Stocks & Shares ISA Calculator (UK — 2026/27 Allowance)

A UK Stocks & Shares ISA is the most tax-efficient general-purpose investment account available to UK retail investors. The 2026/27 annual allowance is £20,000 — unchanged since the 2017/18 tax year, when George Osborne lifted it from £15,240. Everything held inside the wrapper grows free of UK income tax, dividend tax, and capital gains tax, with no minimum withdrawal age and no penalty on access. This calculator projects how much your S&S ISA will be worth in 5-40 years, quantifies the tax saved versus a general investment account, and lets you stress-test the impact of fees, contribution rate, and real-vs-nominal return assumptions.

Why the S&S ISA is the UK-specific compounding sweet spot

The £20,000 annual allowance is unusually generous by international standards. The US Roth IRA caps at $7,000/year (2026, plus a $1,100 catch-up over 50). The South African TFSA caps at R36,000/year with a R500,000 lifetime cap. The Canadian TFSA caps at C$7,000/year (2026, indexed annually). None of those wrappers offer the combination the UK S&S ISA does: a large annual cap (£20,000), no lifetime cap, no minimum withdrawal age, full tax-free growth, and tax-free withdrawals at any age.

The UK has two other tax-advantaged investment wrappers, and the relative strength of the S&S ISA is clearest in contrast with them:

• Lifetime ISA (LISA): £4,000/year cap (which counts within the £20,000 ISA total, not on top of it), 25% government bonus (up to £1,000/year of free money). But: penalty-free access is restricted to a first home up to £450,000 or age 60. Any other withdrawal incurs a 25% charge that more than wipes out the bonus. LISA is a 60+ retirement vehicle or a first-home savings vehicle, not a general-purpose investment account.
• SIPP / Workplace Pension: £60,000/year annual allowance, marginal-rate tax relief on contributions, but funds are locked until the Normal Minimum Pension Age (55 today, rising to 57 on 6 April 2028). Best mathematical return for higher-rate taxpayers because of the 40% relief on the way in, but the locked-until-NMPA constraint disqualifies it as a flexible vehicle.

The S&S ISA is the only fully-flexible tax-free wrapper. For any savings goal with a 5-year-plus horizon that isn't a defined retirement plan, it's the default account.

The S&S ISA growth maths

The calculator implements the standard future value of an annuity formula, which is the workhorse of regular-contribution investment projections:

FV = P × (1 + r/n)^(nt) + PMT × [((1 + r/n)^(nt) − 1) ÷ (r/n)]

Where P is your starting balance, PMT is your monthly contribution, r is the annual rate, n is the number of compounding periods per year (12 for monthly), and t is the time horizon in years. The calculator runs monthly compounding by default, which matches how UK investment platforms (Vanguard, iShares, Fidelity, AJ Bell, Hargreaves Lansdown, Interactive Investor) typically report performance.

Realistic UK return assumptions

The most common mistake in S&S ISA projections is plugging in a nominal return — for example, the FTSE All-Share's ~7-8% nominal long-run average — and then comparing the terminal balance to today's prices. That mixes apples and oranges; a £1 in 2056 is worth far less than a £1 in 2026. The calculator exposes a real-vs-nominal toggle precisely so you can avoid this:

• UK equities (FTSE All-Share, since 1900): ~5% real, ~7-8% nominal. Source: Barclays Equity Gilt Study.
• Global equities (MSCI World, GBP-hedged, since 1970): ~5.5% real, ~7.5-9% nominal.
• 60/40 global blend: ~3.5-4.5% real, ~6-7% nominal.
• UK gilts (intermediate): ~1-2% real, ~4-5% nominal.
• Cash ISA (current rate cycle): 4-5% nominal, ~1% real after a 3-4% inflation environment.

The headline finding from these assumptions: contribute the full £20,000 ISA allowance every year at 5% real return for 30 years (equivalent to £1,666.67/month, monthly compounded) and the terminal balance is approximately £1,387,000 in today's pounds — with £600,000 of that being your own contributions and £787,000 being tax-free growth. UK investors who want to model the same wrapper holding a single US-market tracker (rather than a global blend) should run the dedicated UK S&P 500 calculator, which applies the 0.05-0.07% OCF of VUAG/CSPX/SPXP and the GBP/USD currency drag on top of the compound-growth engine.

