A Risky Job

Being a Roman Emperor was not the safest job in the world. In the 503 years between Augustus and Romulus Augustulus, the last Roman Emperor of the Western Empire, there were 73 emperors with an average reign of 8 years, 9 months.

Of the 73 emperors, 28 died from natural causes, and 45 died violent deaths.

OK, you've got me on the maths. 503 ÷ 73 is actually 6 years and 9 months. The problem is that there are times when the crossover between reigns was not straightforward due to the split between West and East, dual rulers, rebellions, pretenders, and other complications. The total number of years Emperors reigned is given as 640 in the table here.

One of the most dangerous times to be an emperor was between the death of Severus Alexander and the accession of Diocletian, here's a breakdown of the "Crisis of Empire". It's hard to grow attached to any job if it carries a life expectancy of 2.45 years

Methodology

To arrive at the figures I followed the dating of Cary and Scullard, together with their list of Emperors I have tried as much as possible not to include Emperors of the Eastern empire, unless they reigned over the western half at any particular time. If an Emperor is listed as being a co-ruler (as in the case of Lucius Verus) then I have included him. I have followed convention by considering the last emperor of the West to be Romulus Augustulus.

To find the cause of death I have used the following sources where appropriate:

Cary, M/Scullard, H: A History of Rome http://www.amazon.co.uk/exec/obidos/ASIN/0333174402/ref=sr_aps_books_1_1/026-3571555-2338057

De Imperatoribus Romanis http://www.roman-emperors.org/

The Later Roman Empire, A.D.353-378: Selections (Classics S.)
Ammianus Marcellinus http://www.amazon.co.uk/exec/obidos/ASIN/0140444068/ref=sr_aps_books_1_1/026-3571555-2338057

In an ideal world then the individual reigns of emperors would have been added together by year, month, and day. This is not possible however as we are not told the precise date of the accessions or deaths of many emperors. To keep it simple I have kept to the year only, thus when we consider the death of Nero (AD68) to the start of Vespasian's reign (AD69) Galba is credited with reigning one year, whilst Otho and Vitellius are stated as reigning for none. This is of course nonsense, but if we add up the amount of time then it gives the three of them overall a year between them. Vespasian then gets his 10 years from 69 to 79. Although this method is inaccurate when considering the reign of a single Emperor, it works for statistical purposes.