Why your iPhone and Apple Watch show different step counts

The short answer

• Your iPhone misses the steps you take without it. In two studies of everyday life, iPhones counted 12% and 21.5% fewer steps than a pedometer worn all day, mostly because they weren't carried all the time.
• Your Apple Watch counts while you wear it and knows when it is on your wrist.
• Health combines the two by your data source order, so its daily total is not iPhone plus Watch.
• Apps that read steps from Health, Stepdy among them, show Health's combined total.

How each device counts

iPhone has counted steps with a motion coprocessor since the iPhone 5s in 2013. It measures motion continuously, even while the phone is asleep, but only the motion of the phone itself.

Apple Watch counts the steps you take while you wear it and uses a sensor to detect that it is on your wrist; with wrist detection off, some Activity measurements are not available. If your watch's steps are missing from Health, check that Fitness Tracking is on in Health under your profile, Devices, Apple Watch, Privacy Settings.

How the Health app combines them

When several sources record the same kind of data, Health uses the order of your sources: by default data you enter yourself comes first, then your iPhone and Apple Watch, then other apps and devices.

To see or change the order, open Health, tap Browse, then Activity, Steps, Data Sources & Access and Edit, drag a source up or down, and tap Done.

How accurate is step counting on iPhone?

On a treadmill at a steady pace, smartphone apps were within 6.7% of the steps actually taken (Case and colleagues, JAMA, 2015). An iPhone SE in a trouser pocket, a shoulder bag or a backpack was off by less than 3% at normal walking speeds (Höchsmann and colleagues, 2018).

In everyday life the gap is larger, because a phone isn't always with you. Over a week of normal life, people's own iPhones counted 12% fewer steps than a waist pedometer: the gap was largest for those who seldom carried their phone (Amagasa and colleagues, 2019). Another study found iPhones within 5% in the lab but 21.5% lower in free living (Duncan and colleagues, 2018).

Why your step app may show another number

An app that reads steps from Health shows Health's combined total, so it should match the Health app for the same day. Some features count differently: in Stepdy, the world and country rankings use the Health total, while groups and races use the step counts of your iPhone and Apple Watch directly, so a group board can differ slightly from your Steps screen.

A day's total can also change after the day ends. An Apple Watch that was charging or out of range hands its steps over later, so Health's number for yesterday can still rise; Stepdy keeps the last three days open so that those late steps still count toward your streak.

Getting the numbers closer

• Keep your phone with you, or wear your watch, for as much of the day as you can.
• Leave wrist detection and Fitness Tracking on.
• Check the order of your step sources in Health.
• Calibrate your Apple Watch with about 20 minutes of Outdoor Walk with good GPS. Apple says this improves distance, pace and calorie accuracy; it doesn't say it changes step counts.