Clearing out an old folder I found a half written article I thought I'd post:
The American Civil War (ACW) presents an interesting challenge when assessing combat due to the very different reporting systems used in America. For example, the British of the Napoleonic Wars report army strength in terms of Effectives, the number of rank and file in the infantry and cavalry, excluding their officers, excluding those detached (for example to the Wagon Train) and excluding the Ordnance (mainly Artillery) and other attached arms such as the wagon drivers and hospitals. In the ACW however, strengths stated are typically those of all arms, including those detached to the wagon train etc., and sometimes even including the Sick List as combatants.
The digitised records of the 91st Pennsylvania Volunteer Infantry here, which is a very useful resource.
The figures given in the morning state of 23rd September 1863 are in summary:
15 Combat Officers (including the Adjutant)
2 Medical Staff (Assistant Surgeon and Hospital Steward)
22 Blacksmiths and Artificers
= 239 Present for Duty, of whom 168 (70%) are “effectives” (i.e. men in the firing line)
On Special, Extra or Daily Duty: 7 (1 offr and 6 ORs)
On Detached Service: 56 (6 Offrs and 50 ORs)
Thus, a Regiment reporting a total strength of 433 Officers and Men can only put 168 Riflemen (39%) of the men borne on the Rolls into combat.
This problem is not unnoticed by high command. Sherman notes in his Memoirs (and indeed in the Official Records) that on taking command of the Army of the Tennessee fully half his army consisted of non-combatants, and he took steps to limit this to 75%. Typically most of these non-combatants were in the wagon train (which averaged 7-8 men per wagon, thus a typical 5,000 wagon army train could consume 40,000 men, those men still being borne on the rolls and often included as “combatants” in many books), although there were also hospital staff, provosts, officers servants, cooks etc., none of whom were in the firing line.