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ISA Growth & Allowance Tracker

Track this year’s £20,000 UK ISA allowance and project Cash or Stocks & Shares growth

📋 ISA Details

Allowance: £20,000 per tax year  ·  Growth: A = P(1 + r/n)nt
Tax year —
This tax year — subscriptions

Only new money paid in this tax year counts. Transfers and growth inside an ISA do not use the allowance.

£
£
£
£

From 6 April 2027, under-65s can pay at most £12,000 a year into Cash ISAs. The overall pot stays £20,000. 65+ keep the full Cash room.

Existing pots (already in ISAs)

Current balances. Do not include money you have not paid in yet. These figures do not use this year’s allowance.

£
£
Ongoing plan
£
%
%
%
%
yr

Your entries are stored in this browser only and restored next time you open the page.

📊 This year’s allowance

Remaining this tax year
—
Used this tax year
—
Days left until 5 April
—
Adjust inputs to see when this year’s remaining room would be used.

📈 Projected growth

Future combined ISA balance
—
Of which Cash ISA
—
Of which Stocks & Shares
—
New contributions in period
—
Growth / interest earned
—
📐 Rule of 72
Years for S&S pot to double
— years
At S&S rate
7%
Quick estimate: the Stocks & Shares pot roughly doubles every — years at this rate. Cash at 4% would take about — years.
Combined balance
Cash ISA only
Starting pots + new contributions

📅 Tax-year breakdown

Point New subs Unused allow. Cash S&S Total
Adjust inputs to see results

How to Use the ISA Growth & Allowance Tracker

This free UK-specific tool does two jobs: it shows how much of this tax year’s £20,000 adult ISA allowance you still have, and it projects how Cash and Stocks & Shares ISA pots could grow if you keep contributing. Figures update as you type. Nothing is sent to a server.

Step-by-Step Guide

1. Enter this year’s subscriptions

Start with money already paid into ISAs since 6 April of the current tax year:

2. Enter existing pots and a monthly plan

3. Read the results

Growth uses the standard compound formula, applied separately to Cash and Stocks & Shares:

Formula: A = P(1 + r/n)nt

Common Scenarios & Examples

Scenario: Use the leftover 2026/27 allowance before 5 April

Goal: see how much room is left this tax year and when a monthly plan would use it up.

  1. Enter Cash subscriptions of £5,000 and Stocks & Shares subscriptions of £3,000.
  2. Leave Lifetime ISA and IFISA at £0.
  3. Set a monthly contribution of £250.
  4. Read “Remaining this tax year” and the note that estimates the date the leftover room would be used.
Sample:
  • Used so far: £8,000
  • Remaining: £12,000
  • At £250 a month the leftover room lasts 48 months — longer than this tax year — so you will not use the full £20,000 unless you raise contributions or add a lump sum before 5 April.

Unused allowance does not roll over to the next tax year.

Scenario: Cash versus Stocks & Shares over 15 years

Goal: compare a cautious Cash rate with a long-term Stocks & Shares assumption.

  1. Current Cash balance £12,000, Stocks & Shares £8,000.
  2. Monthly £250 with a 40% Cash / 60% Stocks & Shares split.
  3. Cash interest 4%, Stocks & Shares return 7%, monthly compounding, 15 years.
What to look at:
  • The indigo line is the combined ISA.
  • The teal line is Cash only — usually smoother and lower over long horizons.
  • The dashed green line is money paid in (starting pots + new subscriptions). The gap above it is growth.

These rates are illustrations, not forecasts. Stocks & Shares values can fall as well as rise.

Scenario: Under-65 from April 2027 — £12k Cash + £8k S&S

Goal: keep using the full £20,000 after the Cash cap arrives.

  1. Set age band to Under 65.
  2. Leave “Spill unused Cash room into Stocks & Shares” switched on.
  3. Project at least two years so the table includes 2027/28.
From 6 April 2027 (under 65):
  • Cash ISA new subscriptions are capped at £12,000 that tax year.
  • The other £8,000 can still go into Stocks & Shares (or other non-cash ISA types).
  • Savers aged 65 or over keep the full £20,000 Cash room.

Existing Cash balances are not reduced by the cap — it applies to new subscriptions only.

Disclaimer & Limitations

Not professional advice: This calculator is an educational estimate. It is not tax, investment or financial advice. ISA rules and allowances can change. Check GOV.UK and your provider, and consider a regulated adviser for decisions that matter to you.

Assumptions: Constant rates of return, regular monthly contributions from today, no platform fees, no tax inside the ISA, and no LISA 25% bonus or withdrawal charge. Transfers between providers are ignored because they do not use the annual allowance. The April 2027 Cash ISA cap for under-65s follows the Autumn Budget 2025 announcement and could be amended before it takes effect.

Estimation only: Real Cash rates change and Stocks & Shares values are volatile. Inflation adjustment is a simple deflator, not a full real-return model. Junior ISAs (£9,000, separate pot) are not modelled here.

Bug Reports and Suggested Improvements

Please email suggestions and bug reports to: isagrowthcalc@personalfinances.me, thank you.