How Do Lenders Calculate Mortgage Interest? Daily vs Monthly Explained
The way your lender calculates mortgage interest affects how much you pay overall and how effective your overpayments will be. Most UK lenders now use daily interest calculation, but some still use monthly. Here's what you need to know.
Why Does Interest Calculation Method Matter?
When you make a mortgage payment or overpayment, the timing of when that payment reduces your balance affects how much interest you're charged. With daily interest calculation, your overpayment starts saving you money immediately. With monthly calculation, you might not see the benefit until the following month.
Understanding your lender's method helps you:
- Time your overpayments for maximum benefit
- Understand why your interest charges might seem higher or lower than expected
- Compare mortgage products more accurately
- Make informed decisions about when to make large overpayments
Daily Interest Calculation
Daily interest calculation (also called "daily rest" or "daily reducing balance") is now the standard for most UK mortgage lenders. Interest is calculated each day based on your outstanding balance at the end of that day.
How Daily Interest Works
Daily Interest Formula
Daily Interest = (Outstanding Balance × Annual Interest Rate) ÷ 365
Example: With a £200,000 balance at 5% interest:
Daily Interest = (£200,000 × 0.05) ÷ 365 = £27.40 per day
Advantages of Daily Interest Calculation
- Immediate benefit from overpayments: If you overpay on the 5th of the month, you start saving interest from the 6th
- Fair and transparent: You only pay interest on what you actually owe each day
- Rewards early payments: Paying your mortgage early in the month saves more than paying late
- Better for irregular income: You can make overpayments whenever you have spare cash
UK Lenders Using Daily Interest
The majority of UK lenders now use daily interest calculation, including:
- Nationwide Building Society
- Halifax
- Santander
- Barclays
- NatWest
- HSBC
- Lloyds
- Most building societies
Since around 2009, daily interest calculation has become the industry standard following pressure from consumer groups and regulators.
Monthly Interest Calculation
Monthly interest calculation (also called "annual rest" or "monthly rest") calculates interest once per month (or sometimes once per year) based on your balance at a specific point. This was common before 2009 but is now rare for new mortgages.
How Monthly Interest Works
Monthly Interest Calculation
With monthly interest, your balance is typically assessed on a set date each month (often the 1st or your payment date). Any overpayments made during the month won't reduce your interest until the next calculation date.
Monthly Interest = (Outstanding Balance on calculation date × Annual Interest Rate) ÷ 12
Example: With a £200,000 balance at 5% interest:
Monthly Interest = (£200,000 × 0.05) ÷ 12 = £833.33 per month
Disadvantages of Monthly Interest Calculation
- Delayed benefit from overpayments: An overpayment on the 2nd of the month won't reduce interest until next month
- Less fair: You pay interest on money you've already repaid
- Timing matters more: You need to strategically time overpayments to maximise savings
- Can cost more overall: The same overpayment strategy saves less with monthly calculation
Annual Rest: The Worst Option
Some older mortgages (particularly from before 2000) used "annual rest" where interest was calculated just once per year. With annual rest, an overpayment in January wouldn't reduce your interest charges until the following January. These mortgages are now extremely rare, but if you have one, remortgaging to a daily interest product could save you significant money.
Daily vs Monthly: The Real-World Difference
Let's compare how the same overpayment performs under each system:
Comparison Example
Scenario: £200,000 mortgage at 5% interest. You make a £10,000 overpayment on the 5th of the month.
Daily Interest Calculation:
- Interest for days 1-5: 5 days × £27.40 = £137.00
- Interest for days 6-30: 25 days × £26.03 = £650.75 (based on £190,000 balance)
- Total month's interest: £787.75
Monthly Interest Calculation:
- Interest calculated on balance at start of month: £200,000
- Total month's interest: £833.33
- The £10,000 overpayment only helps from next month
Difference this month: £45.58 more interest with monthly calculation. Over a 25-year mortgage, these small differences compound significantly.
