Compound Interest Calculator with Monthly Contributions

Investment details

Starting investment

£0£100,000

Monthly contribution

£ / month

£0£5,000

Annual return

% / year

−10 %20 %

Investment period

years

1 yr60 yr

Additional scenariosOptional

Adjust for inflationShow purchasing power in today’s money

Include a tax estimateApply an adjustable rate to positive gains at the end

Projected result

——Before estimated tax, nominal value

Your contributionsStarting amount plus monthly deposits

—

Investment growthProjected return before estimated tax

—

— contributions · — growth

Contributed each month—

Growth on contributions—

Time invested—

Note: Results are illustrative and use the values you enter. Actual returns, inflation and tax treatment can differ; platform, fund and currency fees are not included.

Growth forecast

How your investment could grow over time

Investment valueYour contributions

How to use the compound interest calculator

Build a realistic scenario in three simple steps.

Enter your starting amount

Add the money already available to invest. You can start from zero if you only plan regular contributions.

Set a monthly contribution and return

Use a long-term annual return assumption rather than a single unusually strong year.

Choose the time horizon

Compounding becomes more visible over longer periods because earlier returns can generate further returns.

How your investment could grow over time

What is compound interest?

Compound interest means returns are calculated on both your contributions and earlier returns. Reinvesting growth can therefore make the curve accelerate over time.

Monthly contributions and compounding

Regular investing spreads contributions across the full period. This calculator adds the contribution at the end of each month and then continues compounding the new balance.

Nominal value versus today’s money

A future balance is a nominal amount. Turning on inflation shows an estimate of what that balance could buy in today’s money.

Choosing an annual return

The return is an assumption, not a guaranteed rate. Testing cautious, central and optimistic cases gives a more useful range than relying on one forecast.

Compound interest calculator FAQ

Answers to common questions about returns, contributions and inflation.

How is compound interest calculated with monthly contributions?

The annual return is converted to an equivalent monthly rate. Each month, growth is applied to the current balance and the monthly contribution is then added.

Are contributions added at the start or end of each month?

This calculator adds them at the end of each month, which is a slightly more cautious assumption than adding them at the start.

Does the calculator include fees?

No. Fund charges, platform fees, trading costs and currency conversion fees are not included.

How does inflation change the result?

The projected balance is divided by cumulative inflation over the selected period to estimate its purchasing power in today’s money.

Is tax charged every year?

No. The optional estimate applies your chosen rate once to positive gains at the end. Annual taxation would compound differently.

Can investment returns be negative?

Yes. You can enter a negative annual return to explore a downside scenario, although real markets rarely deliver the same return every year.

How your investment could grow over time

How to use the compound interest calculator

Enter your starting amount

Set a monthly contribution and return

Choose the time horizon

How your investment could grow over time

What is compound interest?

Monthly contributions and compounding

Nominal value versus today’s money

Choosing an annual return

How the tax estimate works

Compound interest calculator FAQ

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