Simple vs compound interest for business: where each one hides in your debts

Put $10,000 into a savings account and leave it, and the interest you earn in year three is a little larger than in year one, because last year’s interest has joined the pile. Leave $10,000 of debt unpaid and the same thing happens in reverse. That one idea, interest earning interest, is the whole difference between compound and simple interest.

For business owners the useful question isn’t the textbook definition. It’s where each kind turns up in your finances, because each turns up in surprising places: a tax debt, an overdraft you keep topped up, a short-term advance priced as a flat sum. This guide walks through each with dollar examples and no rates.

What is simple interest?

Simple interest is calculated on the original amount only. If the interest for the first year is $1,000 on a $10,000 balance, it is $1,000 in the second year and $1,000 in the third, so three years costs $3,000 whatever happens in between. The cost grows in a straight line.

True simple-interest loans are rare in business lending, but the idea matters because some products behave like it. A flat-cost advance, where the lender fixes the total repayable up front, doesn’t build on itself during the term. We cover those in what a factor rate is.

What is compound interest?

Compound interest is calculated on the original amount plus any interest that has already been added. Take the same balance and let the unpaid interest join it each year:

Year	Balance at start	Interest charged	Balance at end
1	$10,000	$1,000	$11,000
2	$11,000	$1,100	$12,100
3	$12,100	$1,210	$13,310

(Each year’s interest is in the same proportion to the balance as year one’s; the table shows the effect, not any particular lender’s price.)

After three years the compound path has cost $3,310 against $3,000 for simple interest. That’s a $310 difference on a modest sum over a short time. Stretch the period, or the balance, and the gap widens quickly, because each extra year builds on a bigger base.

Two details decide how far compounding goes. One is the frequency: interest added daily compounds a little faster than interest added annually. The other is whether interest ever stays unpaid. Compounding only happens to interest that stays on the balance.

Does my business loan compound?

Mostly, not in the way people fear. A standard term loan uses a reducing balance method. Interest is worked out on the outstanding balance, often daily, and is charged with each repayment, which pays the interest first and reduces the principal with the rest. Because the interest is cleared every instalment, there is almost nothing left to compound. Our guide to business loan interest without the jargon shows the split, and the glossary entry on amortisation explains how the interest share shrinks over time.

Compounding becomes a real feature of a business loan in a few cases:

• Interest-only periods that capitalise. If interest isn’t paid but is added to the balance, you borrow against it. See what capitalised interest means for you.
• Missed or reduced repayments. Shortfalls roll into the balance, and default interest may apply on top. The glossary covers default interest.
• Overdrafts and credit cards. If you carry a balance month to month, unpaid interest usually joins it.

Why is ATO debt the clearest example of compounding?

This is where many owners meet compound interest for the first time. Per the ATO, the general interest charge (GIC) compounds daily on whatever is overdue. The rate changes each quarter, so it pays to check the ATO’s current figure, but the structure is what matters here: every day an unpaid amount sits, the interest on it is added and then earns interest itself.

There’s a second sting. The ATO says you can no longer deduct GIC incurred on or after 1 July 2025, even when the debt itself comes from an earlier year. Before that date it was deductible, which softened the cost. For a business with a tax debt running into the tens of thousands, losing the deduction raises the after-tax price of leaving the debt alone.

Three practical consequences:

1. Small debts become larger debts. Leaving a BAS or tax bill unpaid is the most expensive form of borrowing many owners use, because the compounding is automatic and there’s no negotiation.
2. A payment plan doesn’t necessarily stop the clock. Our guide to ATO payment plans covers how interest continues while you pay it down.
3. Refinancing the debt may cost less. Moving it into a structured loan with fixed repayments can replace daily compounding with a reducing balance and a clear end date. See paying tax debt with a business loan for the trade-offs, and how to deal with an ATO debt for the first steps.

Can a flat-cost product be worse than compounding?

Yes, and this is the counter-intuitive one. A flat-cost product sets the total repayable at the start, say $40k advanced and $50k to be repaid. There is no compounding because the cost doesn’t grow. But it also doesn’t shrink as you repay. If you repay the $50k over 20 weeks, you’ve paid $10k for money that was only partly outstanding for most of that time. The same $10k spread over 52 weeks is a much lighter burden per week of borrowing: the 20-week deal costs about 2.6 times as much for each week the money is out.

So “simple” isn’t automatically “cheap”. Compare products on total dollars and on how long the money is actually outstanding, not on whether the interest is compound or simple. The factor rate calculator shows the effect, and early repayment savings and penalties explains why paying early rarely helps with this structure.

Which type of interest works for your business, and when?

Compound interest isn’t good or bad in itself. It depends on which side of it you’re on.

• Saving or holding surplus cash: compounding helps. A cash buffer in an interest-bearing account earns on its earnings.
• Borrowing with regular repayments: you mostly avoid its worst effects because interest is paid as you go.
• Carrying unpaid balances: compounding works against you, fastest where it’s daily and unpaid for months.

If you want to see how a loan’s interest and principal split across the term, the business loan repayment calculator shows it repayment by repayment.

If you have a balance that’s compounding right now and you’re not sure what to do, tell us about it; there’s no need to have a plan first.

When it helps to talk it through

Knowing the maths is one thing; deciding what to do with a growing balance is another. Be it an ATO debt, an overdraft that never quite clears or an advance that is draining your takings, an outside view helps.

Our enquiry takes about a minute. Asking won’t register on your credit file, because there’s no credit check at this stage, and your details aren’t passed on to a list of lenders. A real person reads what you’ve written, considers your situation and calls you. Fill it in as accurately as you can, including the size of the balance and who it’s owed to, so the first option we raise has a good chance of being the right one.

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