There is general consensus that there were roughly 400,000 U.S. deaths in WW2, and 80,000 MIA:
Of the 16 million Americans who served in World War II, more than 400,000 died during the war. At the end of the war, there were approximately 79,000 Americans unaccounted for. This number included those buried with honor as unknowns, officially buried at sea, lost at sea, and missing in action. DPAA
Does the 400,000 deaths number include the 80,000 MIA number? Or could you technically say that 480,000 Americans were killed or missing in WW2?
You have to remember that you are looking at sets of numbers that are telling the same story from different standpoints, creating two different issues, accounting for combat and non-combat deaths versus a missing status.
In general, it can be said that the Missing in Action and the Missing (two different categories which describe persons for whom there was no accounting; just “Missing” and not “Missing in Action” fall into a category of non-combat losses) are, indeed, counted among the total deaths reported by the various services.
Take the US Army for example. The Defense POW/MIA Accounting Agency reports that there were a total of 36,822 US Army missing, including the USAAF, about 51% the total for all US missing. They report these as Missing in Action because there are still efforts to recover individual remains. But one should note that the presumption is that these missing are, indeed, dead. See: https://www.dpaa.mil/Our-Missing/World-War-II/
Now if you turn to the Adjutant General's report ““Army Battle Casualties and Nonbattle Deaths in World War II” published by the Department of the Army in 1953, you can find wording and statistics that shed some light on the above number. You may obtain a copy here: http://cgsc.contentdm.oclc.org/cdm/compoundobject/collection/p4013coll8/id/130/rec/2 It is in four parts, so you will have to download them one by one for the complete report.
This report provides discussion on the accounting for the numbers of missing in action and missing. Without going into an examination of some 115 or so pages of tabular statistics, from that report, we can draw definitions of things to keep in mind when discussing the missing and the dead.
There is commentary on sources of data: “The statistics presented herein include all changes processed in the card file records through 31 December 1949. Processing after 31 December 1946 consisted primarily of revisions from declared dead to other reportable death categories, from a current wounded or injured in action to a wounded or injured in action disposition status, and from a current missing in action to a declared dead or other reportable death or battle casualty disposition status. Many changes also resulted from the audit of death cases conducted jointly by the Casualty and Strength Accounting Branches, AGO.” (page 1)
Note the phrase “… missing in action to a declared dead or other reportable death or battle casualty disposition status.” This means, essentially that the missing in action who could not be determined through any other means were, at some point, declared to be dead and added into the total deaths accounting.
Next, in relation to this issue there is this passage: “The totals for the major battle casualty categories "Killed in action,” 'Wounded and injured in action,” and "Captured and interned" represent all persons who were ever reported in those categories (exclusive of erroneous reports). This is not the case, however, for the “Missing in Action” total. Missing in action was essentially an unknown status and consisted of personnel whose whereabouts or actual fate could not be determined and whose disappearance was presumed to be the result of enemy action. Most of the cases originally reported in this category were transferred to a killed in action, wounded and injured in action, or captured and interned status, as established by subsequent information. The remainder are shown in this report under the missing in action dispositions of declared dead, died of other causes (nonbattle), or returned to duty.” (page 2)
On page 3 we can find a definition for the purpose of the report of the term of Battle Casualties: “All persons killed in action, dead as a result of wounds or injuries received in action, wounded or injured in action, missing in action, captured by the opposing forces, or taken into custody by the authorities of a neutral country as internees. The term 'in action' characterized the casualty status as having been incurred as a direct result of enemy action during an engagement or otherwise, or sustained while immediately engaged in, going to, or returning from a combat mission whether or not due to enemy action. Psychoneurosis and other mental disorders developed under battle conditions were specifically excluded from battle casualties by War Department Circular No. 195, dated 1 September 1943. This 1943 action represented not a change in policy but rather a clarification and explicit statement of the original intent. War Department Circular No. 142, dated 14 May 1945 excluded from battle casualties, injuries due to the elements (frostbite, trench foot, immersion foot, etc.). Since however, this circular was issued after V-E Day and after essentially all of the periods of high incidence of cold injury had passed, its effect is subject to question. It is known that in some theaters, directives were in effect for parts of the war period requiring the reporting of severe frostbite actually incurred in combat as battle casualties but excluding trench foot from such reporting.”
Battle deaths are described on the same page: “All persons killed in action, dead as a result of wounds or injuries received in action, or declared dead from missing in action. This term excludes nonbattle deaths of personnel in a battle casualty status of captured, interned, or missing in action.”
