How many people worked on the Manhattan Project?

by Alex Wellerstein, published November 1st, 2013

Everyone knows the Manhattan Project was big. But how big was it? There are lots of ways to try and convey the bigness. The size of the buildings and sites, for example. Or the cost — $2 billion 1945 USD, which doesn't sound that big, even when converted to modern numbers (e.g. around $30 billion 2012 USD, depending on the inflator you use), since we're used to billions being tossed around like they are nothing these days. But consider that the USA spent about $300 billion on World War II as a whole — so that means that the atomic bombs made up for a little under 1% of the cost of the entire war. Kind of impressive, but even then, it's hard to wrap one's head around something like "the cost of World War II."

General Groves speaks to a group of Oak Ridge service personnel in August 1945. From the DOE. There are lots of great Oak Ridge photos from the 1940s in this Flickr set.

General Groves speaks to a group of Oak Ridge service personnel in August 1945. From the DOE. There are lots of great Oak Ridge photos from the 1940s in this Flickr set.

Another approach is to talk about how many people were involved. There are a number of various estimates floating around. Instead of focusing on those, I want to jump directly to the source: a once-secret postwar report on Manhattan Project personnel practices that includes some raw numbers on hiring.1

This report has two very interesting graphs in it. The first is this one, showing total employment by month, broken into the various important Manhattan Project categories:

Manhattan Project contractor employment by month

Let's just take a moment to marvel at this. They went from pretty much just talking about a bomb, in theory, on paper, in late 1942, and had a project with 125,310 active employees at its peak, 22 months later. That's a huge ramp-up.

I like this graph because it helps you see, very plainly, the progress of the project. You can see that Oak Ridge (CEW) and Hanford (HEW) construction both got rolling pretty quickly but took about a year to hit their maximums, and that all construction peaked in early 1944. At which point, operations became the main issue — running the plants. It's interesting to compare how many more people were required for Oak Ridge operations than Hanford operations, and that the "Santa Fe Operations" — Los Alamos, et al. — barely registers on the graph. A couple thousand people at most.

You can also see how rapidly that curve starts to drop off in September 1945 — over 10,000 people left at the end of the war, a significant chunk of them being Oak Ridge operations personnel. There is then a long slumping decline until late 1946, when you start to get an up-tick. This maps on pretty well with what we know about the history of the Manhattan Project in the period before the Atomic Energy Commission took over: Groves' hard-built empire decayed under the uncertainty of the postwar and the dithering of Congress.

This is where we get the number one usually sees cited for the Manhattan Project: 125,000 or so employees at its peak. Which is impressive... but also kind of misleading. Why? Because peak employment is not cumulative employment. That is, the number of people who work at any given company today are not the number of people who have worked there over the course of its lifetime. Obvious enough, but if one is wondering how many people did it take to make the atomic bomb, one wants to know the cumulative employment, not the number on hand at any one time, right?

Digging around a bit more in the aforementioned personnel statistics of the Manhattan Project (a thrilling read, I assure you), I found this rather amazing graph of the total number of hires and terminations by the project:

Manhattan District Contractors Hires and Terminations through 31 December 1946

Now that number on the left, the total hires, is a pretty big one — over 600,000 total. Unlike the other graph, I don't have the exact figure for this, but it looks to be around 610,000. That's a huge number. Why would the numbers be at such odds? Because at the big sites — Oak Ridge and Hanford — there was a pretty high rate of turnover, as the "terminations" bar indicates: over 560,000 people left their jobs on the Manhattan Project by December 1946.

Some of this, of course, is because the job was done and they went home — once the construction was done, you didn't need as many people working on construction anymore. But it's also because even during the war, there was a considerable amount of people either quitting or getting fired. People left their jobs all the time, at all times during the war. As the report indicates, the reasons and rates varied by site. For construction at Hanford, they had an average monthly turnover rate of 20%, with a ratio of resignations to discharges set at 3 to 1. Of those who resigned, 26% did so because of illness, 19% were to move to another location (which could be a lot of things), 13% cited poor working conditions, 13% said there was an illness in the family, 14% had got another job somewhere else, 7% cited the poor living conditions, 6% got drafted or otherwise joined the military, and 2% complained about wages. Of those who were discharged, about a quarter of the time it was because they were an "unsatisfactory worker," and the rest of the time it was because of chronic absenteeism. For construction at Oak Ridge, the average turnover rate was 17%, with mostly the same reasons given, though the resignations to discharge ratio was 2 to 1. (More people, by percentage, complained about the living conditions at Oak Ridge than at Hanford.) For the operations at Oak Ridge, the turnover rate was 6.6%, with a resignations to discharge ration of 1.3 to 1 — of those who left, a little over 40% did so because they were fired.

