How to Calculate Your Stocks and Shares ISA Returns (Properly)
Let’s be honest: most ISA investors have no real idea what their stocks and shares returns actually are. They calculate their ISA returns by looking at the green number on the screen and saying woo I made £200 this week.
You log in, see a green number, maybe a percentage if the platform feels generous, and that’s about it. But what does that number really mean? Is it good? Is it better than the market? Does it include dividends? Timing? Anything?
If you’ve been drip-feeding money into your ISA over time—or switching funds, taking income, or reinvesting—then that dashboard figure is often misleading at best, and flat-out wrong at worst.
So let’s fix that.
Here’s the straight-up, no-jargon guide to calculating your Stocks and Shares ISA returns properly in 2025. Whether you’re new to investing or trying to figure out if your platform is actually doing a good job, this will help.
What Do We Actually Mean by “Returns”?
Sounds obvious, but most people don’t stop to think about it. When you say “returns,” do you mean:
- Total return – How much your ISA has grown overall, including dividends?
- Personal return – What you earned based on when and how much money you added?
- Annualised return – How much it’s grown per year, on average?
- Fund return – How the investment itself performed, not taking into account your timing?
Each tells a slightly different story.
If you stuck £10,000 into the market in one go and left it alone, total return works fine. But most people don’t invest like that. You add bits over time. You buy at highs and lows. You maybe take income or switch funds. That means you need to look at the full picture—not just the number your ISA dashboard flashes at you.
The Simple Way (If You Invested Once)
Let’s say you put £5,000 into your ISA a couple of years ago and haven’t touched it since. It’s now worth £6,250. Great.
To work out your return, just look at the profit — that’s £1,250 — and compare it to what you put in. So, you’re up 25%. Job done.
This only works if you made a single contribution. But let’s be real — most people don’t.
The Real-Life Scenario (Multiple Contributions)
This is where it gets messy. Say you added:
- £2,000 in April
- £3,000 in August
- £5,000 more in February
And now your ISA is worth £11,200. It’s tempting to say: “I invested £10,000, now it’s worth £11,200 — so I made £1,200.” Technically, yes. But not all that money had the same time to grow.
Your early contributions had more time in the market. Later ones, less. That skews the number and makes a simple percentage kind of meaningless.
What You Actually Want to Know
So, how do you really work out how you’re doing? There are two main ways platforms (and pros) measure this:
1. Money-Weighted Return (AKA: What You Earned)
This takes into account the timing of your deposits. It’s the best way to measure your actual return.
Good ISA platforms will show you this automatically. It might be called “money-weighted return” or “personal performance.”
It tells you how well your money has grown, based on when you added it and how much you put in.
If your platform doesn’t show this? You’re flying blind. (And it’s a red flag.)
2. Time-Weighted Return (AKA: What the Fund Did)
This ignores your deposits completely. It just measures how the investment performed.
That’s great if you’re trying to compare different funds or strategies, but less useful if you want to know whether you made good decisions.
3. Don’t Forget Dividends
Most ISA investors forget this part — and it matters.
If you’re invested in ETFs, funds, or shares that pay dividends, those payments are part of your return.
If you’re using “accumulation” funds, the dividends are automatically reinvested — meaning they’re already baked into your ISA value.
But if you’re using “income” funds and taking cash out, you’ll need to add that income back in when calculating your real returns. Otherwise, you’re underestimating what your ISA actually earned.
Why Annualised Returns Matter
You might think your ISA’s done well because it’s up 30% overall. But over how long?
If that took one year, brilliant. If it took five, not so much.
Annualised return is the number that tells you how much your ISA is growing per year, on average. That’s the number to watch if you want to compare your performance to the stock market or inflation.
For example: if your ISA has grown 30% over three years, that’s roughly 9% per year. If the FTSE 100 grew 5% a year over the same period, you’ve done well. If the market did 12% and you’re paying 1% in fees… you might want to have a rethink.
Why Your Dashboard Might Be Misleading
A lot of ISA platforms show a return figure that looks official — maybe a nice clean “+7.8%” next to your portfolio value.
But unless they’re clear about what that number actually includes, it can be completely misleading.
Some dashboards:
- Ignore the timing of your deposits
- Don’t include dividend income
- Only show fund performance, not personal returns
- Use funky “since inception” metrics that are totally useless if you joined later
Bottom line: if it doesn’t clearly say “money-weighted” or “personal performance,” take it with a pinch of salt.
The Smart Way to Keep Track
You don’t need spreadsheets. You just need to:
- Know what you’ve put in — Keep a rough log of your total contributions.
- Check what it’s worth now — Use the platform value, including reinvested dividends.
- Think about the time — Did this growth happen over one year or five?
- Compare it to something — Check what the FTSE or S&P 500 did over the same time.
- Don’t get lost in the short term — Ignore the day-to-day swings. That’s not the game.
Final Thoughts
Your ISA isn’t just a fancy savings account — it’s a long-term wealth builder with huge tax advantages. But if you don’t understand your real return, how do you know if it’s actually working?
Most people either overestimate their performance or completely ignore it.
But if you:
- Know what you’ve contributed
- Factor in dividends
- Understand what the percentages really mean
- And check how your ISA stacks up against the wider market
…you’ll be ahead of 90% of investors.
And if your platform doesn’t give you the tools to see that clearly?
Switch.
You deserve to know what your money is actually doing.
As a UK trader / investor for over 10 years and avid ISA investor and saver; James decided to build ISA Interest Calculator to help everyday British savers with calculating potential ISA returns. Having worked for a large FTSE100 company building AI tools for over 5 years, he brought his expertise to finance and quickly launched several highly-respected and successful finance & investing sites for UK savers and investors.