Note that Missing in Action with these definitions are included as battle casualties and we see that missing in action can be accounted in a declared dead status.
On page 4 we find the definitions of “Missing” and “Missing in Action”. Missing is define: Persons whose whereabouts or actual fate could not be determined, who were not known to be in an unauthorized absence status, for whom there was no conclusive evidence of death or circumstances leading to a logical conclusion of death, and who were presumed not to have been within the purview of “battle casualties,” as defined above, at time of disappearance. Deaths of personnel initially reported in this status, including declared dead cases, are recorded in the nonbattle death columns of the death tables of this, but not in the battle casualty tables.”
Missing in action is defined: “A classification similar to that of missing, except that personnel in this status came within the purview of "battle casualties,” as defined above, at time of disappearance. In the battle casualty tables of this report, persons initially reported in this status and later found to have been actually in another battle casualty status were removed from missing in action and placed in the other status. However, those who were declared dead or were returned to duty, and those who died of nonbattle causes are shown as subsequent dispositions from a missing in action status. In the death tables, persons missing in action who were declared dead are included in battle deaths, while those who died of nonbattle causes are included in nonbattle deaths.”
Nonbattle deaths are defined: “In the battle casualty tables, this category consists solely of persons who died of disease or other nonbattle cause while in a captured, interned, or missing in action status. In the death tables, this category consists of all nonbattle deaths, Army-wide, and is comprised of those who died of nonbattle causes while in a battle casualty status of captured, interned, or missing in action, as well as all other nonbattle deaths.” (page 4)
So, how and why are the missing or missing in action moved from those status?
Also found on page 4: “All persons previously reported as missing or missing in action, who were no longer presumed to be living, and in whose cases a finding of death was made by the Chief of the Casualty Branch, AGO, acting for the Secretary of War, pursuant to Section 5 of the “Missing Persons Act,” Public Law 490, 77th Congress, 7 March 1942, as amended. Findings of death were made upon or subsequent to 12 months in a missing or missing in action status, and were withheld so long as the person was presumed to be living. They included the date upon which the death was presumed to have occurred for the purposes of termination of crediting pay and allowances, settlements of accounts, and payments of death gratuities. Such date was never less than a year and a day following the day of expiration of the 12 month period. The declared dead columns in this report include figures for those persons classified as declared dead from a missing in action status only. Persons declared dead from a missing status (other than missing in action) are included in the nonbattle death statistics. in the death tables, but are not separately identified.”
Now we see that there was legal authority to place those in a missing or missing in action status in a declared dead status (in the battle casualties or nonbattle casualties categories) not less than 12 months after being in a missing status.
And they were.
Thus, the dual issues of missing and missing in action versus the legal status of those in those categories and how they are accounted for in casualty reporting. It is clear that the US Army (and, indeed, the US Navy and US Marine Corps) followed the law on declaring those in the missing status to be dead not less than a year after their report missing. (One of the longest waiting periods of which I am aware was the officers and crew of USS Jarvis, lost without a trace - later, long after the war, determined to be sunk by Japanese aircraft - on 9 August 1942 and not declared dead until 12 July 1945.) The Army, at least, then moved these missing into the nonbattle deaths category and the missing in action into the battle deaths category.
You can actually go to the DPMAA site and pluck a name, let's say LCDR John H Armstrong, from the mentioned USS Jarvis - he was the ship's executive officer - and he shows up here https://www.dpaa.mil/Portals/85/Documents/WWIIAccounting/united_states_navy.html with the entry reading ARMSTRONG, JOHN H Jr. - CDR O-063388 - 08/09/1942 - UNITED STATES NAVY - KENTUCKY - SOLOMON ISLANDS
And then go here http://www.naval-history.net/WW2UScasaaDB-USNBPbyName1.htm and scroll down until you find him and there he is as declared dead:
“ARMSTRONG, John H, Jr, LCDR, 63388, USS Jarvis, near Savo Island, August 9, 1942, (CasCode 6221) missing, later declared dead, dd July 12, 1945 (bp1)”
I can't explain the DPMAA rank of Commander versus the naval-history.net rank of Lieutenant Commander, I suspect that the DPMAA rank could simply be a not uncommon typo. John Hord Armstrong, Jr. shows up in USNA records, class of 1930, with a final rank of Lieutenant Commander.
Anyway, so, bottom line, yes, the missing and the missing in action are included in casualty accounting and should not be considered in addition to same. Adding the numbers of missing from such official sources as the DPMAA, to total casualties is double counting them.