A 1944 "Stay on the job" rally at J.A. Jones Construction Co. in Oak Ridge. The workers seem a little unimpressed. Source.

A 1944 "Stay on the job" rally at J.A. Jones Construction Co. in Oak Ridge. The workers seem a little unimpressed. Source.

Of course, these numbers run through the entire tenure of the Manhattan Engineer District. When most people want to know how many people it took to make the bomb, they want to know up until August 1945 or so. I don't have exact numbers on this. However, if we take the data from the report and the graphs, and assume an average monthly turnover rate of about 17% for the entire project, we end up with about the right number total.2 Subtracting all of the people added after August 1945, we get around 485,000 total people required to make the bombs during World War II. Given how much of that employment was front-loaded (again, with a peak in June 1944), I don't think it's too far off to assume that probably half a million people were employed to make the bomb. Which, to put that in perspective, means that during World War II, approximately 0.4% of all Americans worked on the bomb project — about one out of every 250 people in the country at the time.

Which is pretty impressive. By contrast, I've seen estimates that said that the Soviets used about 600,000 people total to make their atomic bomb. Which is not too different a number, actually — a bit less impressive than one might think if one is only comparing it to the peak of the Manhattan Project. The Soviets had around 170 million people at the time, so it works out to be a pretty similar percentage of the total population as the American project. Of course, one suspects that fewer of the Soviet workers were able to quit because they didn't like the wage and working conditions. Though I'm sure they had their own form of grim "turnover."

  1. Manhattan District History, Book I – General, Volume 8 – Personnel (dated 19 February 1946 but with numbers that suggest later additions were made. []
  2. If you want to play with the data yourself, I've uploaded it here as a CSV file. Some of it is extrapolated from the top graph. []

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29 Responses to “How many people worked on the Manhattan Project?”

  1. John Maurer says:

    Great post! Quick question on the number of people through August 1945: given the data contained in the first graph, showing the heavy downsizing that occurred following the end of the war in August 1945, can we assume a monthly turnover rate of 17% up to that point? If turnovers increased significantly after August 1945, shouldn’t the monthly turnover rate prior to that point be somewhat less than the average turnover rate for the entire 1942-1946 period?

    • The month-to-month turnover rate varied a lot over the course of the war. So for construction at Oak Ridge, for example, it was as high as 42% or so at the end of November 1945. So rather than try to take that into effect, I just went with what looked like a smooth average, based in part on the idea that since the peak employment was well prior to the end of the war, the turnovers there would have added up to more raw numbers even if there was higher turnover percentage at the end of the war. That the percentage (17% or so) that produced the closest fit to the number of total hires was something of an average of the average percentages of the two biggest sites struck me as indicating that there was something close-enough-to-correct about it.

      To clarify, the 17% or so that I’m using in my calculations is really a replacement rate, assuming that to get to the employee levels they have each month, they are replacing some number of lost workers.

  2. Doug Dransfield says:

    Some more very interesting research. “The Girls of Atomic City” by Denise Kiernan is an interesting read about the Oak Ridge employee recruitment, living conditions, and work restrictions.

  3. This has very important implications for understanding how many workers overall were exposed to radiation in the course of their jobs, to the extent that the “extra” personnel you’ve uncovered worked in or near reactors, processing and/or waste handling facilities.

    • Pat Byrnes says:

      I suspect that the turnover rates would be somewhat different for operations personnel (probably lower) than for construction workers, and that construction workers (and most clerical workers as well) would generally be less at risk for radiation-related exposures than operators. Some job categories would be much more at risk than others.

      • At some point I’ll post a bit about the job risk question, because it is an interesting one (with a lot of documentation). The short version is, the construction workers were usually at more risk than anyone else, but not for radiation-related reasons — it’s because big construction projects have inherent risks associated with them. So there were people who, unbeknownst to them, were working on a fancy high-tech atomic energy project who died because they got run over by a cement truck, and things like that.

