Free ISA Calculator UK 2026: Cash, Stocks & Shares, Lifetime & Junior ISA
Project your ISA balance for any of the four UK account types, see your government bonus where it applies, and check your figures against real allowance limits — all in one tool.
Cash ISA: models guaranteed interest with no market risk — enter your account’s actual advertised rate for the most accurate result.
| Year | Paid In | Bonus | Growth | Balance |
|---|
| ISA Type | Rate Used | Bonus | Final Balance |
|---|
This calculator provides estimates for planning purposes only. It assumes a constant rate of growth and monthly contributions applied at the end of each month. Actual ISA returns, especially for Stocks and Shares ISAs, will vary. This is not financial advice — for guidance specific to your situation, speak to a regulated financial adviser.
How an ISA Calculator UK Actually Works (And the Inputs That Matter Most)
An isa calculator uk tool runs on a simple compound growth formula, but the accuracy of what it shows you depends entirely on five inputs:
- Opening balance — what you already hold in the ISA today
- Regular contribution — monthly or annual top-ups
- Growth or interest rate — different for cash vs. invested ISAs
- Time horizon — how many years until you need the money
- ISA type — Cash, Stocks and Shares, Lifetime, or Junior
Change the time horizon by even five years, or the contribution by £50 a month, and the final number can shift by tens of thousands of pounds. That’s compounding at work, not a rounding error — which is exactly why it’s worth understanding each ISA type separately before trusting a single blended figure.
Cash ISA Calculator UK: Modelling Guaranteed Growth
A cash isa calculator uk is the most straightforward version because there’s no market movement to account for — just your rate, your deposits, and time.
When running your own numbers:
- Use the current advertised rate on your specific account, not a rate you saw in an old article — cash ISA rates move with Bank of England base rate decisions
- Check whether it’s fixed for a term or variable and can change at any point
- Compare the rate against current inflation — a return below inflation still erodes your real purchasing power, even though your balance is technically growing
Worked example: £10,000 deposited today with no further contributions, left to compound for 10 years, grows meaningfully larger purely from interest reinvestment — and adding just £150 a month on top roughly triples that outcome over the same period. The exact figure depends on your account’s actual rate, so it’s worth running your own numbers through a calculator rather than relying on someone else’s example.
Why Your ISA Calculator Might Be Wrong If You Ever Withdraw Money
Almost every calculator on the market assumes contributions only ever flow in one direction. In reality, plenty of savers dip into an ISA mid-year and want to top it back up later — and what happens next depends entirely on one word buried in your provider’s terms: flexible.
“Flexible ISA” isn’t marketing language — it’s a specific HMRC designation a provider has to opt into, and not every popular Cash ISA has it.
| Scenario | Flexible ISA | Non-Flexible ISA |
|---|---|---|
| Withdraw £2,000, then redeposit it in the same tax year | Doesn’t count against your £20,000 allowance again | Counts as a brand-new contribution — could push you over your allowance |
| How it’s confirmed | Provider states “flexible ISA” explicitly in the account terms | Never assumed — if it’s not stated, treat it as non-flexible |
| Best suited for | Using an ISA as part of an emergency fund | Pure long-term, no-withdrawal saving |
If you’re treating your ISA as part of an emergency fund and expect to move money in and out, it’s worth checking this before you start — otherwise a projection that “says” you’re within your allowance can quietly be wrong the moment a withdrawal happens.
Stocks and Shares ISA Calculator UK: Why the Growth Rate Is an Assumption, Not a Promise
A stocks and shares isa calculator uk works differently to a cash one because you’re projecting an average long-term return, not a guaranteed rate.
Historically, UK and global equity markets have delivered positive average returns over long holding periods, but that history is not a guarantee of future performance — any calculator using a fixed growth percentage is simplifying a genuinely bumpy ride into a smooth line.
Three things skew these projections if you’re not careful:
- Platform and fund fees compound too. A 0.45% annual fee versus a 0.15% one is a meaningfully different outcome after 20 years, even at identical market returns.
- Short time horizons are risky. If you might need the money within 5 years, a Stocks and Shares ISA calculator will overstate how “safe” that number really is.
- Sequence of returns matters. Two savers with the same average return can end up with very different final balances depending on when the good and bad years happen.
If you’re deciding how much of your annual allowance to put into investments versus cash, running the numbers through our Investment Return (ROI) Calculator alongside your ISA projection gives you a clearer side-by-side comparison of annualised return and total gain across different scenarios.
