Hocosa Big Boys Organic Wool-Silk Short-Sleeved Undershirt 70/30% blend of soothingly soft European KbT- certified organic merino wool & silk. Wool for warmth. Silk for strength. For boys or girls. Light-weight, yet warmth-giving under-layer. Natural fibers breathe, maintaining a comfortable temperature in warmth or cold. Wicks moisture away from skin, keeping your child warm & 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. See Product Description below for sizing guide.