What is 1U?
Rack Unit, abbreviated U or RU is a unit of measure for rack systems equal to 44 millimeters or 1.752 inches. Therefore, 1U would indicate 1.752 inches in a rack system. If a rack system has 3U or 4U listed you would simply multiply the number by 1.752, so 3U would be 5.256 inches and 4U would be 7.008 inches.