If you want to measure the length of your hand spun yarn, just place your device with this app under your yarn swift (with mounted magnet) and start rotations.

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Mount a magnet (e.g. fridge magnet) on the end of one of your Swift's shoulders bottom side.

Measure the length of one loop of your Swift and type in the value in meters or yards. Note that corresponding unit (yard or meter) will be updated automatically. You need to do it only once. When you open the app next time the loop length will be recalled from the app's memory. If you use a different Swift, the loop length value needs to be updated.

Place your device with the Yarn Yardage app under the swift so that when it rotates the magnet goes above the device .

Turn the shoulder with the magnet away from the device.

Press "START" and start rotations. Each rotation will be detected by the app and loops, meters, yards numbers will be updated on the app screen.

When done measuring the yarn yardage, press "STOP". This does not clear the calculated number of meters/yards. The number of meters/yards is cleared when start is pressed again.