works
When you invest £1,000 at 5% annual interest compounded annually, after year one you have £1,050. In year two, you earn interest on the full £1,050, not just the original £1,000, giving you £1,102.50.
The Power of Time
The earlier you start investing, the more time compound interest has to work. Even small regular contributions can grow significantly over decades due to the compounding effect.
UK Savings Account Considerations
Annual Equivalent Rate (AER)
In the UK, savings accounts display the Annual Equivalent Rate (AER), which shows what you would earn over a year including the effect of compounding. This makes it easier to compare different savings products regardless of how often they pay interest.
Tax on Savings Interest
UK residents benefit from several tax allowances on savings interest:
- Personal Savings Allowance: £1,000 for basic rate taxpayers, £500 for higher rate taxpayers
- Starting Rate for Savings: 0% tax on up to £5,000 of interest if your income is below £17,570
- ISA Allowance: Up to £20,000 per year in tax-free savings
FSCS Protection
UK savings are protected by the Financial Services Compensation Scheme (FSCS) up to £85,000 per person per authorised institution. From December 2025, this limit will increase to £110,000.
Important Considerations
Interest rates can change, particularly for variable rate accounts. Always check the terms and conditions of your savings product. Consider inflation when evaluating real returns on your savings.
Maximising Your Savings Growth
Regular Contributions
Adding money regularly to your savings amplifies the compound effect. Even modest monthly contributions can significantly boost your long-term wealth accumulation.
Shop Around for Rates
Interest rates vary significantly between providers. Use comparison sites and check challenger banks and building societies, which often offer competitive rates.
Consider Fixed-Term Accounts
Fixed-rate bonds typically offer higher interest rates than easy-access accounts, though your money is locked away for the term period.
Frequently Asked Questions
What’s the difference between AER and gross interest rate?
Gross interest rate is the rate before any compounding effects, while AER includes the effect of compound interest over a year. AER gives you a more accurate picture of annual returns.
How often should I check my savings rates?
Review your savings rates at least annually, or when the Bank of England changes the base rate. Many providers reduce rates over time, so staying alert can help you maximise returns.
Should I prioritise ISAs or regular savings accounts?
If you’re within your Personal Savings Allowance, the choice depends on interest rates offered. However, ISAs provide tax-free growth regardless of your tax status, making them valuable for higher earners.
Can I lose money with compound interest?
With deposit accounts protected by FSCS, your capital is secure up to £85,000 per institution. However, if interest rates fall below inflation, the real value of your money may decrease over time.
How accurate are compound interest calculations?
Calculations assume consistent interest rates and contribution patterns. Real-world results may vary due to rate changes, fees, and varying contribution amounts.
References
- Bank of England. “Bank Rate.” Monetary Policy decisions and supporting material. Accessed September 2025.
- Financial Services Compensation Scheme. “Deposit Protection Limits.” FSCS protection for deposits. Accessed September 2025.
- HM Revenue & Customs. “Tax on savings interest.” Gov.uk guidance on Personal Savings Allowance and Starting Rate for Savings. Accessed September 2025.
- Prudential Regulation Authority. “CP4/25: Depositor protection.” Consultation on FSCS protection limit changes. March 2025.
- Financial Conduct Authority. “Understanding AER and interest calculations.” Consumer guidance on savings products. Accessed September 2025.
- UK Finance. “Annual Equivalent Rate (AER) Practice Note.” Industry guidance on AER calculations and disclosure. Accessed September 2025.