The Platform Fee Cap Problem: When Your Calculator’s Fee Drag Is Just Wrong
Every generic calculator applies its fee assumption as a flat percentage of your whole balance. That’s reasonably accurate for small pots — but it quietly breaks down as your Stocks and Shares ISA grows, because several major UK platforms cap their platform fee at a fixed £ amount once your holdings in shares and ETFs pass a certain size.
| ISA Balance | Effective Fee Under a % Model | Effective Fee Under a Capped Model |
|---|---|---|
| £10,000 | Roughly matches the stated % | Roughly matches the stated % |
| £50,000 | Fee scales up in £ terms with the balance | Fee growth starts flattening once the cap is reached |
| £200,000 | Fee continues rising in line with balance | Effective fee percentage keeps shrinking as the balance grows |
Expert note: Fund-level charges (the OCF, or ongoing charges figure) still apply on top of the platform fee regardless of any cap — a distinction most calculators conflate into one blended number. If you’re comparing platforms, separate these two costs before you compare, and recalculate your effective fee percentage annually as your balance grows — under a capped-fee platform, it silently shrinks over time, which most projections never reflect.
You Can Now Open Multiple ISAs of the Same Type — Here’s Why That Changes Your Strategy
Since April 2024, you can pay into multiple Cash ISAs (or multiple Stocks and Shares ISAs) with different providers in the same tax year. Before that, the rule was strictly one ISA of each type per tax year — and a surprising amount of still-circulating advice online was written under the old rule and never updated.
| Rule Version | What Was Allowed |
|---|---|
| Old rule (pre-April 2024) | One Cash ISA and one Stocks and Shares ISA per tax year, from one provider each |
| Current rule | Multiple ISAs of the same type, across different providers, in the same tax year |
This is a provider-flexibility change, not extra allowance — your combined £20,000 cap across all ISAs still applies regardless of how many accounts you spread it across.
Practical scenario: lock a competitive 6-month fixed-rate Cash ISA in January, then open a separate best-buy easy-access Cash ISA in September when a better rate appears — both within the same tax year, both counting toward the same £20,000 total. Always confirm your specific provider has implemented the new rules correctly before assuming this flexibility applies.
Lifetime ISA Calculator: Don’t Forget the Government Bonus
A lifetime isa calculator needs to account for something standard ISA tools miss entirely: the 25% government top-up.
Contribute up to £4,000 a year into a Lifetime ISA (which counts toward your overall £20,000 allowance) and the government adds up to £1,000 on top — a guaranteed uplift before any market or interest growth even enters the picture. A few rules to keep in mind:
- You must open the account between ages 18 and 39
- Funds can go toward a first home worth up to £450,000, or be withdrawn penalty-free from age 60
- Withdrawing for any other reason triggers a 25% government charge — which effectively claws back your bonus plus a slice of your own money
Myth vs Reality — Is the Lifetime ISA Bonus Really “Free Money”?
Myth: “Worst case, if I withdraw early, I just lose the 25% bonus I was given.”
Reality: The 25% early withdrawal charge applies to your entire withdrawal amount — contributions, bonus, and any growth combined — not just the bonus portion. That asymmetry means you can end up with less than you originally paid in.
Worked example: You contribute £4,000 and receive a £1,000 government bonus, bringing your balance to £5,000. If you withdraw this for an unauthorised reason, the 25% charge applies to the full £5,000 — a £1,250 charge — leaving you with £3,750. That’s £250 less than the £4,000 you originally put in.
The only penalty-free exits are a first home purchase, reaching age 60, or terminal illness. Outside of those, the Lifetime ISA works best when you’re genuinely confident about your house-purchase or retirement timeline — treating it as a flexible general savings account is where people quietly lose money.
Junior ISA Calculator: Why Time Beats Contribution Size
A junior isa calculator highlights something parents consistently underestimate: how much time in the market does compared to the size of the contribution.
The annual Junior ISA allowance is £9,000, but you don’t need to come close to that to see the effect of compounding over 18 years. Starting small and early, then increasing contributions as income allows, tends to outperform starting late with larger amounts — purely because of how many years the money has to grow.
Common ISA Calculator Mistakes That Quietly Skew Your Numbers
Even a well-built calculator produces misleading results if the inputs are wrong. Watch for these:
- Entering a gross rate instead of a net one. Always check whether the growth rate includes or excludes platform fees.
