How many people worked on the Manhattan Project?

How to get close to this number

Reason from the physical plant. The project built entire industrial cities to enrich uranium and produce plutonium, and enrichment at that scale required facilities among the largest buildings in the world at the time. Running several industrial complexes plus research laboratories needs a workforce in the tens of thousands, not the thousands.

The secrecy detail in the explanation is what makes the number hard to believe: the overwhelming majority had no idea what they were building. Compartmentalisation meant a worker knew their task and nothing about its purpose, which is how a project employing a hundred and thirty thousand people stayed secret.

Whole towns were constructed that did not appear on maps, with tens of thousands of residents who could not tell their families where they lived. That is the practical consequence of the number: secrecy at that scale is not about keeping a few people quiet, it is about designing an organisation in which almost nobody has enough information to reveal anything.