Because GPS is accurate to a few meters, not to the centimeter. Your watch records your position again and again and adds up the straight lines between the points, and every point is a little off in its own way. On a winding route, under trees or between tall buildings, those small errors add up to a different total on every run. A difference of a few percent is normal. More than that usually has a cause you can fix.
How much difference is normal?
A few percent. GPS in a phone is typically accurate to within about 5 meters under open sky, and less accurate near buildings, bridges and trees. In a 2020 study of eight sport watches, including an Apple Watch Series 4, the average distance error was 3 to 6%, depending on the watch. In the city and in the forest the watches generally measured short. On a running track most measured slightly long.
On a 5 km loop, 3% is 150 meters. One run showing 4.92 km and the next 5.08 km on the same loop is ordinary.
Why is it different every time?
| Cause | What happens |
|---|---|
| Buildings and trees | The signal is blocked or bounces, so the track wanders or cuts corners |
| A winding route | Points are joined with straight lines, so curves come out a little short |
| Where you run | Crossing the road, taking corners wide or tight, weaving around people. Your real distance changes too |
| A fast start | If the watch has not settled on a good signal yet, the first stretch can be off |
| Fewer GPS readings | A battery-saving setting records your position less often |
Does the watch use anything besides GPS?
Yes. Where the GPS signal is weak or missing, Apple Watch estimates distance from its motion sensors, using the stride length it has learned on your earlier outdoor walks and runs. Apple calls this calibration. A stretch through a tunnel or under dense trees is only as accurate as your calibration.
How do I get more consistent distances?
- Start in the open. Open the workout outside, away from tall buildings, and give the watch a few seconds before you set off.
- Keep calibration working. In the iPhone’s Settings, under Privacy & Security, Location Services, make sure Motion Calibration & Distance is on. The watch keeps learning your stride on every Outdoor Walk and Outdoor Run. Apple suggests about 20 minutes at your normal pace on flat, open ground with a clear sky.
- Reset it if distances stay wrong. In the Watch app on iPhone, tap Privacy, then Reset Fitness Calibration Data, and calibrate again.
- Check the battery setting. On the watch, Settings, Workout, Fewer GPS and Heart Rate Readings saves battery on long outdoor workouts by recording less often. It also turns off splits and current pace, which is a quick way to tell it is on.
- On a running track, pick your lane. When Apple Watch detects that you are at a track during an Outdoor Run, it asks for your lane and measures the distance from it.
Why is my race longer than the official distance?
Certified race courses are measured along the shortest line a runner could take, cutting every corner, and then made 0.1% longer so that no one runs short. Nobody runs that line exactly, least of all in a crowd, so a watch usually shows a little more than the official distance.
So how do I compare runs on the same route?
By time, not by pace. If you start and finish in the same places, you covered the same distance whatever the watch says, and your time is exact. Pace is your time divided by the measured distance, so it carries every GPS error with it. A run that measured 3% short shows a pace about 3% slower than you really ran. At 5:00 per kilometer that is 9 seconds per kilometer, enough to look like a bad day when it was only the GPS.
Look at heart rate next to the time. The same time at a lower heart rate means you got fitter. See How do I know if I’m getting better at running?.
What does Kiiro show?
Kiiro recognizes your routes by the shape of the GPS track, and matching allows for normal GPS drift and small differences in where you start and stop. Runs on a route you repeat land in the same history, with pace, heart rate and effort side by side, your personal bests, and the trend over weeks and months. In the app this is called Route progress. When one run’s pace looks out of line with the rest, check its distance before you trust it. See route comparison in Kiiro and How do I compare runs on the same route with Apple Watch?.
Related questions
- How do I compare runs on the same route with Apple Watch?: Apple Watch has Race Route, which lets you race your last or best time on a route you have run before. To look back at every run on a route, you need an app. Here is how both work.
- How do I know if I'm getting better at running?: Compare like with like. The clearest sign is the same route at a lower heart rate, or faster at the same heart rate. Runs that feel easier, a rising VO2 max and a falling resting heart rate back it up over months.
- How do I analyze my Apple Watch runs?: In three places. The Fitness app shows each run in detail, the Health app shows long-term trends such as VO2 max and resting heart rate, and an app that reads Apple Health shows what your runs add up to.
