Glossary
Interest Rate
An interest rate is the price of money: what you earn for lending it and what you pay for borrowing it. The RBA's cash rate sits at the top of the chain, and every savings account and variable mortgage in Australia moves with it.
What it is
An interest rate is the percentage charged or earned on money over time - the price of borrowing and the reward for lending. In Australia the chain starts at the Reserve Bank's cash rate, the rate banks pay to borrow overnight. When the RBA moves the cash rate, variable mortgage rates and savings account rates move with it within weeks. Fixed rates price off what markets expect the cash rate to do, not what it is today.
For a saver, the rate is income: $10,000 at 4.5 percent earns $450 a year. For a borrower, the rate is cost: a $500,000 mortgage at 6.0 percent costs roughly $2,998 a month. The same loan at 5.5 percent costs roughly $2,839 - about $1,900 a year for half a percentage point. The rate is the single number every loan, savings account and term deposit in Australia is priced from, which is why the RBA's meetings get more attention than they deserve and why your mortgage statement deserves more than it gets.
Key takeaways
- An interest rate is the price of money: earned for lending, paid for borrowing.
- The RBA's cash rate anchors the chain; variable rates move with it within weeks.
- Fixed rates price off expectations of future cash rate moves, not today's rate.
- Half a percentage point on a $500,000 mortgage is roughly $1,900 a year.
- The rate on your savings is income; the rate on your mortgage is a cost that compounds.
How it works
Walk the chain. The RBA sets the cash rate - the overnight rate between banks - and announces it after each board meeting. Retail rates follow: variable mortgage rates and savings account rates adjust within weeks, because the banks' own funding costs move with the cash rate. Fixed rates work differently: a three-year fixed mortgage is priced off the market's expectation of where the cash rate will be over those three years, which is why fixed rates can rise while the cash rate falls.
The arithmetic runs one of two ways. Simple interest pays only on the original principal: $10,000 at 4.5 percent simple earns $450 a year, every year. Compound interest pays on the accumulated balance - the compound-interest entry covers the exponential version - and it is how savings accounts, super and reinvested dividends actually work. Borrowing runs the same arithmetic in reverse: mortgage interest is calculated on the outstanding balance, typically daily and charged monthly, so early repayments save more interest than late ones.
The mortgage arithmetic is where the rate bites hardest. A $500,000 loan over 30 years at 6.0 percent costs roughly $2,998 a month; at 5.5 percent, roughly $2,839. Half a percentage point is about $1,900 a year - run your own numbers through the interest rate calculator or the compound interest calculator.
Why it matters
Every product you own or are considering is priced off an interest rate: your mortgage, your savings, your term deposit, your HECS indexation, and the discount rate behind every valuation. The RBA's cash rate decisions flow through to variable rates within weeks. A rate-cut cycle moves savings account rates and mortgage rates down together - the savings account less enthusiastically than the bank's marketing suggests.
A colleague named Brent (yes, that Brent) fixed his mortgage for three years at the peak of the rate cycle because 'fixed felt safe'. He then watched the cash rate fall while his fixed rate sat two points above the variable market. The safety was real; so was the cost. The highest-leverage thing most Australians can do is know which of your rates are variable and which are fixed, check what your savings account actually pays against the cash rate, and run every borrowing decision through the interest rate calculator and the cash rate entry before signing anything.
Real-world examples
$10,000 in a HISA at 4.5 percent for one year
A $10,000 balance in a high-interest savings account at 4.5 percent earns $450 over a year - simple, visible, and paid into your account monthly on the daily balance. The same $10,000 in a transaction account earns nothing, and the difference is the price of money doing its job. If the RBA cuts the cash rate, expect that 4.5 percent to follow within weeks - savings rates are variable by design, and the advertised number is a snapshot of the current cycle.
$500,000 mortgage at 6.0 percent versus 5.5 percent
A $500,000 loan over 30 years at 6.0 percent costs roughly $2,998 a month. The same loan at 5.5 percent costs roughly $2,839 - a difference of about $159 a month, or roughly $1,900 a year, for half a percentage point. Over the full 30-year term the gap compounds into tens of thousands of dollars, which is why refinancing decisions deserve arithmetic rather than loyalty. The rate is the price; the term is the multiplier.
