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Compound interest

Compound Interest Calculator UK

Use this UK compound interest calculator to see how starting amount, monthly contributions, time, and a nominal or real return assumption can shape a future pot. It does not calculate when work becomes optional; Undefeated’s main calculator turns assumptions into your Financial Independence Year.

Question this answersHow can compounding change a future pot?
Simple termsEstimate, not adviceUses your entered inputs
  1. 01
    What it means

    Start with the plain meaning.

  2. 02
    How it affects the year

    See why the estimate can move.

  3. 03
    What to check

    See what the estimate leaves out.

UK compound interest calculator

Calculate a compound-interest estimate.

Enter a starting amount, monthly contribution, time period, and expected annual return to see how monthly compounding may shape a future pot. This page does not calculate when work becomes optional; use the main calculator for a Financial Independence Year.

Return basis
Nominal or real, as entered
Fees
Fees can reduce the return assumption before it is entered
UK wrappers
ISA and SIPP rules are not modelled here
Based on these assumptions£54,880

Returns build on the previous pot, so time and monthly contributions can change the projected pot non-linearly.

Projected pot
£54,880
Total added
£30,000
Estimated growth
£14,880
Growth share
27.1%
Monthly cadence
120 months
Contribution timing
End of month
Annual return
5.0%
Time
10 years

This result shows compounding only. It does not calculate when work becomes optional. Educational only, not financial advice. Projections are based on assumptions and do not promise future returns.

Why this matters

What this means.

Compounding makes time visible. Returns can build on earlier returns as well as on the starting amount, so small assumption changes can become easier to see over longer periods.

Year impact

How this can move the year.

A larger starting amount, higher monthly contribution, longer time period, or higher return assumption can increase the projected pot. In the main Undefeated calculator, those assumptions can affect the Financial Independence Year.

How it is used

How the calculation uses it.

This calculator uses monthly compounding, adds contributions at the end of each month, and converts the annual return assumption into an effective monthly rate. It does not include tax, platform fees, fund fees, inflation changes, ISA or SIPP wrapper rules, pension access, or changing contribution patterns.

Estimate limits

What can make the estimate change.

The estimate may change if returns, contribution timing, fees, inflation, ISA or SIPP tax treatment, access rules, or time horizon differ from the assumptions entered.

Checks

Checks before you use the number.

Use these prompts to read the estimate carefully. They are not recommendations.

  • Check that the expected annual return is being read as an assumption, not a promise.
  • Check that monthly contributions are the amount added at the end of each month.
  • Use this page to understand compounding, then use the main calculator for a Financial Independence Year.

Financial Independence Year bridge

Calculate your Financial Independence Year

Undefeated’s main calculator uses spending, contributions, invested assets, return setting, withdrawal rate, and buffer assumptions to estimate the year work becomes optional.

Calculate your Financial Independence YearSee how the year is calculated

FAQ

Questions people usually bring to this page.

What is compound interest?

Compound interest means returns can build on earlier returns as well as on the starting amount.

How does this example work?

It uses monthly compounding, adds contributions at the end of each month, and converts the annual return assumption into an effective monthly rate.

How does this relate to the Financial Independence Year?

Compounding helps explain why time, contributions, and return assumptions can move the year work becomes optional.

Is this financial advice?

No. It is educational only and not financial advice.

Read next

Read one assumption at a time, then use the calculator when you want to test the inputs.

Next in this path 3 of 4Monthly savingsMonthly savings is the investable amount added during each projected year.

Related assumptions

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