Using this site
Currency over time
You may read in a person's notes that he/she left an estate of £50 in 1600, and you want to convert £50 in 1600 to today's values? Not easy, and I'll let you read a full discussion of this subject at the Current Value of Old Money site to find out why and how. Purchasing power can be calculated at Measuring Worth giving you £6,767.49 using the retail price index. But this is a fairly meaningless exercise. Keep in mind that this does not measure comparative wealth. In modern economies we measure wealth in monetary terms. We put a price on a person's assets and income and come up with their "worth". Money plays a much more important role in our economy than it did in former years (or does in other cultures today). Also our "standard of living" includes a lot of stuff that people of former centuries did not have or need within their "standard of living". Compare, for instance, how far $A75 would go in Sydney, Australia, against its value in those places with a farming/ hunting/ craft/ exchange way of life, where $A75 might well be the annual money income for people - and they survive and some are "comparatively wealthy".
So making currency (purchasing power) comparisons is probably not all that helpful. Other things can be used to compare a person's comparative wealth within their time and place in history - land owned, type of housing, etc. See also
Australian and British currency
Australian currency used to be based on the old English currency or "sterling", and indeed aligned with it. Australia went decimal in 1966. It converted the pound into two dollars:
There were twenty shillings to the pound, so 10 shillings became $1.
The Australian pound was tied to sterling (one for one) until the Australian dollar was "floated" against other currencies in 1983. So add consideration of exchange rates and the passage of time, and you will realize that you cannot say that A$1 is today worth 50p just because, in 1966 Australia, the conversion was made on that particular formula. At the time of writing (June 2010) the Australian dollar could buy something like 58p (and £1 could buy A$1.75).
Other metric conversions for area, distance, weight, etc. can be done at World Wide Metric. I have tried as a rule to provide metric measurements with their equivalent in older systems, but I may have missed some.