How to calculate an FX rate: A complete guide for your business

Key takeaways

• To send AUD overseas, multiply the AUD amount by the exchange rate. To check a foreign invoice against AUD, divide the foreign amount by the rate.

• Banks typically add a margin of 4–6% above the interbank rate¹. On A$50,000 of annual supplier payments, that's up to A$3,000 lost to hidden costs each year.

• Airwallex offers the interbank rate plus a margin from 0.5% for major currencies. On that same A$50,000, the margin drops to as little as A$250 – a potential saving of up to A$2,750 a year.

If you run an Australian business, exchange rate movements can affect your bottom line more than you realise. Paying overseas suppliers, settling foreign invoices, and funding international transfers all expose you to currency risk – and to hidden costs most providers don't disclose.

This guide breaks down the maths behind currency conversion, shows you how to calculate the real margin your provider charges, and covers practical ways to manage FX risk and automate your workflow.

Why exchange rates are hard to calculate

Two things make exchange rate calculation difficult for finance teams: rates move constantly, and most providers don't show you the real one.

Australia operates a floating exchange rate system – the value of the Australian Dollar is set by market supply and demand, not government intervention. Currency markets never really stop moving: global FX turnover averaged US$9.6 trillion a day in April 2025, according to the Bank for International Settlements, an increase of 28% from three years’ earlier². That constant movement makes it hard to budget for imports or price exports with confidence.

The second challenge is transparency. Banks and brokers rarely quote the true market rate. Instead, they build their margin straight into the rate they show you, so it's difficult to see what you're actually paying.

What is the mid-market exchange rate?

The mid-market exchange rate – also known as the interbank rate – is the wholesale benchmark banks use to trade currency among themselves. It sits halfway between the wholesale buy and sell price.

Because the wholesale market is liquid and trades 24 hours a day on weekdays, it's the fairest available valuation of a currency pair, though banks generally reserve it for institutional trades, routing standard business payments through a marked-up retail rate instead. 

How to calculate the exchange rate between two currencies

Every currency pair has a base currency (listed first) and a quote currency (listed second). In AUD/USD, AUD is the base, and the rate tells you how much USD buys one AUD.

Australian businesses typically run this calculation two ways: converting AUD out to pay a supplier, or converting a foreign invoice back to AUD to reconcile the books.

How to convert AUD into a foreign currency

Formula: Foreign amount = AUD amount × exchange rate

Worked example – paying a US software vendor: you need to pay a A$1,000 licensing bill in USD. At an AUD/USD rate of 0.65:

A$1,000 × 0.65 = US$650

Your vendor receives US$650.

How to convert a foreign invoice amount back to AUD

Formula: AUD cost = foreign amount ÷ exchange rate

Worked example – reconciling a USD invoice: you receive a US$650 invoice from an overseas partner. At the same AUD/USD rate of 0.65:

US$650 ÷ 0.65 = A$1,000

Accounting teams use this formula to process accounts payable and check that a completed transfer settled at the rate they expected.

Spot rate vs forward rate

A spot rate is today's market rate for immediate settlement, usually within two business days. A forward rate lets you lock in an exchange rate today for a transfer that happens later – typically in 30, 60 or 90 days.

Australian importers use forward contracts to protect upcoming supplier payments from short-term currency swings. The forward rate is calculated using interest rate differentials between the two currencies:

Forward rate = Spot rate × (1 + r_quote × t) ÷ (1 + r_base × t)

Where r_quote is the annualised interest rate of the quote currency, r_base is the annualised interest rate of the base currency, and t is the number of days until settlement divided by 365.

Airwallex offers forward contracts for eligible customers to help manage this risk.

How to reverse-calculate the rate applied to a past transaction

Formula: Rate applied = foreign currency received ÷ total AUD deducted

This is a useful audit technique – banks often mix flat fees with a marked-up rate, so the rate on your statement doesn't tell the full story.

Worked example: an invoice payment of US$10,000 resulted in a total deduction of A$15,800. The rate actually applied was:

US$10,000 ÷ A$15,800 = 0.6329

Comparing that 0.6329 against the historical mid-market rate for that date shows you exactly what the provider's markup cost you.

Why your bank's exchange rate differs from the mid-market rate

Banks make money on international transfers by quoting a retail rate that's typically worse than the interbank rate. The gap is their margin, on top of any flat transfer fee.

That margin looks small as a decimal, but it compounds fast. A 3% margin on A$50,000 of annual supplier payments costs you A$1,500 a year. At A$500,000 in annual volume, the same 3% margin costs A$15,000.

How to calculate the percentage margin your provider charges

Formula: Margin % = ((Provider rate − Mid-market rate) ÷ Mid-market rate) × 100

Worked example: your bank quotes an AUD/USD rate of 0.6240. The mid-market rate that day is 0.6500.

