ISA Growth Projection Calculator: What a Projection Shows
Updated:
An ISA growth projection calculator takes a starting balance, regular payments, an assumed growth rate and a number of years, and returns the balance those assumptions would produce for the 2026-27 tax year. It shows what the assumptions add up to, not what an ISA will be worth: investment values can fall as well as rise. The worked example below sets three assumed growth rates side by side so the effect of the assumption is visible.
The site's rules data was last checked against gov.uk on 26 September 2026. Figures typed in this guide may not yet reflect a later change, and the descriptions of gov.uk pages are summaries that may also lag. The current figures are on the ISA rules at a glance page. General information only, not financial advice.
What does an ISA growth projection calculator need?
At its simplest, a projection uses four inputs.
- Starting balance. The value of any existing ISA savings. For a new ISA this is zero.
- Regular payment. The amount added each month or year. gov.uk says that in the 2026-27 tax year the maximum you can save in ISAs is £20,000, and that you can split the allowance across multiple accounts, so total payments across all ISAs are capped at that figure. You do not have to use all of it.
- Assumed growth rate. The most uncertain input. For a Cash ISA it is the interest rate on offer, which can change on a variable account. For a Stocks and Shares ISA it is an assumption, because returns are not guaranteed.
- Time horizon. The number of years. A longer period gives compounding more time to work, and also more time for values to fall.
The output is a projected balance, often split into money paid in and assumed growth. gov.uk says you do not need to declare any ISA interest, income or capital gains on a tax return, so an ISA projection does not take tax off the growth. A general savings calculator may not be set up that way. The site's free ISA Calculator needs no sign-up.
A worked example: one set of payments at three growth rates
This example uses an assumed starting balance of £2,000 and £350 paid in at the end of every month for 20 years. Growth is worked out monthly at one twelfth of the yearly rate, with no charges. The three rates are assumptions chosen to show the arithmetic. They are not forecasts and are not sourced averages.
| Assumed yearly growth | Total paid in | Projected balance after 20 years | Of which assumed growth |
|---|---|---|---|
| 0% | £86,000 | £86,000 | £0 |
| 4% | £86,000 | about £133,000 | about £47,000 |
| 6% | £86,000 | about £168,000 | about £82,000 |
| 8% | £86,000 | about £216,000 | about £130,000 |
Total paid in is the £2,000 start plus 240 payments of £350. The spread between the 4% and 8% rows is about £83,000 on identical payments, which shows how much of any projection rests on the rate. The 0% row is the amount paid in, with no growth at all. These are illustrations, not forecasts.
The same calculator can answer other questions. What if growth is lower? What if payments rise part-way through? What if payments start earlier or later? Running each question as a separate scenario shows the change on the assumptions used.
How can charges and inflation change a projection?
Two things often missing from a first projection are charges and inflation.
- Charges. If an ISA is on an investment platform, platform fees and fund charges reduce the return. In the worked example, allowing for an annual charge of 0.5% by using 5.5% instead of 6% gives about £158,000 instead of about £168,000, a gap of about £10,000. The rate of charges varies by provider, so the figure is only an example.
- Inflation. A projected balance is usually in future pounds. At an assumed 2% a year of inflation, the 6% result of about £168,000 after 20 years would be about £113,000 in today's money. Inflation is an assumption too, and some calculators have a setting for it.
How do projections differ by ISA type?
- Cash ISA. Interest is the return. A projection compounds the rate entered, annually or monthly depending on the account. The Cash ISA calculator models this.
- Stocks and Shares ISA. Returns come from investments and are not guaranteed. The Stocks and Shares ISA calculator takes an assumed annual return.
- Lifetime ISA. gov.uk says the government will add a 25% bonus up to a maximum of £1,000 per year, and that you can put in up to £4,000 each year (GOV.UK Lifetime ISA guide). It also says you must be 18 or over but under 40 to open one, and that a 25% charge applies to withdrawals for any other reason than those it lists (GOV.UK). The bonus is part of the balance in a projection, and the Lifetime ISA calculator includes it.
What does a growth projection leave out?
A projection is a planning illustration. It is not a forecast, a guarantee of returns or regulated advice. It cannot account for:
- Market volatility. An average of 6% does not mean 6% every year, and some years markets fall.
- Changing interest rates. Cash ISA rates can rise or fall, often with little notice on variable accounts.
- Rule changes. gov.uk sets the allowance for each tax year and the rules can change. A projection uses the rules as entered.
- Provider terms. Charges, early access penalties and eligibility conditions are outside a standard calculator.
- Changes in behaviour. Missed payments or early withdrawals are not modelled. gov.uk says you can take your money out of an ISA at any time without losing any tax benefits, but withdrawn money no longer grows.
What to check when reading a projection
- The inputs. Whether the balance, payment, rate and years match the account being modelled.
- The spread of rates. A single projection can look more certain than it is. A lower, a middle and a higher rate show the range, and a rate set much higher than the others tends to produce results that are not a guide to likely outcomes.
- Charges and inflation. Whether the result is before or after them.
- The allowance. Whether payments across all ISAs stay within £20,000 a year.
- Updating. A projection can be re-run at the start of each tax year or when circumstances change.
For several ISA types over many years, the Advanced ISA Planner is a one-off purchase of £14.99 with no subscription. The result is still an illustration.
Frequently asked questions
What is an ISA growth projection calculator?
It is a tool that estimates the balance an ISA would reach from a starting balance, regular payments, an assumed growth rate and a number of years. The result shows what those assumptions would produce, not what the ISA will be worth.
Why do growth projections show more than one growth rate?
Because the growth rate is the most uncertain input. In this guide's example, the same payments reach about £133,000, about £168,000 or about £216,000 after 20 years at assumed rates of 4%, 6% and 8%, so one figure can look more certain than it is.
Is a projected ISA balance a forecast?
No. It is the arithmetic result of the assumptions entered. Investment values can fall as well as rise, and the projection does not allow for market falls, rule changes or provider terms.
How much can a 0.5% annual charge change a 20-year projection?
In this guide's example, using 5.5% instead of 6% to allow for a 0.5% charge lowers the projected balance from about £168,000 to about £158,000. The actual effect depends on what the account charges.
This guide does not constitute financial advice. Your personal situation, tax position, and savings goals may mean different strategies are appropriate. Consider speaking to a regulated financial adviser if you are uncertain.
