When you're out travelling the globe, it's a pain to always be taking out your phone to calculate the exact currency exchange rate. If all you want is a quick estimate, an article on Gadling offers rules of thumb for converting several currencies into dollars.
Photo by ricardo
If you want to know what the exact exchange rate is, definitely look it up (eg, at Xe.com). But these rules of thumb are handy when you just want to roughly know how much money you're taking out from the foreign country's ATM in Australian dollars or whether a tchotchke sold at the market is worth the price.
The mental maths for most countries is pretty easy: in Belize, divide prices by half, in the UK multiply by 1.5 and round up, and in the US and Switzerland no maths is necessary for your estimate (they're roughly 1-to-1 with the Australian dollar). Unless you like doing maths, however, some countries are more calculator-worthy (for Colombia, for example, you have to divide by half, then by 1000, then add back 20 per cent).
Check out the full list of currency converting rules of thumb at the link below. Note the article has a US focus, but given the near-parity of the Australian dollar, the conversions are still valid.