The doesn't require an Apple Watch in order to be useful; a step counting ability is built right into an iPhone, making use of the accelerometer sensors to identify and record when a step is taken, even if the app isn't open. Since an iPhone is often in a pocket or in the hand, it can record a very accurate measurement of how much walking takes place over time.
To find this information, the can go to the iPhone's Home screen then swipe down to open the search bar and type 'Health' to find and open the app. some health and fitness metrics on the 'Summary' page but if steps aren't seen, they can be found by tapping the 'Browse' tab and navigating to the 'Steps' subcategory. Steps can also be added to favorites to ensure it appears on the 'Summary' tab when the app is opened. Tapping Steps on the 'Summary' or 'Browse' tab will open a page full of information, including a graph of steps over time.
How Accurate Is The iPhone's Step Counter?
The iPhone uses its accelerometer sensor to know when a step is taken. It's relatively easy for an iPhone to record this information over time and build the bar charts and statistics shown on the 'Steps' page. The iPhone also makes use of the pedometer along with other data, such as GPS and altimeter readings, to calculate interesting information like Walking Steadiness, Step Length, Flights Climbed, and more.
When it comes to accuracy, a few studies in recent years seem to have landed on a shared conclusion: its tracking abilities are decent, as long as the person actually has the iPhone on their person for the entirety of the walking time they want tracked. Researchers comparing the iPhone’s real-world results to those captured by pedometers/accelerometers worn on the waist found that the iPhone underestimated steps by anywhere from 12-21 percent on average. This was largely due to periods in which the person didn’t have the phone on them. In lab conditions, iPhone still underestimated steps, but to a milder degree.
As to be expected, there are third-party fitness apps, and even applications devoted to step counting, that might meet specialized purposes better. However, Apple's built-in pedometer that's tucked away inside the Health app is free and should meet most s' needs well. Of course, the iPhone needs to be in the hand or pocket in order to record steps, so it's worth always carrying the phone (if not wearing an Apple Watch) to make sure each step is counted.
Source: Apple, JMIR Mhealth Uhealth, Journal of Sports Sciences