- Ignoring inflation entirely. A return that beats a bank savings rate can still lose you real value if inflation is higher.
- Forgetting the combined £20,000 cap. You can split contributions across Cash, Stocks and Shares, Lifetime, and Innovative Finance ISAs, but the total across all of them can’t exceed your annual allowance.
- Assuming a flat rate forever. Interest rates and market conditions shift — rerun your projection at least once a year with updated figures.
If you’re balancing ISA contributions against existing debt, it’s usually worth checking both sides of the ledger — our UK Debt Repayment Calculator shows whether clearing higher-interest debt first gets you ahead faster than maximising your ISA allowance this year, using either the avalanche or snowball method.
Advanced Strategy: Using “Bed and ISA” to Shelter Existing Investments From Capital Gains Tax
Advanced — assumes you already understand the basicsEverything so far assumes you’re starting from zero. But if you already hold, say, £80,000 in a General Investment Account (GIA) — from investing before you opened an ISA, or from an inheritance — moving all of it into an ISA in one go isn’t realistic, since you’re capped at £20,000 a year. This is where “Bed and ISA” comes in.
What it means: selling assets in your GIA and immediately repurchasing the same (or equivalent) assets inside an ISA wrapper, sheltering future growth from tax going forward.
- Confirm the transfer uses up your £20,000 annual allowance just like a cash contribution — a large GIA can take several tax years to fully shelter
- Check your gains against the annual Capital Gains Tax exempt amount before selling, since selling triggers a taxable event if gains exceed it
- Time sales across separate tax years where possible to avoid an unnecessary CGT bill in one go
- Factor in the bid-offer spread and transaction costs on the sell-and-rebuy — many platforms offer a combined “Bed and ISA” service to reduce time spent out of the market
- Prioritise which holdings to shelter first — generally, highest expected future growth or dividend-heavy holdings benefit most, since all future ISA growth is tax-free
Expert note: This isn’t a one-off decision — it’s a multi-year workflow. The mistake most people make is trying to move everything indiscriminately in year one. Decide your shelter order based on which assets have the most future tax exposure to lose, not simply which ones are easiest to sell.
A Quick Framework for Choosing the Right ISA
Rather than guessing which type suits you, work through this in order:
- Need the money within 5 years? → Model it with a Cash ISA calculator
- Investing for 10+ years and comfortable with ups and downs? → Use a Stocks and Shares ISA calculator
- Saving specifically for a first home or retirement, and aged 18–39? → Run a Lifetime ISA calculator
- Saving on behalf of a child? → Use a Junior ISA calculator
Many savers split contributions across two or three ISA types in the same tax year — fully allowed, provided the combined total stays within £20,000. For a full picture of how your take-home pay, tax position, and ISA contributions fit together, our UK Income Tax Calculator and UK Take-Home Pay Calculator are useful companions when planning how much you can realistically set aside each month. For the full range of UK-specific tools in one place, visit the UK Financial Calculators Hub.
Official rules on allowances and account types are set out on gov.uk’s Individual Savings Accounts guidance, which is worth checking directly before making decisions, since allowance figures are reviewed and can change each tax year.
Frequently Asked Questions
Is an ISA calculator UK tool accurate for predicting my actual returns?
It’s accurate for the arithmetic, not the market. The tool correctly compounds whatever rate and contribution schedule you enter, but the rate itself is always an assumption — actual Stocks and Shares ISA returns will vary year to year, sometimes significantly.
How much can I contribute across all my ISAs in one tax year?
The combined limit across all ISA types — Cash, Stocks and Shares, Lifetime, and Innovative Finance — is £20,000 per tax year. The Lifetime ISA has its own sub-limit of £4,000 within that total, and the Junior ISA allowance (£9,000) is separate and applies only to under-18s.
Does an ISA calculator automatically include the Lifetime ISA government bonus?
Not always — many basic calculators only project growth on what you deposit. Check whether the tool explicitly adds the 25% government bonus, or you’ll need to add it manually before running your projection.
Can I use one calculator for both a Cash ISA and a Stocks and Shares ISA at the same time?
You can, but it’s clearer to model them separately. Blending a fixed cash rate with a variable market return in a single projection tends to produce a misleading combined figure — calculate each ISA type on its own, then add the totals together for your overall picture.