A tachi is a traditional Japanese sword that was primarily used by samurai during the earlier periods of Japanese history. The tachi is characterized by its curved, single-edged blade and is typically longer and heavier than the katana.

The average length of a tachi blade is around 30 to 36 inches (76 to 91 centimeters), although there can be variations depending on the specific sword and historical period. The handle (tsuka) adds additional length to the overall measurement.

Compared to the katana, the tachi has a longer cutting edge and is designed to be worn with the cutting edge facing down. It was typically used as a battlefield weapon, especially on horseback, and had a slightly different construction and wearing style than the later-developed katana.

It's important to note that the measurements provided are approximate and can vary between individual tachi swords due to variations in historical examples, regional traditions, and personal tastes of sword owners or collectors.

In summary, the blade length of a tachi typically falls within the range of 30 to 36 inches (76 to 91 centimeters), although variations can exist. The tachi is a traditional Japanese sword that predates the katana and is known for its longer and heavier blade.