Precision Utility
Compound Interest
Calculator
Strategy
Rule of 72
Principle
Power of Compounding
See how your savings and investments grow over time with the power of compound interest. Enter your initial deposit, monthly contribution, interest rate and investment period — the calculator shows your future value, total contributions, interest earned and a full year-by-year growth schedule.
Investment Details
Future Value
£0
Total Contributions
£0
Total Interest Earned
£0
Future Value
£0
Total Deposited
£0
Interest Earned
£0
Rule of 72
— yrs to double
How the compound interest calculator works
Start by entering your initial deposit — the lump sum you're investing or saving today. Then set your monthly contribution, the amount you plan to add every month going forward.
Next, enter the annual interest rate and choose how often it compounds — monthly, quarterly or annually. Monthly compounding earns you slightly more because interest is calculated and reinvested more frequently.
Set your investment period (1–40 years) and hit calculate. The calculator shows your total future value, how much of that is your own contributions and how much is earned interest. The payment structure bar gives you a visual split at a glance.
Open the year-by-year growth schedule to see exactly how your balance builds over time. In the early years most of your balance is contributions — over time, compound interest takes over and accelerates your growth.
What you need to know about compound interest
Compound interest is fundamentally different from simple interest. Simple interest only earns on your original deposit. Compound interest earns on the deposit plus all previously earned interest — so your money grows exponentially rather than linearly.
Key facts for UK savers:
- The annual ISA allowance is £20,000 — interest earned inside an ISA is completely tax-free
- Outside an ISA, the Personal Savings Allowance lets basic-rate taxpayers earn £1,000 in interest tax-free (£500 for higher-rate)
- Compound frequency matters — monthly compounding earns more than quarterly, which earns more than annual
- The Rule of 72 gives a quick estimate: divide 72 by the interest rate to find roughly how many years it takes to double your money
- Starting early is more powerful than saving more — a 20-year-old investing £100/month at 7% will have more at 60 than a 30-year-old investing £200/month
This calculator uses the standard compound interest formula with regular contributions. Real-world returns on investments like stocks and shares ISAs will vary year to year — the rate you enter represents an average annual return.
Frequently asked questions
What is compound interest?
Compound interest is interest calculated on both the initial principal and the accumulated interest from previous periods. Unlike simple interest, which only earns on the original amount, compound interest lets your money grow exponentially over time.
How does compound frequency affect my returns?
The more frequently interest compounds, the more you earn. Monthly compounding produces slightly more than quarterly, which produces more than annual compounding. The difference is more noticeable at higher interest rates and over longer periods.
What is the Rule of 72?
The Rule of 72 is a quick way to estimate how long it takes to double your money. Divide 72 by your annual interest rate to get the approximate number of years. For example, at 6% interest your money doubles in roughly 12 years (72 / 6 = 12).
Is compound interest taxed in the UK?
Interest earned on savings is subject to income tax, but most people can earn up to £1,000 tax-free under the Personal Savings Allowance (£500 for higher-rate taxpayers). Interest earned inside an ISA is completely tax-free, up to the £20,000 annual ISA allowance.
How much should I save each month?
A common guideline is the 50/30/20 rule — save at least 20% of your net income. Even small regular contributions benefit enormously from compound interest over time. Starting early matters more than the amount because compounding accelerates the longer your money is invested.
What is a realistic interest rate to use?
UK savings accounts currently offer around 4–5% for easy access and up to 5–6% for fixed-term accounts. For stocks and shares ISAs or index funds, a long-term average return of 7–8% before inflation is commonly used, though returns are not guaranteed and can vary year to year.