Compound Interest Calculator
Use this compound interest calculator to estimate how savings or investments could grow over time. Enter your starting balance, annual rate, time period, and compounding frequency. You can also add an optional monthly contribution to model more realistic long-term saving.
This page is useful for ISA planning, pension growth, emergency fund goals, investing projections, and comparing the impact of different rates, time periods, and contribution levels. It gives you a quick future value estimate without needing a spreadsheet.
Best for savings goals, investing projections, pensions, ISA growth, and long-term planning.
Calculate future value
Result explanation
The future value shows the estimated ending balance after your chosen number of years. Total contributions combines your starting balance and any monthly deposits you make over the period. Interest earned shows how much of the final total comes from growth rather than money you personally added.
The growth multiple is useful when comparing scenarios. For example, a 1.50× multiple means the final pot is one and a half times the total amount contributed. This helps you compare whether time, rate, or contributions make the biggest difference to the outcome.
How compound interest works
The core formula is A = P(1 + r/n)nt, where P is the starting balance, r is the annual rate as a decimal, n is the number of compounding periods per year, and t is the number of years.
When monthly contributions are included, each new deposit also starts earning interest. That makes regular saving especially powerful over longer periods because new money and previous gains both continue compounding.
Worked examples
Example 1: One-off investment
- Initial amount: £1,000
- Rate: 5% per year
- Time: 10 years
- Compounding: monthly
The ending balance is around £1,647.01. This is a useful simple example of how compounding grows a one-off deposit.
Example 2: Monthly saving plan
- Initial amount: £5,000
- Monthly contribution: £200
- Rate: 6% per year
- Time: 15 years
Regular contributions can push the final balance far beyond what rate alone would achieve because every monthly deposit also starts earning future interest.
When to use this calculator
Use this page when you want to estimate how a balance grows over time with compounding. For non-compounded growth, use the Simple Interest Calculator. For monthly savings planning, the Savings Calculator is a good companion page. To compare overall growth rates over a period, use the CAGR Calculator.