The tax-saved-by-ISA quantification

Outside an ISA, the same £20,000/year invested in a general investment account (GIA) would have paid tax three different ways. Assume 5% nominal growth split as 2% dividend yield + 3% capital appreciation, the post-April-2026 UK tax regime (10.75% / 35.75% / 39.35% dividend tax for basic / higher / additional rate; 18% / 24% CGT on shares for basic / higher rate; £500 dividend allowance; £3,000 CGT annual exempt amount):

• Higher-rate taxpayer: Dividend tax of 35.75% on the 2% dividend yield drags annual return from 5% to roughly 4.285% before exit. At a 30-year horizon, that's a pre-CGT terminal balance of about £1,217,000. On disposal, CGT at 24% (less the £3,000 AEA) reduces the figure further to approximately £1,070,000. The S&S ISA terminal at £1,387,000 is ~23% higher than the GIA equivalent — a saving of roughly £317,000 on the same contributions and the same gross investment return.
• Basic-rate taxpayer: Dividend tax of 10.75% drags annual return to about 4.785%; CGT at 18% on disposal. Pre-CGT terminal balance ~£1,333,000, post-CGT ~£1,202,000. The S&S ISA edge here is ~13%, or roughly £185,000 saved over the 30-year horizon.

The range depends on dividend yield (a global equity ETF at 1.5% yield reduces drag versus a UK income fund at 4% yield), portfolio turnover (passive index funds trigger less CGT than active funds), and your marginal tax band. The headline figure: the S&S ISA wrapper typically saves 13-25% of terminal value over a 30-year accumulation versus a GIA, with the higher savings concentrated on higher-rate taxpayers and high-yield portfolios.

Allowance utilisation scenarios

A single saver using their full £20,000 ISA allowance every year of working life (age 22 to 55, 33 years) at 5% real return ends with a pot of approximately £1.6 million in today's pounds — well within typical PLSA Comfortable retirement spending territory.

A couple maxing both partners' £20,000 allowances saves £40,000/year tax-sheltered (the LISA component sits inside each partner's £20,000, it doesn't stack on top, so the combined ISA-only allowance is exactly £40,000/year). Over 33 working years at 5% real return with monthly contributions of £3,333.33, the terminal pot is approximately £3.35 million in today's pounds. Stacked with the £60,000/year SIPP allowance per person (£120,000 combined, though most savers do not use the full SIPP cap), the per-couple tax-protected investment envelope can reach £160,000/year — orders of magnitude beyond what any taxable account could deliver on the same contribution and the same gross return.

Sequence-of-returns risk during accumulation vs decumulation

A persistent insight from retirement-research literature (Bengen, Trinity Study, Pfau, Karsten Jeske / ERN) is that the same volatility hurts decumulation and helps accumulation. A 30% market decline in years 1-5 of a 30-year S&S ISA accumulation is mathematically beneficial: you buy more units of the underlying funds at lower prices via pound-cost averaging, and the eventual recovery applies to a larger unit base. The same 30% decline in years 25-30 — when contributions have stopped and withdrawals have begun — is catastrophic, because it locks in losses on the decumulation side.

The practical takeaway for S&S ISA savers in their 20s and 30s: market drawdowns are friends. The practical takeaway for S&S ISA savers approaching age 55: the portfolio's volatility profile matters far more than the absolute return assumption, and a glide-down towards bonds in the final 5-10 years before drawdown is the standard mitigation.

Cash ISA vs Stocks & Shares ISA — when each makes sense

The Cash ISA versus Stocks & Shares ISA decision is fundamentally a time-horizon question. Equity markets carry enough drawdown risk in any 3-5 year window to be inappropriate for short-term goals; cash carries enough inflation drag over 10-20 year windows to be inappropriate for long-term goals. For under-5-year horizons, the safer-cousin variant of this calculator — the UK Cash ISA growth calculator — projects nominal and real returns at current 4-5% best-buy rates.

The rule of thumb most UK financial advisers converge on:

• Horizon under 5 years: Cash ISA (4-5% nominal in the current rate cycle, fully protected by FSCS up to £85,000 per institution, no capital loss risk). Suitable for an emergency fund, a house deposit being saved in the next 24-36 months, or a known near-term expense.
• Horizon 5-10 years: Grey zone. A mixed allocation works (e.g. 60% cash / 40% equity), or a multi-asset fund inside a S&S ISA.
• Horizon 10+ years: Stocks & Shares ISA, with global equity exposure as the core holding. At a 5% real return over 30 years, the same £1,666.67/month contribution that produces £1,387,000 in a S&S ISA produces only about £699,000 in a Cash ISA earning 1% real — roughly half the terminal balance.

