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.

UK-friendly No sign-up Optional local save
What this assumes
  • 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.
Use the result as a planning estimate rather than a guaranteed forecast.

Your details

Enter your starting balance, contribution plan, growth rate, and time period.

Use the amount you already have saved or invested.
Add the amount you expect to save or invest each month.
Use an annual growth rate before tax and fees.
Enter the number of years you want to project forward.
Choose how often growth is added to the balance.
Use this to model increasing monthly contributions over time.
Use this only if you want to compare spending power in today’s money.

Results

Final balance
—
Projected value at the end of the term
Total contributions
—
Initial amount plus all added contributions
Total interest earned
—
Estimated growth above your contributions
Inflation-adjusted balance
—
Shown in today’s money when enabled
Growth timeline —
Checkpoint Contribution Growth Balance
How this calculator works
  • 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.
Calculate to see your compound growth projection
Enter your assumptions and press Calculate. Results appear here without shifting the layout.

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 and interpretation

Compound growth illustrations are most useful when the rate assumption is realistic rather than optimistic.

The calculator assumes the annual rate you enter remains steady across the period and that regular contributions continue as planned. Compounding means each period builds on the one before, so longer timeframes and higher contribution levels usually matter more than trying to fine-tune the final decimal places.

It does not account for tax, account fees, market volatility, or the possibility of negative returns in some periods unless you model those through a lower average rate.

Worked UK example

Example only. Investment and savings returns can vary materially in real life.

Example: starting with £5,000, adding £200 each month and assuming 5% annual growth for 15 years. The calculator shows how much of the final value comes from your own contributions and how much comes from growth building on prior growth over time.

How the Calculation Works

The calculator projects how a pot can grow when returns are earned on both your money and prior growth.

The tool starts with your opening amount, adds any regular contribution and applies the annual rate you entered over the chosen timeframe. Because each period builds on the last, the growth itself starts generating more growth, which is the core compounding effect.

That is why contributions made earlier usually have more impact than the same cash added much later.

Limitations

Constant-rate compounding is a useful model, but real outcomes are usually less smooth.

Investment returns can vary sharply from year to year, and cash savings rates can move quickly. This tool also does not incorporate wrapper rules, tax treatment or fees unless you adjust the rate to reflect them.

Use it for long-term scenario planning rather than as a forecast.

What to Do Next

Use the projection to see whether time, contribution level, or return assumption is doing most of the work.

Compare this model with the ISA Growth Calculator and Pension Growth Calculator if you want wrapper-specific planning. For target-based saving, also use the Savings Goal Calculator.

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.

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