You've found a savings account offering 5% interest. On £5,000, that sounds like £250 a year. But when your statement arrives, you've earned £125. What happened?
This is the interest calculation trap that catches thousands of UK savers every November. With rates finally competitive again, now's the perfect time to understand how banks actually calculate interest—before you miss out on thousands of pounds.
How Banks Actually Calculate Interest
The headline rate is just the start. Banks calculate interest in three main ways, and the method matters enormously for your actual earnings.
Daily interest calculation is what most modern accounts use. The bank calculates interest on your balance every single day, then adds it monthly or annually. This is the fairest method because your money earns from day one.
Here's the reality: if you pay £5,000 into a 5% account on November 1st and leave it until November 30th, you don't earn 5% for a full year. You earn 5% for one month. That's roughly £20.83, not £250.
This is why understanding the timeframe matters. If you switch money around mid-month, you might only earn interest from that day onwards—not from the start of the month.
Annual percentage rate (APR) and annual equivalent rate (AER) are two different things, and banks use this confusion against you. AER shows what you'd earn if you left money untouched for a full year and interest was compounded. APR is simpler—it's just the base rate, usually without accounting for compounding.
For savings accounts, AER is the number you want. It's more realistic for comparing accounts because most people don't withdraw and redeposit interest monthly.
The balance cap trap is where thousands lose money silently. Many high-interest accounts cap how much of your balance earns the advertised rate. This happens far more often than people realise.
For example: "5% on balances up to £500." That sounds fine until you realise the next £4,500 earns perhaps 0.5%. On £5,000:
- First £500 earns 5% = £25
- Remaining £4,500 earns 0.5% = £22.50
- Total: £47.50, not £250
Banks bury this detail in the small print. Always check the maximum amount earning the headline rate.
Stacking Bonuses with Earned Interest
When you switch banks, you often get a bonus (typically £100–£400 at the moment) plus interest on your balance. The maths gets confusing fast.
Let's say you switch £3,000 to a new account offering:
- £200 switch bonus (paid after conditions met)
- 4% interest on the full balance
Your math should be:
- Interest earned over the year: £3,000 × 4% = £120
- Switch bonus: £200
- Total first-year earnings: £320
But here's the trap: most people forget the bonus is paid once, while the interest compounds. If you leave money for two years:
- Year 1: £120 interest + £200 bonus = £320
- Year 2: £3,120 × 4% = £124.80 (because your year-1 interest now earns interest too)
- Two-year total: £444.80
That small compounding effect adds up massively over time. This is why paying bonuses into your account immediately (rather than taking them separately) matters.
The November Timing Problem
Right now in November 2025, you're facing a specific challenge: many accounts offer interest rates that might be cut before the year ends.
If a bank drops its rate from 5% to 3% on December 1st, any money you haven't moved yet loses out for the entire final month of the tax year. Check our live offers page to see which accounts are still offering competitive rates—they're changing fast.
Some people deliberately delay switching until late November to lock in current rates. Others switch earlier to earn interest for longer. The calculation depends on your specific situation:
Early November switch example:
- Switch on November 1st at 5%
- If rate drops to 3% on December 1st, you've still earned 5% for November (that's a full month at a better rate)
- December earnings at 3% are still better than leaving money in a 0.5% account
Late November switch example:
- Switch on November 25th at 5%
- You earn 5% for only 6 days (November 25–30)
- Then you'd earn 3% for December
- Overall: slightly worse than switching earlier
The rule: switch when the rate advantage is large and you're confident it won't drop in the next 7 days.
How to Actually Compare Accounts
With multiple accounts offering different rates, balances caps, and timing considerations, comparing is complex. Here's your system:
Step 1: Find your timeframe. Are you moving money for 3 months (until you earn next regular saver), 1 year, or longer? This changes which rate matters most.
Step 2: Check the balance cap. If you're moving £5,000, it's useless comparing accounts if the 5% rate only applies to the first £1,000.
Step 3: Calculate actual interest, not headline rates. Use this formula:
- Balance × Rate × (Days in account / 365) = Actual interest earned
Step 4: Add bonuses. But only count them if you'll meet the conditions (usually keeping money in the account for 3 months and setting up a direct debit).
Step 5: Factor in switching costs. They're usually free, but check for any "restrictions" that might cost you—like withdrawal limits that force you to keep money locked.
Real-World November Scenario
You're considering three switches for £4,000:
Option A: 4.5% with no balance cap, no switch bonus
- Annual interest: £4,000 × 4.5% = £180
Option B: 5% on first £2,000, 2% on remaining £2,000, plus £150 switch bonus
- Interest on capped portion: (£2,000 × 5%) + (£2,000 × 2%) = £100 + £40 = £140
- Plus bonus: £150
- Total first year: £290
Option C: 3.5% with no cap, plus £200 switch bonus
- Interest: £4,000 × 3.5% = £140
- Plus bonus: £200
- Total first year: £340
Option C wins—despite the lowest rate—because the bonus makes up for it. But this is why you can't just look at rates alone.
The Tax Reality
Interest earned on savings is technically taxable, though the personal savings allowance means most people don't pay tax on it. If you're earning interest across multiple accounts, track it carefully. By April 2026, you'll need to know your total interest earned for tax purposes.
Your switch bonus is not taxable. This is another reason why stacking switches alongside interest-bearing accounts is so powerful—you get tax-free money plus interest.
Common Questions
Can I really earn that headline rate on my full balance? Only if there's no balance cap and you leave money in for the full year. Check the terms carefully—most high-interest accounts cap at £500 or £1,000.
What happens if a bank cuts its rate mid-year? Your rate usually drops with it (unless stated otherwise). Always read the terms about rate changes. Some accounts lock rates for a fixed period—those are worth slightly lower rates if you value certainty.
Should I move money out early if a rate is about to drop? Not necessarily. You lose the interest you'd earn during the switching period (usually 7 days), so only move if the rate drop is huge. Check the numbers before deciding.
How do I know if I'm earning interest daily, monthly, or yearly? The account terms state this clearly. Daily is best (you earn from day one), monthly is good, yearly is rare and usually only on fixed-rate accounts.
Can I compare rates across my multiple accounts easily? Not automatically—you have to calculate it yourself using the formula above, or use our rate comparison tool to see what you should be earning versus what you are.
The bottom line: headline rates are marketing. Real returns depend on balance caps, timeframes, and whether you actually meet the bonus conditions. Spend 10 minutes calculating your actual earnings now, in November, and you'll make far better decisions than 95% of savers who just chase the biggest number.