Use one currency consistently inside a scenario. The calculator does not convert between currencies.
The interface may show a dollar symbol, but the figures mean the currency you chose to use. If your household works across currencies, convert amounts to one chosen currency before entering them and note the exchange rate assumption in the item or scenario name.
Tip: Do not mix a New Zealand dollar income with a United States dollar expense as if the numbers were equivalent.
Subscription billing currency is separate. It is selected from your billing country on the subscription page.