Soundwave ships all Sontro OTC Hearing Aids with one size fits most receivers. The receiver should lay flat against the side of your head. Occasionally your ear size may require a longer or shorter receiver.To find the correct size Receiver, use the Receiver Measurement Tool. The receiver measurement tool is located in your Sontro OTC Hearing Aid box, inside the clear cellophane envelope.  Place the tool over the appropriate ear, see diagram. Pick the number that is the closest to the top of the ear canal opening. If you are between numbers, we recommend you choose the higher number. It is ideal to have a friend help you read the number.