Tower, SFF or mini? Desktop PC sizes explained

Most people do not know what the box under their desk is called, and there is no reason they should. The industry uses at least three words for every shape, two of the manufacturers use the same abbreviation to mean different things, and one of the abbreviations people type does not exist at all. Here is the whole thing in plain English.

The four shapes, before any abbreviations

• The big upright box. Stands on the floor, or beside the monitor if the desk is deep enough. In the UK this is often just called the base unit, or the tower.
• The slim flat box. Sits on the desk, usually under the monitor, roughly the size of a thick hardback lying down. This is the commonest shape in offices.
• The little box. About a litre in volume, the size of a hardback standing up. Frequently clipped to the back of the monitor or tucked behind it, which is why people often do not realise they have one.
• The screen with the computer inside it. No separate box at all — an all-in-one.

What each manufacturer calls them

Same three sizes, three different vocabularies. This is the table worth having:

• Dell — Tower (MT), Small Form Factor (SFF), Micro (MFF).
• HP — Tower (TWR) or Microtower (MT), Small Form Factor (SFF), Mini. HP called the small one Desktop Mini, or DM, before shortening it.
• Lenovo — Tower, Small Form Factor, Tiny.

Lenovo does something genuinely helpful here: it puts the letter in the model name. An M70t is the tower, an M70s is the small form factor and an M70q is the Tiny. If your machine ends in a letter, that letter is the shape.

Two traps worth knowing

MT means different things to Dell and HP. Dell’s MT is Mini Tower. HP’s MT is Microtower. They are roughly the same size class, and the abbreviation gives no clue which maker you are looking at.

There is no such thing as an SSF. It is one of the commonest typing slips in this whole subject, and what is meant is always SFF — Small Form Factor. Nothing is wrong with your machine if you have been searching the wrong three letters.

The older shapes still coming out of storage

These are exactly the machines that turn up when a cupboard gets cleared, so they are worth naming:

• Dell USFF — an ultra-small desktop, discontinued around 2014 and replaced by the Micro.
• Dell DT — the flat box the monitor used to sit on top of, before monitors got too light to need one.
• HP USDT — HP’s ultra-slim desktop, the same idea from the other side.
• HP CMT — the convertible minitower from the HP Compaq era, which could stand either way up.

The two nobody expects

Dell’s OptiPlex Ultra hides a flat module inside the monitor stand, so there is no visible computer anywhere on the desk. People clearing an office have been known to send the monitors for recycling without realising the computers went with them.

Lenovo’s Tiny-in-One takes the other approach and clips a Tiny into the back of a matching monitor, so the pair looks like an all-in-one but comes apart into two things.

Does the size change what it is worth?

Less than people expect. What is inside the case decides the figure, not how big the case is — a small form factor machine with a good processor is worth more than a large empty tower, and the shape mostly tells you what could physically fit inside rather than what did.

Where the shape does matter is what can be added: a tower takes a full-size graphics card, a small form factor takes a low-profile one at most, and a Micro, Mini or Tiny takes none at all. So on the bigger shapes it is worth checking whether there is one in there before you assume there is not.

Finding out what yours is

Read the badge and the shape word off the case and you have most of it. From there: selling a Dell desktop covers OptiPlex and Precision and why the shape is half the model name, selling an HP desktop untangles HP’s three naming eras, and selling a custom built PC is the one for a machine with no badge at all.

If none of the words on the case match anything here, selling an old computer takes it from the shape alone.