Compound interest calculator
Use this calculator to project how a savings pot or investment could grow over time. It combines a starting balance, regular monthly contributions, compound growth, optional annual increases to contributions, and an optional inflation adjustment so you can compare nominal and real-value results.
- The annual growth rate stays constant for the whole projection.
- Monthly contributions are added at the end of each month.
- Contribution increases are applied once each year.
- Inflation, tax, fees, and market volatility are simplified.
Your details
Enter your starting balance, contribution plan, growth rate, and time period.
Results
Growth timeline
| Checkpoint | Contribution | Growth | Balance |
|---|
- Starts with your opening balance.
- Adds monthly contributions and optional yearly increases.
- Applies growth using the compounding frequency you selected.
- Optionally converts the final balance into today’s money using inflation.
How to use this compound interest calculator
A simple way to model long-term growth from regular saving or investing.
- Enter your starting balance and the amount you expect to add each month.
- Add an annual interest rate and choose monthly or annual compounding.
- Set the projection length in years and optionally model yearly contribution increases.
- Enter inflation and switch on inflation-adjusted results if you want to compare spending power in real terms.
Assumptions
Keep these limits in mind when interpreting the projection.
- The growth rate is treated as constant across the whole period.
- Monthly contributions are assumed to be made at the end of each month.
- Annual contribution increases are applied every 12 months.
- The model does not include tax, account charges, investment volatility, or contribution limits.
- Inflation-adjusted results are a simple real-terms estimate and not a forecast of future prices.
Worked example
A practical example to sense-check the projection.
- Initial amount: £5,000.
- Monthly contribution: £200.
- Annual growth rate: 5%.
- Investment length: 10 years.
- Compounding frequency: monthly.
- Contribution increase: 3% per year.
- Inflation rate: 2% with inflation-adjusted results switched on.
FAQs
Common questions about compound growth and real-value projections
What is compound interest?
Compound interest means your balance can grow on both the money you have contributed and the growth already added in earlier periods.
Should I choose monthly or annual compounding?
Choose the option that best matches the account or investment you are modelling. Monthly compounding credits growth more often, while annual compounding adds growth once each year.
Does this include market volatility?
No. The calculator uses a fixed annual rate for planning purposes, so real returns could vary materially year to year.
What does the contribution increase field do?
It increases your monthly contribution once each year by the percentage entered, which can help model pay rises or step-ups in saving.
What is an inflation-adjusted balance?
It estimates what your final balance may be worth in today’s money after allowing for inflation over the full projection period.
Why can inflation-adjusted growth look much lower?
Even when the account balance grows in pounds, inflation can reduce how much that balance may actually buy in the future.
Can I use this for an ISA or pension illustration?
Yes. It is a general planning tool for any pot where you want to combine a starting balance, regular contributions, and a long-term growth assumption.
Are the results guaranteed?
No. Rates, returns, inflation, and contribution patterns can all change, so treat the projection as an estimate only.