All right. This is taken heavily from 67th's work (based on Carman), but hopefully I can summarize.
In all cases estimates are made of Effective strength, which is the lowest measure and is appropriate for Antietam due to all the straggling from forced-marches.
Confederates.
Using Carman as a base, and adding back in units Carman designated as "not engaged", plus catching a few other issues, 67th puts it as:
In Place 15th
- "Evan's Division" - 3,111 infantry and 419 arty
- DR Jones' Division - 3,810 infantry and
- DH Hill - 7,773 infantry and 346 arty
= 14,694 infantry
On the 16th
- 12,370 infantry (above)
- Jackson: c. 1,600 infantry
- Ewell: 4,401 infantry
- Walker: 3,764 infantry
- (both Ewell and Walker may straggle in more slowly)
= c. 24,449 infantry
Morning of the 17th
- 24,449 infantry (above)
- stragglers of Jackson: 2,830
- McLaws: 2,823
- RH Anderson: 5,000
= 35,112 infantry
Arriving during the 17th
- (5-6,000 stragglers from the Harper's Ferry marches, no way to verify and not counted yet)
- AP Hill: 3,914
= 39,026
deduct casualties to estimate total at end of the day = 36,702-9,919 = 26,783 (this overestimates as shattered units have lost cohesion)
Morning of the 18th
- Deduct casualties in infantry: - 9,919 (undercount)
- RH Anderson stragglers (say): 1,200
- McLaws stragglers: 1,700
- AP Hill stragglers: 1,300
= 32,810 (probably 2-3,000 less due to unaccounted for casualties)
Total Confederate Strength On the 17th
Infantry (inc. AP Hill): 39,026
Artillery (from Carman directly): 3,629
Cavalry (from Carman directly): 4,500*
Combatants on the 17th: 47,155 (42,454 exc/ AP Hill)
Total Confederate Strength On the 18th
Infantry: 32,810
Artillery (3,629-314, from Carman): 3,315
Cavalry (4,500 - 49, from Carman): 4,451
Combatants on the 18th: 40,576
The link provides breakdowns by division and in some cases by brigade.
Total infantry effectives over whole battle about 42,500, of which 39,000 were around on the 17th.
Union
This is spread over several posts because some corps are examined separately. (2nd and 9th corps get their
1st Corps
Official Strength: 14,856
Meade's statement of those carried into battle: ca. 9000
Approximate infantry strength: 8,619 (Carman)
Casualties: 2,590
Approximate infantry strength the morning of the 18th: 5,327 (Sears, p300)
2nd Corps
(here 67th looks regiment by regiment via Fox's regimental losses)
Richardson's Division
1st Brigade 1,107
2nd Brigade 1,354 (assume the duty men and the officers of the 88th NY cancel out)
3rd Brigade 1,173 (if a 25% deduction is taken for the regiments which appear to be PF the figure reduces to 845)
DIVISION INFANTRY: 3,306 (including reduction)
Sedgwick's Division
1st Brigade 1,701 PFD (but excluding the officers of 1st Minnesota). This is slightly higher than Carman, probably because he excluded the MN SS. A reasonable deduction of 25% for non-effectives suggests a combat strength of about 1,275.
2nd Brigade 1,635 (very rough)
3rd Brigade 1,946, deducting 25% for the PFD regiments leaves 1,664.
DIVISION INFANTRY: 4,584 (including reductions)
French's Division
1st Brigade 1,541 (and probably high, I believe the 132nd PA straggled badly and wasn't on the firing line in anything like the above strength)
2nd Brigade 1,315 (but highly questionable)
3rd Brigade 2,191 PFD, a 25% reduction would give them 1,643 engaged.
DIVISION INFANTRY: 4,499
Total infantry of 2nd Corps: 12,389
Add Artillery of 859 (Carman's figures): 13,248 (70.4% of present)
5th Corps
Official Strength: 12,939 (2 Divisions only)
Buchanan's brigade: 1,640 (Carman, but no supporting text could be found)
Lovell's brigade: 1,060 PFD consolidated into 4 battalions on the 20th (OR)
5th NY: less than 115 (effective strength at 2nd Bull Run minus casualties at said battle, battlefield marker)
10th NY: unknown
Morell: 5,407 PFD
Estimate: 6,990
6th Corps
Official Strength: 12,300 (of which 75-80% were combatants, B&L II, pp 595)
Force engaged at Crampton's Gap (Slocum's Div and Brooks' and Irwin's Bdes of Smith's Div): ca. 6,500 (of which 533 were casualties)
Strength of Smith's Division: ca. 4,500 (B&L II, pp 596)
PFD of Hancock's Bde (not included in the 6,500 engaged above, but in the strength of Smith's Division): 2,114, or ca. 1,600 combatants
Irwin's brigade: 1,684 (Carman)
Infantry Strength of 6th Corps: 8,324 (75% of present minus 901 for the artillery (figure from Carman)
9th Corps
Bottom line figure:
TOTAL INFANTRY: 8,357
Add in artillery at 979 (Carman): 9,336 (67.6%)
12th Corps: 10,126
Official Strength: 10,126
Carman: 7,239
Total infantry effectives over entire battle about 52,000 (bit less than this as this includes artillery for some corps)
In infantry terms the two sides were about even in
veterans and McClellan also had about one extra corps of
new troops who were not as good.
End of month p
ost-Antietam Lee reported a strength of 52,500 PFD, and he'd suffered about 17,500 casualties (9,200 wounded by his own account and 8,500 prisoners or dead counted by McClellan) implying a pre-casualty PFD of 70,000 once casualties added back.
Immediately post-Antietam he reported 36,000 PFD, and the 16,000 increase is probably stragglers returning - this implies a strength of about 53,000 PFD immediately pre-Antietam given straggling, though not all of these would make it into the line any more than McClellan's did. 53,000 PFD implies about 42,000 Effectives, very roughly.
One other estimate made was to count regiments, which are roughly even in number on both sides. McClellan's regiments include some very fresh ones, which would be (1) bigger and (2) much less well trained, and apart from that the veteran regiments have largely been fighting one another and so have broadly similar average strength.
All this points to infantry veteran counts on both sides being about equal. The reason it's so hard to determine this is that both sides really raced to the battlefield and sometimes counts were taken at bad times (e.g. Jackson's men were counted 30 minutes after arriving and a day before going into the battle, IIRC) or not at all (some Union corps report their previous PFD as their total engaged, which is very unlikely.)