The most precise figure I'm seeing is "407,316 recorded deaths" of US soldiers in WWII combat, although I'm not sure what the authoritative source on that would be. This apparently includes all branches of the US military. There is no reason to think that the 80,000 MIA would be included in that figure.
Here are state-by-state lists of all the names of those killed from the Army / Air Force and the Navy / Marines / Coast Guard
"Recorded deaths" is a quite clear term. It means proved to be dead. MIA does not fit into that category.
Recorded deaths and MIA both have the primary common factor the date of the reporting.
Example: let's say you had yesterday a total of 1100 people isolated and attacked; you may find today that you have 1000 dead in that operation and 100 MIA.
Tomorrow you have 1010 confirmed dead, 10 found alive and 80 still MIA.
At your final statistic after years, maybe 1050 were found dead in total, 40 alive and 10 presumed dead since they were never found after all this time. But not even presumed dead is not 'recorded dead' either.
So that is how the MIA and KIA number can overlap. To conclude: KIA stats do not include MIA stats at a given point in time.
It is still two different issues. Missing and missing in action versus accounting for total losses. One is made up of a great long lists which one usually grabs from the DPAA accounting, and which is, in reality, not listings of missing or missing in action, but, rather, listings of unrecovered bodies… there is a difference. One might note that the DPAA site specifically uses the phrase “service personnel not recovered following WWII” when it presents its listings. It does head those lists as “missing in action”. The second issue, which addresses the questions posed by the OP, is how the services accounted for the total deaths of the conflict, in which the missing and missing in action are counted and included.
The original questions were: “Does the 400,000 deaths number include the 80,000 MIA number? Or could you technically say that 480,000 Americans were killed or missing in WW2?”
Regardless how one, today, might want to count the US missing and missing in action from World War II, the answers are quite clear:
“Does the 400,000 deaths number include the 80,000 MIA number?”
Yes, the missing and missing in action are, indeed, by law, which is clear and unambiguous, included, counted, in the total deaths count.
“Or could you technically say that 480,000 Americans were killed or missing in WW2?”
No, you cannot even “technically” add the missing and missing in action to the total deaths as that would be counting them twice. Well, s'truth, you can if you want to but you would come up with an erroneous total which I am sure someone would be happy to challenge.
US services accounting for the missing or missing in action were covered under Public Law 490 of the 77th Congress which I mentioned in my earlier answer. The US Army casualty report extracts in that answer briefly describe how the Army defined and applied the law. If you wish, you can find the referenced Public Law 490 in its entirety here: https://www.loc.gov/law/help/statutes-at-large/77th-congress/session-2/c77s2ch166.pdf. The various sections of this law cover the potential eventualities and the process to be followed; of specific note is section 9, where it says:
“Within the scope of the authority granted by this Act, the determination by the head of the department concerned, or by such person as he may designate, of the status of a person in the military or naval forces, the Coast Guard, the Coast and Geodetic Survey, the Public Health Service, or civilian officers or employees as defined in paragraph (a) (3) of section 1 of this Act, or his direction relative to continuance, temporary suspension, or resumption of payment of pay and allowances, or finding of death, shall be conclusive.”
“… or finding of death shall be conclusive… ” sounds definitive. That means an individual is no longer listed as missing or missing in action, it means the individual is listed as one of the categories of war dead.
It is all very straight forward. For example, US Army combat deaths:
Total US Army deaths among battle casualties, all theaters and branches including USAAF, as of 31 December 1946 were 234,874. This breaks down as
Killed in action = 189,696
Died of wounds or injuries = 26,225
Evacuated to US, died of wounds or injuries = 84
Captured & interned total = 124,079
Captured & interned, returned to military control = 111,426
Captured & interned, killed in action = 3,102
Captured & interned, died of wounds and injuries = 453
Captured & interned, died of other causes = 9,098
Missing in action total = 30,314
Missing in action, returned to duty = 24,098
Missing in action, declared dead = 6,058
Missing in action, died of other causes = 158
Add up the deaths in these three categories and you get, hmmm, 234,874.
For non-battle deaths there were a total of 92,656
Non-battle accident aircraft = 27,628
Non-battle accident, non-aircraft = 29,224
Non-battle disease = 26,518
Non-battle other = 9,286
There is absolutely no doubt in my mind that the bodies of a not inconsiderable percentage of those noted as killed in aircraft accidents were not recovered, thus making them “missing” (“missing,” not “missing in action”) and probably a percentage of bodies from the accident non-aircraft deaths were likewise not recovered, putting them also in the “missing” category, but for death casualty accounting purposes, they are dead and are recorded as same.
Add up all these non-battle deaths and apply that sum to the battle deaths and we arrive at 318,274.
