How does an iPhone count steps?
From motion, not location. An accelerometer in the phone senses its movement many times a second, and a low-power motion coprocessor watches that signal for the regular rhythm of walking. Each cycle of that rhythm is a step. No GPS, no internet and no app needs to be open; the chip counts on its own and hands the total to Apple Health. The Apple Watch does the same from your wrist, and Health reconciles the two.
What a step looks like to the sensor
Walking is periodic. With every stride the body rises and falls slightly and the phone moves with it, so the accelerometer sees a repeating pattern at roughly one to two cycles a second. The coprocessor waits for that pattern to establish itself over a few steps before it starts counting, which is why the first few steps of a walk can go unrecorded, and why a single jolt does not register as a step.
What the phone misses
The phone counts only when it moves with you. On a desk, charging, or in a bag left on a chair it records nothing, however far you walk. Pushing a trolley or a pram with the phone in a jacket pocket counts less reliably, because the body’s motion is damped. Slow, shuffling walks give a weaker rhythm and are counted less accurately than a normal pace; validation studies generally find phones and watches count ordinary walking well and slow walking less well.
What the Watch adds
The Watch is on your wrist all day, so it catches what the phone misses: the walk to the kitchen while the phone charges, the lap of the office with the phone on the desk. It has its own blind spots. Pushing a trolley with both hands, or walking with a hand in a pocket, gives the wrist little to swing, and the count drops.
Why the two numbers never quite match
Two sensors, two positions on the body, two slightly different algorithms. On the same walk the phone might count 4,000 and the Watch 4,100, and neither is wrong. Apple Health keeps both and, minute by minute, uses one where they overlap, so the day’s total is not the sum of the two. That is the number to trust, and why your steps differ explains it in full. If the Watch’s count is missing from the phone, Apple Watch steps not syncing has the seven checks.
What GPS is for
Distance and pace. A step count is a count; a distance needs to know how far each step went. Without GPS the phone estimates from your stride, which is why treadmill distances drift. With GPS, during a recorded walk, an app can measure the path and report distance and pace. Manpo Steps uses location only when you turn it on for a walk, and never stores where you went.
Why the count still matters
The mortality research measures steps, not kilometres and not speed, and it rewards the total: see how many steps a day do you actually need? A sensor that counts your ordinary walking well, reconciled with the Watch by Health, is all the precision that evidence asks for.
Questions
Does the iPhone count steps in a bag or a pocket?
Yes, as long as the bag or pocket moves with you. The phone counts the rhythm of your walk from its own motion, so a phone in a backpack on your back counts; the same phone in a bag left on a chair does not.
Does step counting need GPS or an internet connection?
No. Steps come from the accelerometer and the motion coprocessor inside the phone, which run whether or not location or data is available. GPS is only involved when an app measures distance and pace for a walk.
Does the iPhone count steps on a treadmill?
Yes, because the count comes from motion, not from moving across the ground. Distance on a treadmill is estimated from stride rather than measured, so it is less exact than the step count.