Use this calculator to determine the gear ratios of any hub gear equipped bicycle.
It can handle all the common hub gears as well as bottom bracket gearing systems and give you output in metres or gear inches. If you want to know how fast you can travel in a given gear ratio choose "mph" or "kph" as the output.
For Bromptons just click the box for Brompton-compatable data only.
For an easy way to compare different internal gear systems try this handy interactive spreadsheet.