Understand before you sign.

How interest works — and why it matters more than you think.

Interest is the price you pay to borrow money. Understanding how it's calculated changes how you think about every loan decision you make.

The basic formula

Interest is calculated as a percentage of the amount you owe. If you borrow $10,000 at 5% annual interest, you owe $500 in interest in the first year. Simple enough — but loans rarely work this simply, because of compounding.

Compound interest: the multiplier

Most loans use compound interest, which means interest is charged on the outstanding balance including previously accumulated interest. The effect snowballs over time.

Why the early years hurt the most

On a standard repayment loan, your monthly payment is fixed — but the split between interest and principal changes every month. In the early years, most of each payment goes to interest, and very little reduces what you actually owe. This is why overpaying early has such a dramatic effect on the total cost of a loan.

APR vs APY — what's the difference?

Two acronyms that look almost identical but mean different things. Getting them confused can cost you money.

APR: the cost of borrowing

Annual Percentage Rate (APR) is the yearly cost of a loan, including interest and most fees (origination fees, broker charges, etc.). It's designed to let you compare loan offers on a like-for-like basis.

When you're shopping for a loan, always compare APRs — not just the advertised interest rate. A loan with a lower rate but high origination fees may have a higher APR than one with a slightly higher rate and no fees.

APY: the return on savings

Annual Percentage Yield (APY) accounts for the effect of compounding — interest earned on interest. It's mainly used for savings accounts and investments, not loans. A higher APY on savings is better. On a loan, you'll almost never see APY quoted.

Understanding amortisation — where does your money actually go?

Every monthly payment you make is split between interest and principal. The split changes every single month — and understanding it explains why repaying early is so powerful.

The amortisation schedule

When you take out a loan, the lender calculates a fixed monthly payment that will pay off the loan exactly by the end of the term. Behind that number is an amortisation schedule — a month-by-month breakdown showing how each payment is divided.

Notice that in month one, $1,587 of your $1,826 payment goes straight to the lender as interest — only $239 reduces your actual debt. This ratio slowly shifts over time, but it takes years before principal overtakes interest in each payment.

Why this matters for overpayments

When you make an overpayment, 100% of the extra amount reduces the principal. That means all future interest calculations are based on a lower balance — compounding the savings over the remaining term. The earlier in the loan you overpay, the greater the effect.

The power of overpayments — why an extra $100/month matters more than you think.

Overpaying a mortgage or loan is one of the most reliable ways to save money. The maths is straightforward, but the results can be surprising.

What happens when you overpay?

Every extra dollar you pay above your required monthly amount reduces your outstanding balance. Because interest is calculated on the remaining balance, a lower balance means less interest charged each month — which means more of each future payment goes to principal, accelerating the payoff even further.

Things to check before overpaying

• Early repayment charges (ERCs): Many fixed-rate mortgages allow overpayments of up to 10% of the outstanding balance per year without penalty. Check your agreement before overpaying more than that.
• Higher-rate debt first: If you have credit card debt at 20% APR and a mortgage at 6.8%, pay off the credit card first. Always target the highest rate debt.
• Emergency fund: Keep 3–6 months of expenses accessible before aggressively overpaying a mortgage. Once the money is in the property, it's not easily liquid.

Should you consolidate your debts?

Debt consolidation can simplify your finances and reduce interest costs — or it can cost you more in the long run. Here's how to tell the difference.

What is debt consolidation?

Consolidation means taking out a new loan to pay off multiple existing debts — credit cards, personal loans, overdrafts — leaving you with a single monthly payment. The pitch is lower interest, simpler finances, and a clear payoff date.

When it makes sense

• The new loan's APR is meaningfully lower than the weighted average rate of your existing debts.
• You're struggling to track multiple payment dates and risk missing payments.
• You want a fixed payoff date rather than minimum-payment treadmills on credit cards.

When it doesn't

• The new term is much longer — a lower monthly payment over 7 years may cost more total than a higher payment over 3 years.
• You don't address the behaviour that created the debt — consolidating credit cards and then running them up again leaves you worse off.
• The new loan has high origination fees that eat into the interest savings.

When does it make sense to refinance?

Refinancing replaces your existing mortgage with a new one. Done at the right time, it can save tens of thousands. Done at the wrong time, it can cost you.

The break-even calculation

Refinancing comes with closing costs — typically 2–5% of the loan amount. To know if it's worth it, calculate how long it takes for the monthly savings to offset those costs. If you plan to move before that break-even point, it probably isn't worth it.

Signs it's worth refinancing

• Rates have dropped by at least 0.75–1% since you took out your mortgage.
• Your credit score has improved significantly, qualifying you for better rates.
• You want to switch from a variable to a fixed rate for payment certainty.
• You plan to stay in the property long enough to pass the break-even point.

Signs to wait

• You're in the early repayment charge period on your current deal.
• You're planning to move within 2–3 years.
• The rate difference is small and closing costs are high.

Fixed vs variable rate — which should you choose?

The choice between a fixed and variable mortgage rate is essentially a bet on where interest rates are heading. Here's a framework for thinking about it.

Fixed rate

Your interest rate stays the same for an agreed period — typically 2, 3, 5, or 10 years. Your monthly payment is predictable, which makes budgeting easier and protects you if rates rise. The trade-off: you won't benefit if rates fall, and you may face early repayment charges if you want to exit the deal early.

Variable rate

Your rate can move up or down, usually in line with the central bank base rate or your lender's standard variable rate. You can benefit if rates fall, but your payment can increase if they rise — sometimes significantly.

A simple rule of thumb

If you need certainty — a tight budget, a young family, a single income — a fixed rate is usually the right choice. If you have financial flexibility and rates appear to be at or near a peak, a variable rate or a shorter fixed term may be worth considering.