My friend asks me about the different between 1U rackmount server with 6U. I search around and find some valuable information from here. I copy-and-paste the table below.

Unit

Dimension
(Widh x Height x Depth)
1U 19″ x 1.75″ x 17.7″
19″ x 1.75″ x 19.7″
19″ x 1.75″ x 21.5″
2U 19″ x 3.5″ x 17.7″
19″ x 3.5″ x 20.9″
19″ x 3.5″ x 24″
3U 17.1″ x 5.1″ x 25.5″
4U 19″ x 7″ x 17.8″
19″ x 7″ x 26.4″
5U 19″ x 8.34″ x 19.67″
19.1″ x 8.75″ x 26.4″
6U 19″ x 10.5″ x 19.5″
7U 17″ x 12.2″ x 19.8″

Make sure you will use the correct server size. Contact your ISP or hosting provider and ask about their rack size. Usually, they accept 1U or 2U, but most of them also will support up to 5U or 6U. Just call them to make sure about this.