There are currently many different lengths of Dennis/TransBus/Alexander Dennis Dart that have been built, they are as below:

Step Entrance
8.5M, 9M and 9.8M

Low Floor SLF
8.8M (MPD), 9.3M, 10.1M, 10.7M and 11.3M (SPD)

Enviro300
12M and 12.5M

I have included the Enviro300 in this list as they are mechanically very similar to the newer Dart SPD.  To clarify too, all step entrance Darts are 2.3M wide, almost all Low Floor Darts are 2.45M wide, though some recent ones for Guernsey, Jersey and even London have been 2.3M wide, whilst all Enviro300s are 2.55M wide.

Initially the lengths of step entrance Darts were easy to work out, based on the chassis number, e.g. Warrington Borough Transport 220 J10 WBT has chassis number 8.5SDL3008/474, from here is it easy to see that it is 8.5M long and it is also from here that the 'SDL' notation comes from, though what exactly it does stand for I don't know! Looking at the chassis number again, it is split into four sections:

N.B. The Chassis Model Type is the exact variation of the Dart chassis, as many minor alterations were made over time, so the higher this number, the newer the chassis.  The Chassis Number is the exact number of that chassis within the overall build of Dennis Darts.  Some other similar examples are as follows:

This all worked fine, but then came the Darts SLFs which had their own numbering system, which was totally different, and to make matters worse, some late build 9.8M step entrance Darts had the new style chassis numbers!

An example of an SLF chassis number is as follows, from Arriva NW&W Dart 7562 R562 ABA, SFD322BR1VGW12036.  This can be similarly split into four sections:

Here the Chassis Length section bears no resemblance to the actual length, the key for them is as follows:

1 - 9.3M
2 - 10.1M
3 - 10.7M
4 - 11.3M (SPD)
6 - 8.8M (MPD)

Some examples are as follows:

These appear to be in sequence apart from 6 for the MPD, but this is because production of the MPD started later than the other lengths, I've no idea why 5 was missed out, what it is, or what it is intended for - The SPD also started later than the main ones (though before the SPD), but as it was longer that the existing ones, it is still in sequence.  Confusingly, as noted earlier, some late build step entrance 9.8M Darts have new style chassis numbers, such as Arriva NW&W's ex South Lancs Transport Dart 1300 P3 SLT, which has chassis number SFD412BR5TGD13371, which would indicate from the table above that it is an SPD, which obviously it isn't, though there shouldn't be too much of a problem here, as the first SPDs were only R reg, with the last few 9.8M step entrance Darts also being R reg, so for R reg buses check, otherwise if its older than R it is step entrance, newer its an SPD!