I'm about to start my first iPhone/iPod Touch app. It will have two input numeric text fields and a YES/NO switch. Once the user has entered all three inputs it will display some calculated output. All of this fits easily on one screen. There's plenty of space for the input fields and either a numeric keyboard or the calculated output. No need for a dedicated data entry screen for each field. However, there is a slight complication. The two numeric fields are weight and height. They should default to kilograms and meters, but give the user the option of entering non-metric measures: pounds and feet & inches. If the user enters non-metric measure it will then be converted to metric. It's envisaged that the user will mostly enter metric measures. Only occasionally will the data conversion be called upon. My question, from someone new to mobile apps, is how best to design for the data conversion. Should I make it so that tapping on the weight field opens a new screen where the user can either enter the number of kg and hit "done", or tap an extra key to enter pounds and then hit "done", returning to the main screen. This is a flexible approach, but it will slow the user down on the majority of occasions when they enter in kg. Or should I add a "conversion" button to the main screen? This button would appear when the keyboard pops up. If pressed, it would take the user to the non-metric data entry screen. But where should this conversion button be? In the main body of the screen, just above the keyboard? On the toolbar? (But then I would need to make sure the keyboard doesn't cover the toolbar.) Not on the Navigation bar, surely? Any ideas, anyone?