UK ISA Growth Calculator
Project cash ISA savings and compare a taxable account at the same gross interest rate. Enter a tax drag that reflects your own circumstances; investment returns and future ISA rules are not promised.
Rule year: 2026/27 · GBP · Sources checked
Projected cash ISA balance
Hypothetical cash-interest scenario, existing eligible ISA balance, no withdrawals, transfer errors, fees or variable rates. The full-year deposit check uses the 2026/27 shared £20,000 allowance only. Later years are scenarios, not a guarantee of unchanged subscription rules; cash ISA changes from April 2027 must be reviewed separately.
Cash ISA and taxable-interest comparison
Future years are hypothetical: hold entered rate, deposits and tax drag constant. Eligibility and future annual subscription rules must be checked; current £20,000 allowance is not guaranteed for later years.
| Year | ISA balance | Taxable balance | Cumulative tax |
|---|---|---|---|
| 1 | £11,621.84 | £11,537.48 | £84.37 |
| 2 | £13,308.56 | £13,124.15 | £181.04 |
| 3 | £15,062.75 | £14,761.60 | £290.40 |
| 4 | £16,887.10 | £16,451.44 | £412.86 |
| 5 | £18,784.43 | £18,195.37 | £548.84 |
For informational purposes only. This estimate is not financial advice, a tax return, a payroll calculation or a lender decision. See the supported circumstances and exclusions below.
How this calculator works
Cash ISA interest is currently sheltered within the ISA framework for an eligible account. A comparison account may have interest tax after allowances, so this tool accepts an effective tax drag rather than guessing your Personal Savings Allowance or income band. It compounds monthly using an effective annual rate, deposits at month end, and removes positive taxable-account interest tax at each modeled year end. Contributions are shown separately from interest.
Formula and rules
Monthly rate = (1 + annual rate)^(1/12) − 1. Compound balance and month-end deposits; taxable-account yearly tax = positive yearly interest × entered effective tax drag.
Assumptions and limitations
Hypothetical cash-interest scenario, existing eligible ISA balance, no withdrawals, transfer errors, fees or variable rates. The full-year deposit check uses the 2026/27 shared £20,000 allowance only. Later years are scenarios, not a guarantee of unchanged subscription rules; cash ISA changes from April 2027 must be reviewed separately.
Worked example
Start with £10,000, add nothing and enter 10% annual interest for one year. The cash ISA reaches £11,000. A 20% tax drag on £1,000 interest leaves £10,800 in the taxable comparison: a £200 difference.
- Projected cash ISA balance
- £11,000.00
- Total contributed capital
- £10,000.00
- ISA interest at entered constant rate
- £1,000.00
- Taxable-account comparison balance
- £10,800.00
Official sources and review
Reviewed against the sources below on 2026-10-07. Rates are held in versioned rule modules. Versioned rules, official source review and year-specific calculation tests.
- GOV.UK: ISA subscriptions ↗
2026/27 adult ISA limit £20,000 shared across account types, with £4,000 separate Lifetime ISA cap.
- GOV.UK: cash ISA changes from April 2027 ↗
Current 2026/27 projection assumptions are not a promise of future unchanged cash ISA subscription limits.
Common questions
Does the calculator decide whether I qualify for an ISA?
No. Eligibility, residence and account terms must be established. The balance is assumed to be legitimately held in an ISA.
Should I enter my marginal tax rate as tax drag?
Only if that reflects the interest actually taxable after your allowances. A user-entered effective rate can be zero; Savings Interest Tax handles supported tax rules separately.
Will the current cash ISA limit apply for the whole projection?
No such promise is made. The current-year check is 2026/27; future deposits and returns are hypothetical and rules change from April 2027.