Sniff around the internet and one can find the total US Army casualties, here's a quickie: https://www.nationalww2museum.org/students-teachers/student-resources/research-starters/research-starters-us-military-numbers and what do we find? Hmmm 318,274… looks right to me.
At that same site we see for the US Navy, US Marine Corps, and US Coast Guard deaths 62,614 and 24,511 and 1,917 respectively. There are available on micro-fiche lists of USN killed, originally produced in the Navy and Marine Corps Awards Manual, but it takes some sleuthing about to use the data in a meaningful manner. And, unfortunately, it does not include all US Navy losses; others may be found in files at NARA covering miscellaneous losses such as naval gun crews assigned to merchant ships and US Army vessels. Working to determine a formal date of death for the individual missing requires some careful researching. Sometimes it is tedious but can be done. We can, however, work backwards through the gross numbers to satisfy ourselves that the missing and missing in action are included in these total numbers.
Looking at https://www.history.navy.mil/research/library/online-reading-room/title-list-alphabetically/u/us-navy-personnel-in-world-war-ii-service-and-casualty-statistics.html
We find the US Navy reports total combat deaths due to enemy (a general category) all officers, enlisted, and officer candidates, as 36,950, about 59% of its total reported deaths.
Killed in action = 30,831
Killed in action, air combat = 3,173
Died of wounds = 1,837
Died POW = 919
Died other deaths due to enemy action = 190
Next, we can find that the total of deaths reported due to other than enemy action was 25,664.
Natural causes = 5,533
Aviation accidents = 8,184
Other deaths, 11,947
Let us see… 36,960 + 25,664 = 62,614… check.
For a couple of micro looks, one might consider the loss of submarines over the course of the war. The USN reported 52 submarines as known lost or lost as overdue from patrol. Lost with these were 3,506 officers and men. Their remains were, for the most part, not recovered and never will be. These individuals appear in the DPAA listings, but when one double checks, one finds that they were declared dead as required by law and are included in the deaths accounting.
For example, plucking a name from those known losses and trying to keep it simple with one of the more well-known such, CDR Samuel D Dealey was commander of USS Harder, lost on 24 August 1944. Dealey appears on the DPAA rolls as unrecovered; he was reported as missing in action. A year later, 25 August 1945, his widow, Edwina, was presented with his posthumous Medal of Honor. We can also find his name on the list of those killed aboard Harder in the US Navy's “United States Submarine Losses World War II” (a nice volume if you can get your hands on one, but the USN Heritage & History site has it available in an on-line format at https://www.history.navy.mil/research/library/online-reading-room/title-list-alphabetically/u/united-states-submarine-losses.html). The bodies of he and his crew were never recovered (in Navy vernacular 'BNR' - Body Not Recovered) and are, therefore, counted among the US Navy's war dead. They, the crew of USS Harder, were all officially declared dead on 2 October 1945 and are memorialized at the Manila American Cemetery, see https://abmc.gov/print/certificate/462598
Another easily ferreted example would be from those killed aboard USS Arizona at Pearl Harbor. The accounting of how many, and who, were lost came not from counting bodies, but from comparing muster rolls to survivors. Thus, and without looking very hard at all, on the first line of the DPAA Navy unrecovered listing is Aaron, Hubert, F2c, lost on 7 Dec 1941 on USS Arizona; his body was not recovered. If you were to check the US Navy's published state by state reporting of casualties, look in Aaron's home state of Arkansas and, lo, there he is, the very first entry under “Killed in Action, Died of Wounds, or Lost Lives as a Result of Operational Movements in War Zones.” State by state loss registers are available from the NARA website.
So, F2c Hubert Aaron, USN, who, as an AS, was received for duty aboard USS Arizona on 1 January 1941, was lost in the attack on Pearl Harbor. He is listed as missing in action on a 31 December 1941 USS Arizona muster roll and currently still listed as remains unrecovered. He was, however, declared dead by the Navy as of 7 December 1941 and is counted as killed in action. He is memorialized at the Honolulu Memorial, see https://www.abmc.gov/print/certificate/476944.
Most of this is a very simple exercise with the knowledge of the data underpinning the numbers. I will not go through the exercise for the US Marines or US Coast Guard, anyone wanting to chase their numbers is invited to do so.
At the risk of being repetitious, the missing and missing in action from US service in WW2 are not carried separately; they were, at some point, declared to be dead, as required by law, and were added to the count of dead for the particular service. With their numbers then included in the total deaths for each service, and it would be a mistake to count those whose bodies were never recovered as being in addition to those counted as service deaths.