Is this before or after tax?

This is the gross return. In an ISA or 401(k)-style tax-advantaged account, growth is tax-free. In a taxable account, deduct dividend and capital-gains tax as it applies to you.

Does it account for inflation?

No — this shows nominal balance. To see real (inflation-adjusted) growth, subtract expected inflation from the annual return before entering it.

Are contributions made at the start or end of the month?

End of the month, immediately after that month's interest is applied. This is the standard convention for a savings simulation.

Is the Compound Interest calculator free?

Yes — free to use, with no signup, no limits and no personal details required. The site is supported by advertising rather than subscriptions.

Is my data saved or sent anywhere?

No. Every calculation runs in your browser, so the numbers you type are never transmitted to us or stored on a server. Closing the tab discards them.

How accurate is the result?

The maths is exact for the inputs you give it. Accuracy in practice depends on those inputs matching your real situation — fees, rate changes and edge-case rules are the usual sources of difference. It is a planning tool, not financial advice.

Can I share or save my results?

Yes. Your inputs are stored in the page URL, so you can copy the link from the address bar (or use the share button) to bookmark a scenario or send it to someone else.

Compound Interest Calculator

See how initial deposits, regular contributions and compounding grow into a final balance.

Your details

Initial deposit

Monthly contribution

Expected annual return

Years

Compounding frequency

Final balance

£132,061

after 20 years

Total contributions

£65,000

Interest earned

£67,061

Balance growth

How this is calculated

Compound interest earns interest on interest. Each period, the interest is added to the balance, so the next period's interest is calculated on a larger amount. Over long time horizons, this snowballs into dramatic growth.

The formula for a single lump sum is A = P(1 + r/n)^(nt), where P is the principal, r is the annual rate, n is the compounding frequency per year, and t is the number of years. With regular monthly contributions we simulate month by month for accuracy.

More frequent compounding produces a slightly higher return, but the difference between monthly and daily compounding is small compared with the rate itself and the length of time invested.

Who this calculator is for

This is aimed at anyone weighing up borrowing, saving or investing decisions before committing to them. It is most useful when you are comparing offers side by side, testing how sensitive a plan is to a change in rate or term, or sanity-checking a figure a lender or adviser has quoted you.

Assumptions and accuracy

Results assume the interest rate you enter stays fixed for the whole term and that payments are made on schedule. Real products may carry arrangement fees, early-repayment charges, variable rates, or different compounding intervals, all of which change the total. Use this to compare options, then confirm the exact figures with the provider.

Frequently asked questions

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