It's a question that has sometimes raised great debate and several websites such as www.measuringworth.com and www.nationalarchives.gov.uk/currency/ have been set up to answer it.
But if you have an iPhone/iPod/iPad you can now get an app for that.
Using records from the National Archives, RevelMob and historian Nick Barratt have developed an app called "Old Money".
In a press release The National Archives said
Our records are not just the first draft of history, but are increasingly being used in smart ways by developers to create innovative products and services for anyone who needs instant access to practical historical information.
The Old Money application enables users to make sense of historical facts and figures which might otherwise be meaningless, such as the 1761 sale of Buckingham Palace for £21,000 or William Shakespeare's annual salary in 1599 of £100 a year.
Details can be found on the RevelMob website at www.revelmob.com/currency-converter.