The Nike+ Running App uses the GPS functionality and your mobile device's built-in accelerometer to accurately measure the distance of your run.* Depending on how and where you use the app, you may want to calibrate. Here's some background on how calibration works in different scenarios, and how to calibrate if you choose to do so.
If you run outdoors with the Nike+ Running App and are able to maintain a GPS signal, the Nike+ Running App will calibrate the accelerometer automatically, with no action required on your part. If you lose GPS signal during your run, Nike+ Running may offer you the option to calibrate your run by adjusting it to match your actual distance.
If you run indoors, please make sure you set your workout up as an Indoor run in the Location section of the Nike+ Running App run setup screen. You'll need to carry your device in your hand as you run.
If you wish to calibrate and haven't been presented with the opportunity to do so, you can calibrate your next run by completing a run of a known distance (over 1 kilometer or 0.62 miles) at a steady pace over flat terrain. If the run is eligible for calibration, you will see a Fine-Tune Your Run message after you end your workout.
Runs eligible for calibration can be calibrated at any time. If you do not follow the Fine-Tune Your Run prompt immediately after your run, you can locate the run in your Activity tab later, tap the run you'd like to calibrate, and then tap the Calibrate icon in the upper right corner of your screen. Use the wheel to adjust your workout distance. Click Done when you've input the correct information.
Please keep in mind that not every run will be eligible for calibration. If your first calibration attempt isn't eligible, review the steps above and try again. Runs may not be eligible for calibration if they're too short (less than one kilometer), contain too many variations in pace, contain too many pauses or stops, etc.
Please note: The GPS functionality is available on the iPhone (3GS and later) and Android devices (Android 2.3 and later). GPS is not available on the iPod touch, which collects accelerometer data only.
Contact Nike+ Support for assistance. You can also give feedback or report a bug.
Based on the information provided and whether it was easy to follow.