Hocosa Big Boys Organic Wool Underwear Shirt with Long Sleeves Made of soothingly soft European KbT-Certified Organic Merino Wool. For boys or girls. Natural fibers breathe, maintaining a comfortable temperature in warmth or cold. Light-weight. Comfortable under clothing or worn alone. Wicks moisture away from skin, keeping your child dry & comfortable. Even when wet, feels warm. Natural wool with lanolin gives self-cleansing properties, minimizing need for washing. Can go weeks without washing, even with daily wear. For best results, wash by hand with lanolin-replenishing woolwash to maintain hygienic effects. For best fit - European size correlates to child's height in centimeters. Child's height in inches x 2.5. For heftier children or room for growth, go up one size. See Product Description below for sizing guide.