How Interest Calculation Affects Overpayment Strategy
With Daily Interest
You have more flexibility. Key strategies include:
- Pay as early as possible: Making your regular payment on the 1st rather than the 28th saves a small amount each month
- Overpay whenever you can: There's no need to wait for a specific date
- Small, frequent overpayments work well: Even £50 extra per week reduces your balance faster than £200 per month
With Monthly Interest
Timing becomes more important:
- Time overpayments before the calculation date: Find out when your lender calculates interest and make overpayments just before
- Lump sums are more effective: One large overpayment at the right time beats several smaller ones spread through the month
- Consider switching lenders: If you're making regular overpayments, a daily interest mortgage could save you money
How to Find Out Your Lender's Method
To find out how your lender calculates interest:
- Check your mortgage offer: Look for terms like "daily interest," "daily rest," or "daily reducing balance"
- Review your annual statement: Some lenders explain their calculation method
- Call your lender: Ask specifically: "Do you calculate interest daily or monthly?"
- Check their website: Most lenders publish this information in their mortgage FAQs
Already on a Monthly Interest Mortgage?
If your mortgage uses monthly interest calculation and you plan to make regular overpayments, it may be worth remortgaging to a daily interest product. The improved interest calculation combined with competitive rates could save you thousands over your mortgage term.
The Maths Behind Mortgage Interest
For those who want to understand the technical details, here's how mortgage interest calculation works:
Standard Repayment Mortgage Formula
Most UK mortgages are "capital repayment" mortgages where your monthly payment covers both interest and principal. The payment is calculated so that you pay off the entire loan over the agreed term.
Monthly Payment Formula
M = P × [r(1+r)^n] / [(1+r)^n - 1]
Where:
- M = Monthly payment
- P = Principal (loan amount)
- r = Monthly interest rate (annual rate ÷ 12)
- n = Total number of payments (years × 12)
How Each Payment Is Split
Each monthly payment is divided between interest and capital. Early in your mortgage, most of your payment goes towards interest. As your balance decreases, more goes towards capital.
Payment Split Example
£200,000 mortgage at 5% over 25 years. Monthly payment: £1,169.18
| Month | Interest | Capital | Remaining Balance |
|---|---|---|---|
| 1 | £833.33 | £335.85 | £199,664.15 |
| 12 | £818.47 | £350.71 | £195,843.37 |
| 120 (Year 10) | £622.40 | £546.78 | £148,832.19 |
| 300 (Final) | £4.85 | £1,164.33 | £0.00 |
This is why overpaying early in your mortgage has a bigger impact—you're reducing the balance that accrues the most interest.
Interest-Only vs Repayment Mortgages
Interest calculation also differs between mortgage types:
Repayment Mortgages
Each payment reduces your balance, so interest charges decrease over time. Daily interest calculation benefits you as your balance drops faster with overpayments.
Interest-Only Mortgages
Your monthly payment covers only interest—the balance stays the same unless you make capital overpayments. The calculation method matters less for regular payments but still affects how overpayments are credited.
Common Questions
Does daily interest mean I pay more overall?
No—daily interest calculation typically means you pay less overall because overpayments and regular payments start reducing your interest immediately. The word "daily" refers to how interest is calculated, not how often it's charged.
Why do some lenders still use monthly calculation?
Legacy systems and older mortgage products may still use monthly calculation. Most lenders have moved to daily for new mortgages following consumer pressure and regulatory guidance.
Does the calculation method affect my quoted interest rate?
No. The interest rate is the same regardless of calculation method. However, the effective cost of your mortgage can be slightly higher with monthly calculation if you make regular overpayments.
Should I remortgage just to get daily interest calculation?
Only if you're planning significant overpayments. For standard monthly payments without overpayments, the difference is minimal. But if you regularly overpay, switching to a daily interest product could save you money—factor in any remortgaging costs when calculating.
Key Takeaways
- Most UK lenders now use daily interest calculation
- Daily interest means overpayments start saving you money immediately
- Monthly interest calculation delays the benefit of overpayments
- If making regular overpayments, daily interest is more beneficial
- Check your mortgage offer or call your lender to confirm their method
- Early overpayments have the biggest impact due to how interest compounds
Calculate Your Interest Savings
See how much interest you could save by overpaying your mortgage with our free calculator.