Investors choosing where to hold their investments face a fundamental question: ISA or a standard taxable account? The choice can mean thousands of pounds in difference over time, purely due to how tax compounds. An ISA shields your returns from Income Tax on dividends and Capital Gains Tax on growth, while a taxable account erodes your wealth year after year through the tax drag. Understanding the maths behind this difference helps you see exactly what that tax-free wrapper is worth.

The Formula in Plain Language

The core difference lies in how returns compound. In a taxable account, you pay Income Tax on dividends above the £500 annual dividend allowance and Capital Gains Tax on profits when you sell above the £3,000 annual CGT exemption (HMRC, 2026). For a higher-rate taxpayer, dividends are taxed at 33.75% and capital gains at 20%. These taxes create a drag on your compounding: instead of reinvesting the full return, you reinvest what remains after tax.

In an ISA, every penny of dividend income and every pound of capital gain stays in your account, compounding tax-free (HMRC, 2026). The formula comparing the two is straightforward. Final value in ISA = initial amount × (1 + growth rate)^years. Final value in taxable account = initial amount × (1 + growth rate × (1 - effective tax rate))^years. The effective tax rate depends on your Income Tax band, how much of your return comes from dividends versus capital gains, and whether you have used your allowances elsewhere.

The time horizon amplifies the difference. Over 10 years, the tax drag is noticeable. Over 20 years, it becomes dramatic, because you are not just losing tax on this year’s return, you are losing the compounding of all the returns you would have earned on the tax you paid in previous years. As covered in foundational texts such as Principles of Finance, the power of compound growth means small differences in annual returns produce large differences in final wealth over long periods.

A Worked Example

Consider £10,000 invested in a globally diversified equity fund returning 7% annually, with 2% from dividends and 5% from capital growth. You are a higher-rate taxpayer who has already used your dividend allowance and CGT exemption on other investments.

In an ISA, after 10 years your £10,000 grows to £19,672. After 20 years, it reaches £38,697. The full 7% compounds every year.

Read also: ISA Versus Taxable Account in the UK: The Maths Behind a Tax-Free Wrapper Over 10 and 20 Years

In a taxable account, dividends are taxed at 33.75%, so your 2% dividend return becomes 1.325% after tax. Capital gains are taxed at 20% when you sell, but assuming you rebalance annually or the fund distributes realised gains, your 5% capital return becomes 4% after tax. Your effective annual return is roughly 5.325%. After 10 years, your £10,000 grows to £16,771. After 20 years, it reaches £28,126.

The difference: over 10 years, the ISA is worth £2,901 more. Over 20 years, it is worth £10,571 more, more than your original investment. That is the compound cost of the tax drag.

If you are a basic-rate taxpayer, the gap narrows but does not disappear. Dividends are taxed at 8.75% and capital gains at 10%, so your taxable account still lags behind, just by less.

What the Wrapper Is Worth

The ISA’s advantage is not a one-off saving. It is a structural benefit that grows every year you hold the investment (MoneyHelper, 2026). The calculator lets you model your own numbers, your tax band, your expected return, your time horizon, so you can see the exact value of that tax-free wrapper for your situation. For long-term investing, the maths is unambiguous: if you have the ISA allowance available, use it.

Disclaimer: This article provides general educational information about ISAs and taxable accounts. Nexzoe is not authorised by the Financial Conduct Authority. Tax rules and allowances change each tax year. Consider speaking to an FCA-authorised Independent Financial Adviser for advice tailored to your personal circumstances. Verify current rates and allowances with HMRC or a qualified tax adviser before making investment decisions.