  4. Cindy Kelly says:

    This is fabulous, Alex. Great job.

    There is a darker side of the story that we don’t always read about. To underscore the challenges of working at Hanford, Rober Bubenzer who was a supervisor for Hanford plant production, commented in an interview in 1986 to S. L. Sanger how difficult life could be.

    “Drunkenness was prevalent. And depression was quite a deal, and this was a big reason for people leaving there. Homesickness, too, it was a depressing sort of place. It was almost like being in prison. Wired in, barb wire. Men separated from the women, even husbands and wives were separated sometimes. We had a number of nervous breakdowns of personnel. It was loneliness and depression and they hit the booze very hard.”

    No wonder there was considerable turnover, and not just from the termination winds.

    • Thanks for your dogged and creative research, Alex, and for finding ways to provide useful context.

      I’m interested in the “nervous breakdown” numbers and protocols for Manhattan Project sites — specifically how the approaches might differ depending on a worker’s clearance. If a clerk or construction worker became depressed or suicidal, were they simply discharged? How about a scientist or manager? Were they shipped off to Walter Reed? Treated by medical personnel at Hanford or Oak Ridge? How and where would one find out more about this aspect of Manhattan Project medical history?

      Thanks for any suggestions!

      • Cindy Kelly says:

        The full interview with Robert Bubenzer that I quoted about depression is on the Atomic Heritage Foundation’s website, “Voices of the Manhattan Project,” ( Denise Kiernan discusses the role of a psychiatrist at Oak Ridge in her book, “Girls of the Atomic City.” There are a few women who comment on how difficult it was to be isolated with limited professional opportunities at Los Alamos in a collection of writings called “Standing By and Making Do.”

        As Alex’s blog points out, there was tremendous difficulty in retaining workers, especially at Hanford. To combat discontent and boredom, Army made a great investment in keeping everyone entertained with dozens of recreational facilities and social events at both Oak Ridge and Hanford. Most of the scientists at Los Alamos seem to have been swept up in their work six days a week with a sense of mission and esprit de corps. They worked hard and played hard.

        Perhaps there are articles on this subject with which someone else is familiar. Let us know!

  5. Peter says:

    Just doing the background and security clearances for the new hires must have been an enormous job. Without computers, everything would have had to be done manually, for instance checking for criminal records by writing to the courthouses in every location where a new hire had lived. Interviewing neighbors and former employers for each of the hundreds of thousands of workers must have required a near-army of people.

    • It was a huge job, and for Hanford and Oak Ridge they rejected a huge number of the applicants.

      The volume of military background investigations that the FBI handled was so high that they took the DC Armory, a sports arena, and turned it into a massive temporary filing facility. “By 1942 the FBI was adding 400,000 file cards a month to its archives, and were receiving 110,000 requests for “name checks” per month. By 1944 the agency contained some 23 million card records, as well as 10 million fingerprint records.” (Source at the end of this ancient post.)

  6. Bradley Laing says:

    —A book about the Betchel Corporation said that a pipeline crossing Canada up to Alaska was advertised by the company as “These conditions area as harsh, or harsher than any in the country. This is no Picnic (in all caps).” But, said the book, “anxious to avoid the draft, workers came like Yahoos to a Monster Truck Rally.”

    —With Gold Stars for dead servicemen in the windows of many homes, it would seem a happy thing to run to work at Oak Ridge.

    • The thing is, though, that unless your job made you irreplaceable, they could still draft you if you were of draft age. So the Manhattan Project as a whole wasn’t really a solid haven for people avoiding the draft; even the young Los Alamos scientists had to have their drafts deferred. I wrote a bit about this here.

  7. Bradley Laing says:

    —i wrote Something new under “worst of Manhattan project leaks,” please read.

  8. Stan Norris says:

    Alex On page 724 of the Hewlett & Anderson The New World is a monthly breakdown of the costs. $2.2 billion total, highest month August 1944.