Platforms to compare on cost: Vanguard Investor (0.15% platform fee capped at £375/year, low-cost LifeStrategy funds at 0.22% OCF), iShares Core (via Interactive Investor / AJ Bell), Fidelity Personal Investing (0.35% platform fee), and Hargreaves Lansdown (0.45% platform fee — higher but with the most extensive fund range). The FCA's platform charges guidance is the authoritative consumer reference for fee comparison.

Fee drag: the underweighted variable

Fees compound just like returns, except they compound against you. At £1,666.67/month for 30 years with a 5% gross real return:

• 0% fees (theoretical): £1,387,098 terminal balance
• 0.5% combined fee drag (platform + OCF): £1,265,644 terminal — a 9% reduction in terminal balance for a small-looking 0.5% annual cost
• 1% combined fee drag: £1,156,749 terminal — a 17% reduction
• 1.5% combined fee drag (typical of older active funds with platform): £1,059,000 terminal — a 24% reduction

For an investor maxing the £20,000 ISA allowance over a 30-year accumulation, the difference between a 1% all-in fee and a 0.25% all-in fee compounds to roughly £168,000 of terminal wealth. The calculator does not enforce a fee assumption — it expects you to input the net real return after fees — but the relationship between fee level and terminal balance is the most underweighted variable in S&S ISA projections. A typical 2026 low-cost setup runs at 0.15-0.20% platform + 0.05-0.22% fund OCF, totalling 0.20-0.45% combined. Anything materially above 0.75% combined warrants scrutiny.

Common S&S ISA mistakes

Five patterns recur in the financial-adviser caseload that the calculator can help surface before you commit to them:

1. Holding cash in a S&S ISA wrapper. Cash deposits inside a S&S ISA earn near-zero interest (because the platform is the custodian, not a bank); if you want cash exposure, use a Cash ISA. Idle cash in a S&S ISA over a 5-year horizon costs you roughly 25% of real terminal value vs being deployed in equity.
2. Concentrating in a single country or sector. Home-country bias to UK equity, or thematic concentration in tech/AI/ESG sectors, is the single largest source of investor underperformance vs the global market portfolio. A global equity index fund (FTSE All-World, MSCI ACWI) is the default low-cost diversification.
3. Performance-chasing. Switching funds every 12-24 months based on recent returns reliably underperforms a buy-and-hold by 1.5-2% per year (the Dalbar QAIB studies show this consistently). The S&S ISA wrapper makes switches free of tax — which is a feature not to abuse.
4. Confusing dividend reinvestment with new contributions. Reinvested dividends inside a S&S ISA do not count against the £20,000 annual allowance — they are part of the wrapper's growth. New contributions are separate from reinvestments. The calculator treats both correctly when you input expected return and monthly contribution as distinct fields.
5. Not using the allowance. The £20,000 allowance is use-it-or-lose-it — unused, it is gone at midnight on 5 April each tax year. Over a 30-year window, the difference between consistently maxing and routinely under-using the allowance is typically £500,000-£1,000,000 of terminal balance.

Calculator inputs and outputs

The calculator takes five inputs: starting ISA balance, monthly contribution (capped at £1,666.67 to enforce the £20,000 annual allowance), expected annual return percentage, time horizon in years, and a real-vs-nominal toggle. It outputs five figures: final balance, total contributions, tax-free growth (the difference), the tax saved vs a higher-rate GIA equivalent (assuming a 32% effective tax drag over a 30-year horizon — combining dividend tax on the yield component and CGT on disposal of the capital-gain component), and a year-by-year balance table. The toggle for real-vs-nominal lets you express the terminal value either in today's purchasing power or in nominal future pounds; long-term plans should always be sanity-checked against the real figure.

For underlying rules and current allowances, HMRC's ISA Manager Reference Manual on gov.uk is the authoritative regulator reference. The FCA's Conduct of Business sourcebook governs platform charges and consumer-disclosure rules. The Barclays Equity Gilt Study (published annually) is the standard reference for historic UK equity and gilt returns.