A 4.5" core of natural rubber is covered with a quilted outer which is a layer of thin organic cotton fabric, a thick layer of organic cotton batting, a thick layer of organically processed wool from organically raised happy sheep, and a layer of our premium European organic cotton fabric all quilted together to prevent shifting. This provides a 5" supportive mattress with a nice even top for safety. This mattress exceeds all the higher CA flame retardancy guidelines without any flame retardant chemicals. Fits all standard cribs 28" x 52". Approximately 5" thick.