iPhone step tracker.... Do they work?

I've been using the one built into my iPhone (which is an iPhone 5). In the app go to "More" then "Steps" and see if MFP app can find it on your phone.

I like it for days when I do a lot of on-off mild movement (e.g. long shopping trip, or going round a museum).

I sometimes take my phone when running or cycling. But I use an app to measure calories consumed for that - the steps counter records some steps, of course, but then MFP assumes that those were par of the running or cycling I've logged. So that is good: I don't end up double-counting.

That anti-double-counting business backfires slightly, of course if you do some exercise without the phone (martial arts in my case, or swimming maybe...). If you, say, went for along swim & logged that and then went for a long walk, you'd find that you weren't being credited for the calories consumed in your walk. But usually you're not being cheated of all that much . If you do feel cheated enough to bother correcting this, MFP still records the number of steps, so you could figure out how many calories you're due (e.g. look back at a day when you were credited calories for your steps, to work out the number of calories per 1000 steps) and then add those manually.