The fixed rate priced off expectations
Brent fixed for three years at the top of the rate cycle because fixed felt safe. The cash rate then fell through the following two years, and his fixed rate sat well above the variable market for the remainder of the term - the break costs to exit early ran into four figures. Fixed rates price off expectations of future cash rates, not today's rate, which is why fixing at a peak is the most expensive form of safety available.
Common mistakes
Mistake: Assuming your savings account rate will stay where it is.
Why it bites you: Savings rates are variable and follow the cash rate within weeks. The advertised 4.5 percent is a snapshot; check it after every RBA board meeting, because the bank will not tell you twice.
Mistake: Fixing a mortgage at the peak of the rate cycle for safety.
Why it bites you: Fixed rates price off expectations, and fixing at a peak locks in the expensive part of the cycle. Brent's three-year fix cost four figures in break costs when the cycle turned.
Mistake: Comparing loans on the advertised rate instead of the comparison rate.
Why it bites you: Fees and conditions sit outside the advertised rate - the [comparison-rate entry](/glossary/comparison-rate/) covers the true-cost number Australian lenders must publish. A 5.8 percent rate with fees can cost more than 6.0 percent without.
Mistake: Ignoring the interest rate on your HECS-HELP indexation.
Why it bites you: HELP balances are indexed to CPI each June, and the 2023 print added 7.1 percent. The rate on that debt is not one you chose, but it is one that compounds against you.
Ivy's take
I spent an afternoon pulling the last five years of RBA board decisions and holding them against what my savings account actually paid in each window so you don't have to, and the pass-through was real, lagged, and consistently less generous than the marketing. The cash rate is the anchor; the retail rate is the anchor plus a margin the bank keeps.
The framing I use is one line: know which of your rates are variable, because those are the ones that move without your permission. The mortgage at 6.0 versus 5.5 percent is $1,900 a year of pure arithmetic, and refinancing decisions deserve a spreadsheet rather than a feeling. Brent fixed at the peak for safety, and the safety cost him the cycle. Interest is the price of money. Know what you are paying, and what you are being paid, at all times.
Related jargon, defined
- Cash rate
- The RBA's overnight rate between banks, the anchor for every retail rate.
- Variable rate
- A rate that moves with the cash rate within weeks of each RBA decision.
- Fixed rate
- A rate locked for a term, priced off expectations of future cash rates.
- Compound interest
- Interest paid on the accumulated balance, the exponential version of the arithmetic.
- Comparison rate
- The true-cost rate including fees that Australian lenders must publish.
- Indexation
- Automatic CPI adjustment, as applied to HECS-HELP debts each June.
Frequently asked questions
What is an interest rate in simple terms?+
It is the price of money: the percentage you earn for lending it and pay for borrowing it. In Australia every retail rate chains back to the RBA's cash rate, which variable rates follow within weeks.
How quickly do Australian mortgage rates follow the RBA cash rate?+
Variable mortgage rates typically adjust within weeks of an RBA decision, because banks' funding costs move with the cash rate. Fixed rates are different - they price off expectations of future cash rates, not today's.
How much does half a percent matter on a mortgage?+
On a $500,000 loan over 30 years, the difference between 6.0 and 5.5 percent is roughly $159 a month, or about $1,900 a year. Over the full term it compounds into tens of thousands of dollars.
Is a fixed rate safer than a variable rate?+
It is predictable rather than safer. Fixed rates lock out rises but also lock out falls, and fixing at the peak of a cycle is the expensive version of safety. The right choice depends on your budget's tolerance for rate moves.
Why does my savings account rate change when the RBA cuts?+
Because savings rates are variable and priced off the cash rate plus a margin. The bank's funding costs fall with the cash rate, and the advertised rate follows - though historically less generously than the marketing implies.
Disclaimer: This information is general in nature and does not constitute financial or legal advice. Always consult a qualified professional for your specific situation.
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Ivy Sinclair-Wren
Financial Chaos Analyst
Ivy Sinclair-Wren is a Financial Chaos Analyst covering investing, AI, wealth psychology, and the emotional consequences of opening finance apps during market crashes. Based in Melbourne, she specializes in demystifying the Australian tax code and helping users navigate the intersection of spreadsheet logic and human irrationality.