    THE NEW WORLD / 1939-1946
    Monthly Expenditures in the Manhattan Engineer District
    August 1942 through December 1946 (in thousands$)
    		1942 	1943 	1944 	1945 	1946
    January 		2,010 	68,984 	68,276 	19,073
    Feb			6,829 	61,336	63,987 	18,136
    March			6,036 	55,269 	60,156 	18,622
    April			8,533	93,041	47,776 	16,846
    May			14,395	89,283	69,099	18,282
    June			23,173	78,459	54,584	18,664
    July			25,114	73,539	47,164	11,742
    Aug		15,000*	41,286	111,391	43,414	48,152
    Sept		 24	48,344	71,301 	51,219 	11,791
    Oct		 67	54,973	80,779	58,819	17,166
    Nov		138	68,005	81,832	23,885	54,006
    Dec		893	45,889	74,181	21,932	28,563
    Totals 		16,122	344,587	939,395	610,311	281,043
    TOTAL							$2,191,458
    • Transferred to OSRD.
  9. Alex-
    I was telling someone about this post the other day and we got to wondering- what percentage of the US workforce during WW2 did the MP represent?

    • Patrick — An excellent question and one I had been meaning to look up.

      According to Historical Statistics of the United States (Table Ba478-486 – Labor force, employment, and unemployment: 1938–2000), the total US labor force for the WWII period was around 53-56 million (or 57-58% of the total civilian non-institutional population at the time). So that would make the total Manhattan Project employment something like 0.9% of the total labor force at the time, or 0.2% of the labor force at the Manhattan Project’s peak. If we consider the full 610,000 participation number through 1946 (and consider each of those hires a unique individual, which may not be the case), then that would mean that a solid 1-1.1% of the total workforce passed through the Manhattan Engineer District’s rosters at one point or or another between 1942 and 1946.

  10. kme says:

    Apropos of nothing, are there any records on how the name of the Manhattan Project came to be chosen?

    • Indeed: The first headquarters were in New York City, at 270 Broadway on the 18th floor. This was a convenient location to both Columbia University, the Kellex Corporation headquarters, and the Union Carbide and Carbon Building — important players in the early construction work. As such, a new Engineer District was created, and those are always named after their region. Ergo, the Manhattan Engineer District. Later, when Groves took over, the center of gravity moved to Oak Ridge, but the name stuck and even provided convenient cover given that by that point New York was not really the center of things anymore. It had a few other code-names (Development of Substitute Materials was one of the early ones) but Groves preferred the cryptic banality of the MED. Eventually they started using Manhattan Project to refer to the entire endeavor, not just those run by the Army Corps of Engineers.

      The New York Times did a nice piece on this a few years ago, with lots of details (and a wonderful interactive map of the NYC locations) from Stan Norris. The Atomic Heritage Foundation also has a nice little book on the Manhattan Project in Manhattan, which goes into more details.

  11. Dan Downing says:

    Interesting and well put.
    The question can be framed another way. How about if one includes not only the direct employees at MP sites, but the support needed? Looking only at the workers is a bit like counting the number of combat soldiers—rifle companies, bomber crews, fighting ships and so forth—and calling it “The Armed Forces”, while ignoring the 70-90% of people waging war: clerks, supply, personnel, training and perhaps on to munitions workers, food suppliers, merchant mariners, etc.
    Looked at from this total perspective all those people involved in security for the Project were employed in the making of the bomb. So were the manufacturers who supplied the centrifuges, the filters, the raw materials; also the house keepers, food suppliers, housing constructors, transporters, etc. who built the accommodations and kept them running. Without the support crew, nothing happens. With it included, a case can be made for 10% of the American work force being involved in making the bomb at the beginning of 1945.
    The comparison to the USSR shorts the U.S. effort because the bad old commies had two huge advantages: one, they stole the plans and, two, more importantly, they knew it could be done. The Los Alamos crew did not know until they witnessed Trinity.

  12. Dan Billingsley says:

    Long ago I heard from someone alive during WWII that social security numbers were used as a non-obvious way to screen residents from one part of the country where there was a major industrial project going on (eg, Tennessee) from being hired in another part of the country where there was a major industrial project going on (eg, Washington state). The idea being to reduce the number of friends or relatives who might put two & two together.

    Sounds plausible but I haven’t seen anything about it elsewhere. Has anyone previously heard this? Does there seem to be any evidence pro or con?


  13. […] The total number of people who worked on the Manhattan Project — over half a million! […]

  14. […] General Groves speaks to service personnel at Oak Ridge in 1945. The photo is via a very good post in the Nuclear Secrecy blog on how many people worked on the Manhattan […]

  15. […] half of the 500,000 or so people employed by the Manhattan Project were employed in construction. As a result, most of the injuries and fatalities associated with […]