6-9 months = approximately 16-17"

9-12 months = approximately 17"-18"

1T-3T = approximately 18-19"

Please be sure to get an appropriate measurement.
Keep in mind that these beanies have plenty of stretch.

To get an idea of how they fit, my daughter has a 46 cm/18.1 in head at her 9month appointment.
The 6-9month size fits her really well, i'd say perfect. The 9-12 month feels actually a little loose.