An ISA doesn’t change what your investments earn — it changes what you keep. This calculator runs the same contributions, growth and dividends through both wrappers: inside a Stocks & Shares ISA (no dividend tax, no CGT) and in a general investment account, where dividends above £500 are taxed yearly at the new 2026/27 rates and gains above £3,000 face CGT at exit. The result is the wrapper’s value, in pounds. Everything runs in your browser.
Same money, two wrappers
Simplified model: contributions at the start of each year (assumed within the £20,000 ISA allowance), dividends reinvested, £500 dividend allowance fully available each year, everything sold in a single year at the end with one £3,000 CGT allowance (staggering sales across tax years would reduce the CGT). 2026/27 rates. Mathematical context only — not tax or investment advice.
Why the same portfolio ends up with two different values
Outside a wrapper, tax compounds against you twice. First, dividends above the £500 allowance are taxed every year (10.75% basic / 35.75% higher in 2026/27 — both rates rose two points this April), so less gets reinvested each year. Second, when you eventually sell, gains above the £3,000 annual exempt amount face capital gains tax at 18% or 24%. Inside an ISA, both taxes are simply absent — and the gap between the two accounts grows with every year and every percentage point of yield.
What moves the number most
Dividend yield and time. A high-yield portfolio bleeds dividend tax annually — money that never gets to compound — so the wrapper is worth most to income-heavy portfolios held for decades. Low-yield growth portfolios shift the cost toward the CGT bill at exit instead. Either way, the calculator’s assumptions are deliberately kind to the GIA: one clean sale at the end with a full £3,000 allowance. Real life, with reinvested dividends eroding allowances yearly, usually makes the ISA’s case stronger.
What this calculator deliberately doesn’t include
It isolates the wrapper — one clean comparison. It assumes contributions fit within the £20,000 annual ISA allowance, dividends are reinvested, and everything is sold in one go at the end (staggering sales across tax years would reduce the CGT). It doesn’t model platform fees — those hit both wrappers equally, and our platform fee calculator handles that question separately. For the tax mechanics in detail: the dividend tax calculator for the yearly bill, and our free CGT calculator for the exit maths with HMRC’s matching rules.
About this tool
- All computation happens locally in your browser — no data is uploaded, stored or sent anywhere.
- Uses 2026/27 figures: £500 dividend allowance, dividend rates 10.75%/35.75%, CGT allowance £3,000, CGT rates 18%/24%, ISA allowance £20,000. Simplified model (single exit sale, full allowances available). Mathematical context only — not tax or investment advice. Figures refreshed each April.
When you have allowance available and the money is for investing, the ISA side of the comparison never loses — same investments, strictly less tax. GIAs exist for money beyond the £20,000 annual ISA allowance, which is exactly when this calculator’s question matters: how much is that overflow costing?
Dividends above the £500 allowance: 10.75% (basic) or 35.75% (higher) — both rose two percentage points from April 2026. Capital gains above the £3,000 annual exempt amount: 18% or 24%. Inside an ISA, both are zero.
Because dividend tax in a GIA is paid every year, and the tax paid never compounds again. A 4% yield taxed annually costs far more over 20 years than the headline rate suggests — that’s the compounding drag the calculator makes visible.
Yes — the calculator runs entirely in your browser. Nothing you type is uploaded, stored, or sent to any server.