Loan Constant Explained: A Must-Know Metric for Smarter Borrowing in 2025

For Australians navigating the world of loans—whether it’s for a home, investment property, or business—there’s one metric that rarely makes headlines, yet has the power to demystify your true borrowing costs: the loan constant. While interest rates and comparison rates get all the attention, the loan constant offers a unique lens for understanding and comparing loan repayments, especially as lending rules and rates continue to shift in 2025.

What Is the Loan Constant—and Why Should You Care?

The loan constant, sometimes called the mortgage constant, is a percentage that tells you what portion of your loan you’ll repay each year—factoring in both principal and interest—relative to the total loan amount. Simply put, it’s a quick way to see how much cash you’ll need to service your loan annually. The formula:

• Loan Constant = Annual Debt Service / Total Loan Amount

For example, if you borrow $500,000 and your total annual repayments are $32,500, your loan constant is 6.5%. This means you’ll pay 6.5% of the loan balance each year in scheduled repayments.

Why does this matter? Unlike interest rate alone, the loan constant incorporates both principal and interest, reflecting your real cash outflow. It’s a powerful tool for comparing loans with different structures, terms, and rates—especially in 2025, as the Australian lending market grows more complex.

How Loan Constant Helps in Comparing Loans

With dozens of loan products on the market, borrowers often struggle to compare apples with apples. Here’s where the loan constant shines:

• Fixed vs Variable Loans: Two loans might have similar interest rates, but if one has a shorter term or different repayment schedule, the loan constant can reveal which option demands higher yearly repayments.

• Interest-Only vs Principal & Interest: In 2025, tighter APRA rules on interest-only lending mean these loans are harder to access and often pricier. The loan constant for an interest-only loan is lower at first (since you’re not repaying principal), but jumps sharply when principal repayments kick in. For principal & interest loans, the constant is higher but stable throughout.

Real-World Example: Let’s say you’re comparing two $400,000 home loans in 2025:

- Loan A: 5.9% interest, 30-year term (principal & interest). Annual repayment: $28,380. Loan constant: 7.1%.

- Loan B: 6.2% interest, 25-year term (principal & interest). Annual repayment: $31,670. Loan constant: 7.9%.

The higher loan constant on Loan B shows you’ll be paying more each year, even though the interest rate difference seems small. The loan constant cuts through marketing spin to reveal your true repayment burden.

Loan Constant in 2025: What’s Changed?

The economic and regulatory landscape in Australia has shifted rapidly in recent years. Here’s how it affects loan constants in 2025:

• Higher Interest Rates: After several RBA hikes in 2023–2024 to combat inflation, average mortgage rates remain elevated in 2025 (typically 5.7–6.5% for owner-occupiers). This drives loan constants higher, meaning larger annual repayments for new borrowers.

• Shorter Fixed-Rate Offers: Banks have pulled back on long-term fixed rates, with most fixed periods capped at 2–3 years. Shorter terms mean higher loan constants if you want to pay off your loan faster, or more uncertainty if you need to refinance soon.

• Debt Serviceability Tests: APRA’s stricter serviceability buffers (typically 3% above the offered rate) mean borrowers must prove they can handle higher repayments. Lenders increasingly use the loan constant, alongside debt-to-income ratios, to assess eligibility.

• Investment Property Loans: With tighter credit for investors, the loan constant helps property investors compare the cash flow impact of different loan types and terms. Lenders are scrutinising rental yields versus loan constants more closely to ensure sustainable lending.

How to Use the Loan Constant to Your Advantage

Ready to put this metric to work? Here’s how savvy Australians are using the loan constant in 2025:

• Compare Loan Offers: When weighing up different loans, calculate the loan constant for each. A lower constant means lower annual repayments for the same loan size.

• Stress-Test Your Budget: Use the loan constant to see how much of your income will go towards loan repayments each year. This is especially crucial with higher interest rates and living costs.

• Optimise Loan Structure: If you want to pay down your loan faster, a higher loan constant (shorter term, higher repayments) means less interest paid over time. But make sure it fits your cash flow.

• Assess Investment Viability: For property investors, compare the loan constant to expected rental yields. If your yield exceeds the constant, your investment is more likely to be cash-flow positive.

Conclusion: Make Loan Constant Part of Your Borrowing Toolkit

The loan constant is more than just a finance buzzword—it’s a practical tool for navigating Australia’s evolving lending market. By factoring in both principal and interest, it gives you a clearer picture of your true repayment obligations in 2025. Whether you’re buying your first home, refinancing, or growing your investment portfolio, understanding the loan constant can help you borrow smarter and avoid repayment surprises.