William F. Fox, Lt. Col. U. S. V., Regimental Losses in the American Civil War, 1861-1865: A Treatise on the extent and nature of the mortuary losses in the Union regiments, with full and exhaustive statistics compiled from the official records on file in the state military bureaus and at Washington

The number of men killed in a regiment during its term of service has thus far been considered only in respect to the maximum of loss, and the result is of value only so far as it defines the limit of casualties to which regiments of this size are exposed. But, though similar in formation, the regiments varied in numbers according to the recruits or transferred men received. Some regiments received large numbers of recruits to make good their losses, while other commands went through the war with constantly lessening ranks and carried only the original thousand, or less, upon their rolls. Some regiments which reenlisted at the end of their three years term received large accessions from other commands which, returning home, left detachments in the field composed of recruits with unexpired terms, or reenlisted men. Distinction must be made, in the matter of losses in action, between the regiments whose ranks were always kept full, and the ones which received no fresh material.

In short, the proper way to judge of the relative losses of regiments during their term of service is to accompany the statement of the losses with the figures of the total enrollment, and compare the percentages as well as the losses. The regiments in the following list can fairly claim the honor of having encountered the hardest fighting in the war. They may not have done the most effective fighting, nor the best fighting; but they evidently stood where the danger was thickest, and were the ones which faced the hottest musketry. They were all well-known, reliable commands, and served with unblemished records. The maximum of loss is reached in this table:

Killed and died of wounds: maximum percentages of Enrollment.