iPS: We like interval training because you can pack a lot of effective exercise into a short amount of time. In fact, you can use it to do one of the most effective routines in only 20 minutes. Regardless of which method you employ, you need a good timer. Repeat Timer gives you one for free.
Repeat Timer can act simply as a stopwatch or countdown timer, but it can also handle intervals. You just set the main timer to whatever you want, then the interval timer to a separate length, and then choose the number of times you want both timers to repeat. So, for example, if you wanted to bike really fast for eight seconds and bike casually for 12, then repeat 60 times until you finished your 20 minute workout, you could. You'd just set the main timer to 8, the interval timer to 12, and the repeat amount to 60. That may sound a little confusing, but once you use the app you'll soon master it.
The downside is that the free version of Repeat Timer only allows five repeats and one timer. You have to pay $2.99 for the pro version that provides you with much more. Still, if you need a customisable interval timer for the iPhone this is pretty much the best option you've got.