((0.6500 − 0.6240) ÷ 0.6500) × 100 = 4%

Your bank has added a 4% margin to the rate you were quoted.

Where to find the real exchange rate

• Reserve Bank of Australia daily rates: Published at 4.00pm Sydney time (AEST or AEDT) every business day; the official statistical benchmark in Australia.

• Google Finance: Real-time mid-market spot rates.

• Mastercard currency converter: The daily wholesale rate applied to card payments.

• XE.com: Live interbank rates.

• Airwallex transfer calculator: Real-time rates across our full network of supported currencies.

How exchange rates are determined in Australia

Australia has operated a floating exchange rate since the dollar was floated in December 1983. The value of the AUD is set by supply and demand in the global market, not by the RBA. The RBA doesn't target a specific rate, though it can intervene in extreme conditions to keep the market functioning.

The main drivers of the Australian Dollar:

1. Interest rate differentials: The gap between the RBA cash rate and rates set by other central banks, like the US Federal Reserve. Higher domestic rates attract foreign capital, which lifts the AUD.

2. Inflation: Over time, exchange rates adjust to equalise the cost of goods across countries. Faster inflation in Australia tends to push the Australian Dollar down.

3. Terms of trade: Australia's currency tracks commodity prices closely, given exports like iron ore, coal and gas. Rising export prices lift demand for the Australian Dollar

4. Global risk sentiment: As a commodity currency, AUD tends to rise when investors are confident about global growth, and fall when they flee to safe havens like the USD.

How to calculate exchange rates in Excel or Google Sheets

1. Enter your base AUD amount in cell A2 (e.g. 10000).

2. Enter the three-letter ISO code for your target currency in cell B2 (e.g. USD).

3. Pull in a live rate in cell C2: =GOOGLEFINANCE("CURRENCY:AUD"&B2)

4. Calculate the converted amount in cell D2: =A2*C2

5. Drag the fill handle down from C2 and D2 to apply the formulas to your whole ledger.

Common mistakes when calculating an exchange rate

• Ignoring SWIFT intermediary fees: Payments routed through correspondent banks often lose money to processing fees along the way, so your recipient gets less than you sent.

• Accepting Dynamic Currency Conversion (DCC): Choosing to pay in AUD at an overseas terminal or ATM lets the merchant's provider convert your funds at an inflated rate.

• Comparing rates without all fees: A quoted exchange rate means little without factoring in flat fees, percentage markups, and any receiving-bank charges.

Here's what the Big Four currently charge for a standard international payment.

Overseas correspondent bank fees can apply on top of these unless covered by the bank.

Why businesses choose Airwallex for international transfers

Relying on traditional banking channels for regular international payments adds friction and cost.

• Wholesale FX rates: Interbank rates plus a transparent margin from 0.5% for major currencies (including AUD, USD, EUR, GBP, CAD, CHF, NZD, SGD, HKD, CNY and JPY) and 1% for others.

• Global Accounts: Local foreign currency accounts in 20+ currencies, so you can collect client payments like a local business from 70+ countries. 

• Fast transfers: Free transfers to 120+ countries via local payment rails, with about 93% of international transfers settling the same day and 45% instantly.

• Reconcile in seconds: Connect to Xero, NetSuite, QuickBooks, Sage and Odoo to sync international expenses automatically.

Frequently asked questions

Why do currency exchange rates change?

In a floating system like Australia's, exchange rates fluctuate based on global supply and demand for the Australian Dollar. These movements are driven by interest rates, inflation differentials, trade balances, commodity price shifts, and changes in investor risk sentiment.

Is there a good time to convert currency for business?

Timing the currency market is difficult, even for seasoned professionals. Instead of speculating on rate swings, focus on practical risk management – hold multi-currency balances in Global Accounts to settle bills directly, or use forward contracts to lock in rates for large, known upcoming payments. 

Why do exchange rates differ between providers?

Every bank and payment provider adds its own margin on top of the interbank rate to cover operational costs. Digital-first platforms like Airwallex can offer tighter margins by running on modern rails, bypassing SWIFT intermediaries, and avoiding the overheads of a branch network.

What is a good exchange rate for AUD to USD?

There's no single good rate – it depends on current market conditions. Check the mid-market rate on the RBA website or XE.com, and use that as a benchmark to evaluate your provider's quote.

Sources

1. https://www.rba.gov.au/publications/bulletin/2026/feb/on-the-road-to-better-cross-border-payments-how-is-australia-travelling.html

2. https://www.bis.org/statistics/rpfx25_fx.htm 

3. https://www.nab.com.au/business/international-and-foreign-exchange/international-money-transfers 

4. https://www.westpac.com.au/business-banking/international-trade/international-payments/ 

5. https://www.commbank.com.au/business/international-business-payments/international-money-transfer.html 

6. https://www.anz.com.au/content/dam/anzcomau/documents/pdf/anz-business-general-service-fees-charges.pdf