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Head sizes can really vary. To get the best fit possible, it really helps to have a measurement of the child's head. To measure a head, use a sewing measuring tape. If you don't have one you can use anything  that doesn't stretch, such as a string, and then figure out the length by putting that against a ruler or measuring tape. Measure across  the middle of the forehead around the back of the head over the widest part.  From this, we can figure out which size is best.  Please include this information and the child's age in notes to seller for best possible fit.  Thanks!