All of our hats feature an adjustable slider, but head sizes do vary!
Here are our suggested sizes by age:

Sprout: Best fits kids aged 8 to 36 months and measures 46 to 50 cm. 

Seedling: Best fits kids aged 2 to 5 years and measures 50 to 54 cm.

Sapling: Best fits kids aged 6 to 10 years and measures 54 to 58 cm.

Adult: Bigger heads!

If you don't have a soft measuring tape, you can measure a hat that currently fits your child's head with a piece of string, or by placing the string around the child's head comfortably across the forehead and just above the ears. Use a ruler or measuring tape to then measure the string.