Savings Interest Calculator — See It Grow

Think of this as the sums your bank statement never quite spells out for you. You know your balance today, you know roughly what rate you're getting, but working out where that leaves you in three, five or ten years — especially once you factor in adding money every month — isn't something most of us want to sit down and do with a pen and paper. That's the gap this tool fills.

Enter your starting balance, what you can realistically add each month, the interest rate (AER) your account pays, and how long you're planning to save for. The calculator then runs the same compound growth maths a bank would use behind the scenes — or simple interest, if that's what your particular account pays instead. Every monthly deposit is added at the point it actually lands and grows from there, so you're seeing what genuine month-by-month saving produces, not just a lump sum left untouched.

What this calculator gives you

• Your projected final balance and total interest earned over the term you choose
• A year-by-year table so you can see exactly when the growth starts to speed up
• A rough read on whether your interest is likely to stay within your Personal Savings Allowance
• A downloadable PDF summary if you want to keep a record or share it with a partner

One tip: if you're not sure exactly what rate you're getting, check your account's AER rather than a "gross" or "monthly" rate quoted separately — AER is the figure that lets you compare accounts on a like-for-like basis, and it's what this calculator expects.

The pattern worth noticing here is that compound growth doesn't ramp up in a straight line — it starts slow and gets steeper the longer you leave it, because each year you're earning interest on a slightly bigger number than before. Look at the table below and you'll see the annual interest earned barely changes between year 1 and year 3, but by year 20 it's grown by more than the entire original deposit. Here's roughly how £10,000, left untouched with no further deposits, grows at a typical 4.5% AER, compounded annually:

Treat that table as a starting point rather than a promise. Interest rates move — the 4.5% you're getting today might not still be on offer next year, particularly on variable-rate easy access accounts that can be repriced at any time. A fixed-rate bond locks your rate in for the term, which trades flexibility for certainty. In practice, most people we've spoken to end up mixing the two: an easy access pot for whatever they might need at short notice, and a fixed-rate or notice account for money they're confident they won't touch. If you're weighing that decision up alongside a house deposit, our mortgage affordability calculator is worth a look too.

Four things really move the number, and it helps to know which ones you actually control.

Interest rate

This does most of the heavy lifting. Even a difference of half a percentage point compounds into a meaningful gap once you're looking five or ten years out — you saw that in the worked example further down. It's the one factor that's largely out of your hands day to day, but it's also the one most worth shopping around for before you commit any money.

Time

Time in the account matters more than most people expect, and it's genuinely the one lever where starting earlier — even with a smaller amount — tends to beat waiting until you've saved up a bigger lump sum. The reason is simple: money that's been compounding for eight years has had eight years of "interest on interest," and no later lump sum can fully make that up.

Monthly top-ups

These make a bigger difference than people usually assume. Adding even £100 a month consistently can end up contributing more to your final balance than the interest itself over shorter terms, simply because you're adding fresh capital every month rather than relying purely on growth. It's the most controllable factor of the four.

Compounding frequency

Don't overlook this one — an account that adds interest monthly rather than annually will nudge your final balance up slightly at the same headline rate, since each month's interest starts earning its own interest a little sooner. The difference is small on modest balances but adds up on larger ones.

This is the distinction that trips a lot of people up. With simple interest, you only ever earn interest on your original deposit, so the cash amount you get every year stays flat — £10,000 at 4.5% earns exactly £450 a year, every year, no matter how long you leave it. With compound interest, each year's interest gets added to your balance and then earns interest itself the following year, so the growth accelerates the longer you leave it — the same £10,000 earns £450 in year one, but closer to £470 by year five, because it's now growing from a bigger starting point each time.

We've seen savers assume their account works one way when it actually works the other, and only spot the gap when the numbers don't match what they expected years down the line. It's worth checking rather than guessing:

• Compound interest — used by the vast majority of UK savings accounts, cash ISAs and regular savers. Better for you if you're leaving the money to grow untouched.
• Simple interest — less common, but some fixed-term bonds that pay interest out to a separate account (rather than reinvesting it) behave this way in practice, since the interest never gets the chance to compound.

If your account statement or terms don't make it obvious which one you've got, ring and ask — it's a completely normal question and any decent provider will answer it in a sentence.

