The largest military unit in the Mongol army was the tumen. A tumen was designed around 10,000 households and could provide roughly 10,000 soldiers, although its actual strength varied with circumstances, casualties, and recruitment.
The Mongols organized their forces using a decimal system traditionally associated with Genghis Khan's military reforms. Ten arbans formed a jaghun, ten jaghuns formed a mingghan, and ten mingghans formed a tumen. This structure made it easier to assign commanders, assemble units, and coordinate large campaigns across vast distances.
The term tumen referred to both a military formation and a unit of social and administrative organization. Its name comes from a word meaning “ten thousand.” In practice, a tumen was not always exactly 10,000 strong, so the figure describes its intended scale rather than a guaranteed headcount. The smaller mingghan, meaning “thousand,” is a common mix-up because it was also a major Mongol unit, but it ranked below the tumen.