How to Calculate ISA Growth in the UK: Step-by-Step Guide

Introduction

Interest works for you when you save and against you when you borrow. Seeing the numbers laid out helps you decide where each pound does the most good.

This guide explains how to calculate iSA Growth in the UK, step by step. Project the tax-free growth of your Stocks and Shares or Cash ISA. You'll see the formula, a fully worked example and practical tips for UK readers.

What this calculation covers

Project the tax-free growth of your Stocks and Shares or Cash ISA.

Small differences in rate, term or regular contributions compound over years. Understanding them helps you pay off debt faster and grow savings more efficiently.

The method and formula

Each year's contribution is added at the start of the year and grows at your chosen rate. Growth inside an ISA is free of income tax and CGT. The annual allowance is £20,000.

Every figure on our iSA Calculator follows this method, so you can check the working yourself.

Worked example

Here's a typical example using these figures:

Running them through the method gives:

The headline figure is isa value: £66,033.94. Change any input and the result will move with it.

How to use the calculator

1. Enter or choose yearly contribution (£).
2. Enter or choose growth rate (%).
3. Enter or choose years.
4. Read your results. The main figure is shown at the top, with a breakdown underneath.
5. Try different values to compare scenarios side by side.

Tips for UK readers

• Pay off high-interest debt before saving beyond a basic emergency fund.
• Use your £20,000 ISA allowance each year where you can – it doesn't roll over.
• Check whether your savings rate beats inflation; if not, your money is losing value.
• Increasing regular contributions early has a bigger effect than larger contributions later.

Frequently asked questions

What's the quickest way to work out iSA Growth?

Use our free iSA Calculator. It applies this method automatically: Each year's contribution is added at the start of the year and grows at your chosen rate. Growth inside an ISA is free of income tax and CGT. The annual allowance is £20,000.

Are returns guaranteed?

No. Savings and investment projections are illustrations based on the rate you enter. Investments can go down as well as up.

What rate should I use for investments?

Many people test a cautious 4–5% and a more optimistic 6–7% to see a range of outcomes.

Is my savings interest taxed?

Basic-rate taxpayers have a £1,000 Personal Savings Allowance, higher-rate taxpayers £500. Interest inside an ISA is tax-free.

Summary

To work out iSA Growth, follow the method above: Each year's contribution is added at the start of the year and grows at your chosen rate. For a quick, accurate answer with your own figures, use the calculator.

Next, read ISA Growth: Common Mistakes, Tips and FAQs.