1U, 2U or tower: choosing the shape of a server.

What 1U and 2U measure

A rack unit, U, is the vertical pitch of a standard 19-inch rack: 1.75 inches, 44.45 mm, set by the EIA-310 rack standard. A 1U server is one unit tall and a 2U server two; the width is the same and the depth varies by model. A standard 42U rack therefore holds up to 42 1U servers or 21 2U ones before switches, power distribution and cable management take their share.

Every major line builds the same platform in both heights. HPE's DL360 and DL380 are the 1U and 2U pair of each Gen, and Lenovo's SR630 and SR650 are the same pairing in ThinkSystem. Cisco builds the C220 as the 1U counterpart of each C240 generation.

What the second unit buys

Processors and memory are usually the same in both heights of one generation. What the extra unit buys is room: more drive bays at the front, more full-height expansion slots at the back, and more room for air to move through the chassis. On Dell's seventeenth generation the figure is explicit: the specification sheets give up to 20 E3.S bays in the 1U chassis and up to 40 in the 2U one (the 17G row on the PowerEdge page names the sheets).

• Choose 1U when compute density per rack is the constraint, storage lives elsewhere — a SAN, a NAS, a storage cluster — and the machine needs at most a couple of network cards. Web tiers, hypervisor hosts on shared storage and network appliances fit here.
• Choose 2U when the machine carries its own storage, needs GPUs or several full-height cards, or will run hot for long periods. Databases, hyperconverged nodes, backup targets and anything with accelerators belong here.
• When in doubt, 2U. A 2U machine with empty bays is an easier problem than a 1U machine that the second card or the next eight drives do not fit.

When a tower is the right answer

A tower server is a rack server's platform in a free-standing case: Dell's T line, HPE's ML line and Lenovo's ST line. It is the right answer when there is no rack and will not be one — a branch office, a shop, a single-server site — because it runs quieter, needs no rails and sits on the floor of a room people work in.

It is the wrong answer once there are three or four of them. At that point a small rack and rack servers cost less in floor space and cabling than a row of towers, and the next machine slots in. Several towers can be converted to rack mounting with a vendor kit; check that one exists for the exact model before relying on it.

Blades are a chassis decision

A blade server does not stand on its own. It slides into a chassis that supplies its power, cooling and networking, and on some lines the chassis needs its own switching in front of it — Cisco's B-series blades only run inside a chassis with fabric interconnects. Buying a blade therefore means buying into, or already owning, the chassis it fits.

That makes blades the right choice for extending an estate already built on them, and rarely the right choice for a first purchase. If you are matching an existing chassis, send the chassis model with the request; the blade has to match it, not the other way round.

Where to go from here

The form factor narrows the choice; the generation finishes it. Which generation is that server lines the platforms up across vendors. The catalogue is split the same way: rack servers, tower servers and blade servers, and each line page — PowerEdge, ProLiant, ThinkSystem, UCS, Supermicro — lists every model it covers.