The height of data cabinets and server racks are generally measured in increments of size called 'u' increments. One 'u' is 44.45mm high. In most cases a 24 port patch panel or switch will be 1u high.
It is important to consider the equipment you want to use inside your server rack. Not all equipment is rack mountable, and if you want to use equipment that is not you should include the size of it in your measurements and divide the height by 44.45 to give an approximate 'u' value. Once you have an idea of the space it will occupy, you will then need to mount a special shelf to hold this piece of equipment.
It is always wise to purchase a server rack that includes at least 10-15% room for future expansion. If you suddenly decide to expand on the equipment in your server rack, you will have saved a lot of time and potential headaches by planning well in advance. Otherwise you will face additional costs associated with moving your equipment from your existing server rack into a new one - depending on the purpose of your server, these can be considerable.
Because the 'u' value refers to the internal space of the server cabinet, it can give you only a rough idea of the external dimensions of the rack itself. Server cabinets can vary considerably in size and you should make sure to note down both the internal AND external measurements before you buy so you can ensure there will be space in your office or data centre.
Most server racks come in one of two widths; 600mm or 800mm. Regardless of the width you choose, the 19' mounting angles for rack mounted equipment will almost always be the same. So why might you want a wider server rack? The main difference between 600mm and 800mm is the space inside for cable management. If you have a complex server arrangement and you are not extremely short on floor space, we recommend you choose the wider rack as this will make organising your cables much easier.
The depth of server racks is becoming increasingly important as manufacturers begin to favour shorter, deeper builds over taller, shallower ones. As a rule of thumb the actual working space within your cabinet will be around 150mm less than the measurement quoted for depth. In a server rack of 1000mm depth, you will find much closer to 850mm workable space. Make sure that when you are calculating the depth of your server (and whether or not it will fit into a certain rack) you account for this difference or you may end up with an unpleasant surprise.