One fact you can be sure of - no two heads are the same! ALWAYS measure your child's head before purchasing to ensure getting the accurate size.
Step 1: Distract the child with a toy or food (so they don't try and help you!).
Step 2: Use a measuring tape to take measurement. Otherwise use a ribbon to take the size then use a ruler to find the measurement.
Step 3: Purchase the size closest to your childs measurement. These hats are very stretchy and will grow up to 4cm.
FOR EXAMPLE: The child measures 41cms, you will need the 42cm size that will stretch another 4cms up to 46cms - giving plenty of future wear. Purchasing the next size up would be too large and block the childs vision.