UK ISA Calculator & Decision Engine — 2025/26

PlainISA gives you a free interactive UK ISA calculator and decision engine for the 2025/26 tax year. Enter your annual contribution, time horizon, ISA type, and expected return — the calculator projects your balance year by year, showing the impact of the £20,000 annual allowance, the 25% Lifetime ISA bonus (up to £1000/year), and compounding at your chosen return rate. The decision engine asks six short questions about your goal, time horizon, age, and risk tolerance, then recommends a Cash / S&S / Lifetime / Junior split with the reasoning shown.

The total adult ISA subscription allowance for 2025/26 is £20,000. You can split this across Cash, Stocks & Shares, Lifetime, and Innovative-Finance ISAs in any combination, provided the Lifetime ISA element does not exceed £4,000. The Junior ISA allowance is separate: £9,000 per child, on top of the adult allowance — so a parent could subscribe £20,000 to their own ISA and £9,000 to each child's JISA in the same year. The allowance does not roll over; the unused portion at midnight on 5 April is gone.

The government tops up your Lifetime ISA contributions by 25%. Contribute £4,000 in a tax year and HMRC adds £1,000 — paid monthly into the account, not at year-end. The bonus is paid on contributions only (not on transfers in from old ISAs unless they count as new subscriptions). The catch is the 25% withdrawal charge: take money out for any reason other than a first-home purchase under £450,000, age 60+, or terminal illness, and HMRC claws back 25% of the entire balance — which is more than the original 25% bonus because the charge is applied to the topped-up amount.

It depends almost entirely on time horizon and your tolerance for short-term losses. Cash ISAs are the right choice for money you might need within 5 years and for any pot you cannot afford to see drop — the headline rate is guaranteed (currently around 4.5% for fixed-rate accounts) and FSCS-protected to £85,000 per institution. Stocks & Shares ISAs have averaged roughly 5% real returns over 30-year windows (FTSE All-Share Total Return Index, inflation-adjusted) but with substantial year-to-year volatility — they have lost 30% or more in single calendar years and recovered within 18-36 months. The decision engine quantifies this trade-off using your specific inputs; the calculator lets you stress-test different return assumptions.

Yes. Inside an ISA, interest, dividends, and capital gains are not taxable and do not need to be reported on a Self Assessment return. Outside an ISA, basic-rate taxpayers pay 8.75% on dividends above the £500 dividend allowance, higher-rate taxpayers 33.75%, and additional-rate taxpayers 39.35%. Capital gains above the £3,000 annual exempt amount are taxed at 18% (basic rate) or 24% (higher rate) on shares. For long-horizon investors in higher tax bands the ISA wrapper is materially valuable — a £20,000 contribution returning 5% real for 30 years compounds to roughly £86,000 inside an ISA versus roughly £68-72,000 in a taxable general investment account once dividend and CGT are deducted.

Yes. Since 6 April 2024 you can open and contribute to multiple ISAs of the same type in the same tax year — for example, two Cash ISAs with different providers — provided your total contributions stay within the £20,000 overall allowance. The Lifetime ISA is the exception: you may still only subscribe to one Lifetime ISA per tax year, and you cannot hold more than one Lifetime ISA at a time (a transfer is required to switch providers).

No. Every calculation runs in the browser; nothing is sent to a server. We have no account, no email field, no signup. Aggregate page-view metrics are collected via Umami analytics — no cookies, no personal identifiers, no third-party tracking pixels. The decision engine results are computed and rendered without leaving your device.