How much can I put into an ISA each tax year?

This calculator uses a default annual ISA allowance of £20,000, which you can edit if needed. The calculator can also flag when your planned annual contributions exceed the allowance you enter.

What is the difference between a Cash ISA and a Stocks and Shares ISA?

A Cash ISA is typically used for cash savings with an interest rate, while a Stocks and Shares ISA is used for investments that can rise or fall in value. This calculator uses your entered growth rate rather than assuming a fixed return by ISA type.

Should I enter monthly contributions or annual contributions?

You can enter either or both. If you enter both, the calculator combines them into the yearly funding plan and shows whether that total breaches the allowance entered.

Does this calculator include inflation?

Yes. If you enter an inflation rate, the calculator also estimates an inflation-adjusted final value so you can compare future money in today’s terms.

What growth rate should I use?

Use a cautious rate that fits the account or investment you are modelling. For cash savings, that may reflect a quoted savings rate. For investments, many people test several scenarios rather than relying on one number.

Does the calculator guarantee future ISA returns?

No. It is only an estimate based on the figures you enter. Real returns, rates, and account rules can change.

Can I use this for long-term planning?

Yes, it is useful for scenario planning over several years, but it should be treated as an estimate rather than a forecast.

ISA Growth Calculator (UK) – Estimate ISA Value, Contributions & Growth

ISA growth calculator

Estimate how your ISA could grow over time using your current balance, planned contributions, growth rate, and investment period. You can model either a Cash ISA or a Stocks and Shares ISA and check whether your planned contributions exceed the annual ISA allowance.

UK-focused Allowance check No sign-up

Your details

Enter your current ISA balance, contributions, growth rate, and time period.

Current ISA balance

Your starting ISA value today.

Monthly contribution

Optional if you plan to add money monthly.

Annual contribution

Added once per year. Can be used alongside monthly contributions.

Expected annual growth rate

Use a cautious estimate for planning.

Investment period

How long you plan to leave the money invested or saved.

ISA type

Used for labelling and context only.

Annual contribution increase (optional)

Increases planned contributions each year.

Inflation rate (optional)

Used to estimate today’s-money value of the final balance.

Annual ISA allowance

Editable so you can model different tax-year limits.

Save inputs on this device

Results

Estimated ISA value

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Total contributions

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Starting balance plus added contributions

Total growth / interest

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Estimated gain above contributions

Inflation-adjusted value

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Shown when an inflation rate is entered

Yearly breakdown —

Year	Contributions	Growth	End balance

How this ISA calculator works

A simple way to estimate future ISA growth over time.

This ISA growth calculator estimates how your savings or investments could build over time based on a starting balance, contribution plan, and annual growth rate. It is useful for modelling both regular monthly funding and occasional yearly additions.

If you enter an inflation rate, the calculator also shows an inflation-adjusted final figure so you can compare future value in today’s-money terms. It also checks your planned yearly contributions against the ISA allowance you enter.

Assumptions

Important limitations behind the estimate.

The calculator assumes the growth rate remains constant across the period modelled. In reality, savings rates and investment returns can change, and investment values can fall as well as rise.

Monthly contributions are assumed to continue steadily throughout each year. Annual contributions are added once per year. The calculator does not include platform fees, fund charges, withdrawal timing, or changes to ISA rules.

Worked example

A simple illustration of how the calculator can be used.

Suppose you already have £8,000 in an ISA, plan to add £250 per month, expect 4.5% annual growth, and want to leave the money invested for 10 years. The calculator estimates your future balance by combining the starting amount, ongoing contributions, and compound growth.

If you also add a 2% inflation assumption, the inflation-adjusted figure gives a rough estimate of what that future balance may be worth in today’s terms.

FAQs

Common questions about ISA growth planning.

Can I enter both monthly and annual contributions?

Yes. The calculator combines them and checks the total planned annual funding against the allowance you enter.

What happens if I exceed the ISA allowance?

The results area shows a warning note so you can see that your planned annual contributions are above the allowance entered.

Does ISA type change the maths?

No. The calculation is driven by the figures you enter. ISA type is used as context for the scenario rather than to impose a built-in return.

Should I include inflation?

Including inflation helps you understand the likely buying power of your future balance rather than just the nominal value.

Is this suitable for short-term saving?

Yes, but short periods are more sensitive to the exact rate you enter, especially for cash savings.

Can growth be negative in real life?

Yes. Investment returns can vary and markets can fall. This calculator is only a scenario-planning tool.

Why is the inflation-adjusted value lower?

Because it tries to express future money in today’s terms, which reduces the headline figure when inflation is applied.

Last updated: 8 March 2026