Find the right size of armband for your requirements.
Let your arm relax and measure (you may need help to do this) around your bicep, you should select the size that is just smaller than your measurement to ensure a snug fit, the elastic will the expand with your arm movement.
Example: if your arm measures 30cm then select the Small size armband.
Remember to allow for fitting over a shirt or heavy jacket.
CHILD 4/6 yrs 80mm 20cm to 23cm (8in to 9in)
CHILD 7/12 yrs 80mm 23cm to 25cm (9in to 9 ¾in)
SMALL 80mm 29cm to 33cm (11½in to 13in)
MEDIUM 80mm 33cm to 38cm (13in to 15in)
LARGE 80mm 38cm to 42cm (15in to 16¼in)
X LARGE 80mm 42cm to 46cm (16¼in to 18in)
XX LARGE 80mm 46cm to 51cm (18in to 20in)