This is the step people forget when they're mentally spending interest that hasn't actually been taxed yet. Most savers get a Personal Savings Allowance: basic-rate taxpayers can earn up to £1,000 in savings interest a year tax-free, higher-rate taxpayers up to £500, and additional-rate taxpayers get no allowance at all. Interest earned above your allowance is taxed at your normal income tax rate, usually collected automatically by HMRC adjusting your tax code rather than through a separate bill.

Here's where it catches people out in practice: with savings rates where they've been recently, it takes less capital than you'd think to bump into that limit. A basic-rate taxpayer with roughly £22,000 saved at 4.5% is already earning close to £1,000 a year in interest — comfortably within the allowance today, but a rate rise or a bit more saved could tip them over it. If that sounds like your situation, it's worth checking your numbers here and then reading the ISA section below, since none of this applies to money held inside one.

Worth knowing: this calculator shows your gross interest before tax, because your actual allowance depends on your total income and tax band for the year — not just your savings.

An Individual Savings Account (ISA) is simply a tax wrapper — the interest inside it doesn't count towards your Personal Savings Allowance at all, because it's tax-free regardless of how much you earn or which tax band you're in. For the 2026/27 tax year, most UK adults can pay in up to £20,000 across all their ISAs combined, and that allowance resets every 6 April rather than carrying forward if unused.

When a cash ISA is worth it

• You're a higher or additional-rate taxpayer, where your Personal Savings Allowance is smaller or non-existent
• Your savings balance is large enough that the interest would otherwise breach £1,000 (or £500) a year
• You want the certainty of never having to think about savings tax again on that pot, however rates move

Where a standard account can still win

• Smaller balances where you'd never get close to your Personal Savings Allowance anyway
• Some non-ISA accounts pay a slightly higher headline rate, since providers aren't bound by ISA rules
• You've already used your £20,000 ISA allowance for the tax year

This calculator doesn't distinguish between ISA and non-ISA savings — the maths is identical either way, so run your numbers above and then weigh up the ISA angle separately using the guidance here, or check the official GOV.UK ISA rules for a full breakdown of the different ISA types available.

Two things generally trade off against each other: rate and access. Knowing which one matters more for a given pot of money makes the choice a lot easier.

One honest bit of advice that gets glossed over on plenty of sites: if there's a real chance you'll need the money within the term you're locking in for, don't lock it in. The penalty for early access on a fixed bond can easily wipe out the extra interest you were chasing in the first place. Keep an emergency fund somewhere easy access, and only tie up money you're genuinely confident you won't need — three to six months of essential outgoings is the rule of thumb most advisers still point to.

Let's put a real person in this, even a made-up one. You start with £10,000, add £200 every month, and find an account paying 4.5% AER, compounded annually. Over 5 years, you'd have paid in £22,000 of your own money, and the account would have grown to roughly £24,400 — around £2,400 earned in interest on top of what you saved.

Now change one thing: say you find an account paying 5.5% AER instead, with monthly compounding. That single change lifts your interest earned to roughly £2,950 over the same five years — nearly £550 more, just from shopping around for a better rate. Or take the opposite approach and simply wait — hold the same 4.5% account for 10 years instead of 5, and your interest earned jumps to roughly £6,200, since compounding has far longer to work. Try your own numbers in the calculator above and see which story fits you.

Official sources & further reading: read guidance at MoneyHelper, check current allowances at GOV.UK, and compare protected accounts via the FSCS. Browse every tool on the mortgage calculators homepage.

The honest summary is this: your rate and your time in the account do most of the work, but they're not the only things you control. Adding a bit more each month, checking whether monthly compounding is on offer, and making sure you're not needlessly paying tax you could have avoided with an ISA are all things you can act on this week, not someday.

A few practical next steps, roughly in order of impact:

• Run your real numbers through the calculator above, not just a round figure, so the year-by-year table actually reflects your situation
• Check your current account's AER against what's available elsewhere — moving a stagnant balance to a better rate is usually the single biggest lever most people have
• Work out whether you're likely to breach your Personal Savings Allowance, and if so, look at shifting new savings into a cash ISA
• If you're saving towards a mortgage deposit specifically, check the mortgage affordability calculator to see what deposit size actually gets you where you want to be

None of this needs to be complicated. Small, consistent decisions — a slightly better rate, a standing order that goes in every payday, a savings tax check once a year — tend to matter more over time than any single clever move.