How Interest Is Actually Calculated on Your Home Loan

The Most Important Calculation You Need to Understand

If you have a home loan, you need to understand how interest is calculated. Full stop. It's the secret sauce behind every strategy for paying off your mortgage faster.

Your mortgage repayment has two components: principal (the amount you borrowed) and interest (the bank's cut for lending it to you). Over 30 years, the interest adds up significantly. But it's not charged all at once, and it's not calculated the way most people think.

Annual Rate, Monthly Charge, Daily Calculation

When someone talks about an interest rate, they're almost always referring to the annual rate. If you have a $500,000 loan at 6%, it sounds reasonable to assume you'll pay $30,000 interest per year. Divide that by 12, and you'd expect $2,500 per month.

But banks don't calculate it that simply. They calculate interest every single day, then tally those daily amounts at the end of each month.

The daily interest formula is straightforward:

Loan Balance × Annual Interest Rate ÷ 365 = Daily Interest

For a $500,000 loan at 6%: $500,000 × 6% ÷ 365 = $82.19 per day.

Why the Number of Days Matters

Because interest is calculated daily, your monthly interest charge will zigzag — dropping on months with 30 days and rising on 31-day months. February is always the cheapest month for interest.

More importantly, the three variables that determine your monthly interest are:

• Your loan balance

• The annual interest rate

• The number of days in that month

Reduce any of these variables — even just for a single day — and you'll reduce the interest you pay that month.

The Variable You Control

You can't change the number of days in a month. You have limited control over the interest rate. But you have significant control over your loan balance.

Every dollar that reduces your loan balance, even temporarily, reduces the daily interest charge. This is why strategies like offset accounts, extra repayments, and aligning your repayment frequency to your income all work — they reduce the balance that interest is calculated on, as early and as much as possible.

Compound Interest Works Against You

At the end of each month, interest is added to your loan balance. In the next month, interest is calculated on that higher balance — which includes last month's interest. By Month 3, you're paying interest on the interest on the interest.

This is compound interest working against you. The longer your loan runs, the more the bank makes. Understanding this is step one to fighting back.