Stocks & Shares ISA Calculator

A Stocks and Shares ISA shelters up to £20,000 per tax year of investments from income tax on dividends, capital gains tax on profits, and income tax on bond fund distributions. Unlike a Cash ISA with guaranteed returns, growth depends on market performance of the selected funds, ETFs, investment trusts, or individual shares held within the wrapper. Historical UK equity returns have averaged approximately 7-8% nominal over the long term.

Platform fees and fund charges reduce your effective return. The calculation deducts an annual percentage from the portfolio value, typically comprising a platform fee (0.15-0.45% for major platforms) plus the fund's ongoing charge figure (OCF) ranging from 0.07% for passive index trackers to 1.5%+ for active funds. On a £100,000 portfolio, the difference between 0.22% total charges (cheap index tracker on a low-cost platform) and 1.50% total charges costs approximately £1,280/year, compounding to tens of thousands over decades.

The projection models annual contributions at the ISA limit with compound growth at the assumed rate minus charges. Volatility drag means the arithmetic average return overstates actual compound growth — a fund returning +20% then -20% over two years doesn't break even (£100 becomes £96). The calculator uses geometric mean returns for more realistic projections and optionally models Monte Carlo scenarios to show the probability distribution of outcomes.