How Is Term Deposit Interest Calculated? A Guide With Worked Examples

A term deposit rate is quoted as a percentage “per annum” — say 5.00% p.a. — but that figure alone doesn't tell you how many dollars will actually land in your account. The final amount depends on how much you deposit, how long you lock it away, and how often the bank pays the interest. Understanding the mechanics helps you compare two products that look similar on the headline rate but pay out differently.

The basic formula: simple interest

For a straightforward term deposit held to maturity, interest is calculated using simple interest:

Interest = deposit × annual rate × term (in years)

If you deposit $50,000 at 5.00% p.a. for a full 12 months, the calculation is:

• $50,000 × 5.00% × 1 year = $2,500

At maturity you receive your $50,000 back plus $2,500 in interest. Nothing about the rate is hidden — the “per annum” figure is the whole-year return on the amount you put in.

Terms shorter than a year

The rate is always annualised, so a shorter term earns a proportion of the annual figure. A six-month term deposit does not pay the full advertised rate as a lump sum — it pays roughly half, because you've only held it for half a year:

• $50,000 × 5.00% × 0.5 years = $1,250

This is why a headline six-month rate and a headline twelve-month rate aren't directly comparable in dollar terms. Both are annualised, but the twelve-month product has twice as long to earn. Our term deposit calculator does this arithmetic for any amount and term.

How payment frequency changes your return

Banks let you choose how often interest is paid — monthly, quarterly, annually, or at maturity. This choice can slightly change what you earn, and it comes down to compounding.

If interest is paid to a separate everyday account, it's simple interest: you receive the same $2,500 over the year, just in smaller instalments along the way. Useful if you want regular income, but the total is unchanged.

If interest is instead added back into the term deposit, each payment then earns interest itself. On $50,000 at 5.00% p.a. paid and reinvested monthly, you'd earn about $2,558 over the year rather than $2,500 — roughly $58 more, because the earlier interest payments compound. The shorter the compounding interval, the larger this effect, though on a one-year term the difference is modest.

Note that many banks pay simple interest at maturity on terms of twelve months or less, so compounding often only comes into play on longer terms or when you actively choose a periodic-payment option that reinvests.

Longer terms and compounding

On multi-year term deposits, compounding matters more. Take $50,000 at 5.00% p.a. over two years with interest paid annually and reinvested:

• End of year one: $50,000 × 5.00% = $2,500, giving a balance of $52,500
• End of year two: $52,500 × 5.00% = $2,625

Total interest is $5,125, compared with $5,000 if the interest were simple. The extra $125 is the second year's interest earned on the first year's interest. Always check whether a multi-year rate compounds or pays out annually — two products at the same headline rate can return different amounts.

A quick way to sense-check the interest

You don't need a calculator to get a rough figure. One per cent of your deposit is a useful anchor: 1% of $50,000 is $500, so a 5% rate is about five times that — roughly $2,500 for a full year. Halve it for a six-month term, and add a little if the interest compounds. This mental shortcut won't be exact to the dollar, but it's enough to tell instantly whether an advertised return looks right, or whether a “bonus” offer is smaller than it sounds once you translate the percentage into actual money.

What the rate doesn't include

The interest calculation gives you the gross return. A few things sit outside it:

• Tax. Term deposit interest is assessable income, taxed at your marginal rate. Your after-tax return is lower than the gross figure.
• Early withdrawal. If you break the term before maturity, most banks apply an interest reduction, so the calculated amount assumes you hold to the end.
• Inflation. The dollar figure is a nominal return; rising prices erode its real value over the term.

Check the numbers before you lock in

Once you understand the formula, comparing products is straightforward: decide your amount and term, then look at the rate and the payment option side by side. You can see the current highest rates on our best term deposit rates page and the full comparison table on the homepage.

This is general information, not financial advice. Rates and product terms change — confirm the exact figures and interest arrangements with the bank before you open a term deposit.

One thing you can do now: take the amount you're planning to deposit, run it through the calculator at a couple of different terms, and see how the dollar return — not just the headline rate — compares.