Classification Guide by Blade Length: Calculation Methods for Shaku/Sun/Bu Units and Boundaries Between Tachi/Katana/Wakizashi/Tantō

Blade Length as the Fundamental Unit of Japanese Sword Classification

The most fundamental classification indicator for Japanese swords is "blade length" (hachō)—the straight-line distance from the blade edge (hamon) to the blade tip (kissaki). This measurement determines the four major categories: tachi, uchigatana (katana), wakizashi, and tantō.

This article provides a complete explanation of how to measure blade length, convert traditional Japanese units of measurement (shaku, sun, bu), and the boundary values and historical background of each classification.

Traditional Japanese Units of Measurement: Understanding Shaku, Sun, and Bu

Japanese sword dimensions are often expressed in both modern metric and the traditional "shakkanhō" (shakkan system) units. Understanding the shakkan system is essential for sword appreciation.

Basic Unit Relationships

Memorization tip: "1 sun ≈ 3cm, 1 shaku ≈ 30cm" works as a rough estimate. Precisely, 1 shaku = 10/33 meters ≈ 30.30303...cm.

Historical Background of Units

The shakkan system originated from China but Japan developed different standards for different uses—the "magane-jaku" (carpenter's ruler) and "kujira-jaku" (whale ruler), among others. For blades, the "magane-jaku" standard is used.

Blade Length Measurement Method

The correct method for measuring blade length:

1. Identify the blade edge (haba): The haba is the blade's terminal point where it meets the habaki (blade collar) at the base. This is the starting point for measurement.
2. Measure to the kissaki (blade tip): Place the blade on a flat surface and measure the straight-line distance from the haba to the kissaki tip.
3. Do not measure along the curve: Blade length is not the distance following the sword's curved surface but the direct straight-line distance.

Measurement tips: Placing a ruler directly on the blade edge is dangerous. Instead, align the ruler along the blade's spine (mune) or use calipers.

Boundary Values of Each Classification (Official Standards)

Current Legal and Conventional Classifications

The primary classification standards currently used in Japanese sword circles:

Boundary between tachi and katana: Legally there is no distinction by blade length; the traditional distinction is based on "wearing style." Tachi are suspended edge-down from the waist (haitō form), while katana are worn edge-up inserted at the waist (sashitō form).

NBTHK (Society for Preservation of Japanese Art Swords) Standards

The NBTHK uses these standards for registration and authentication:

• Katana (uchigatana): blade length 2 shaku (approx. 60.6cm) or more
• Wakizashi: blade length 1 shaku (approx. 30.3cm) or more, less than 2 shaku
• Tantō: blade length less than 1 shaku (approx. 30.3cm)

Detailed Breakdown of Each Classification

Tantō (under 1 shaku, under 30cm)

Tantō are the smallest Japanese swords, used for self-defense, ceremonies, and religious rituals. Representative forms include aikuchi, yoroi-dōshi (armor piercer), and tantō (worn at the breast).

Notable historical examples:

• "Nikko Ichimonji" by Yoshimitsu: a celebrated Kamakura-period tantō
• Tantō by Masamune: Sagami tradition tantō showcase the intense hamon and concentrated beauty characteristic of Masamune's work

Wakizashi (1 to 2 shaku, approx. 30–60cm)

Wakizashi developed in the Edo period as a companion sword (secondary blade) to the tachi and katana. Warriors wore a "daishō" (pair)—a katana and wakizashi together.

Wakizashi are subdivided by blade length into "naga-wakizashi" (long wakizashi: 1 shaku 5 sun or more), "chū-wakizashi" (medium), and "tan-wakizashi" (short).

Katana/Uchigatana (2 shaku or more, approx. 60cm or more)

The standard form of sword used daily by samurai during the Edo period—the "ai" (larger) of the "daishō" pair.

Standard katana size: blade length of 2 shaku 3 sun to 2 shaku 4 sun (approx. 70–73cm) is most common.

Tachi (Suspended Wearing Form)

The primary sword type from the Kamakura through Muromachi periods. Blade length typically ranges from 2 shaku 3 sun to 3 shaku (approx. 70–91cm). Extant National Treasures and Important Cultural Property designated tachi center around this size range.

Swords of Unusual Length

Ōdachi and Nodachi (Field Tachi)

Swords exceeding 3 shaku (approx. 91cm) in blade length are called "ōdachi" (great sword) or "nodachi" (field sword). These oversized blades became popular during the Nanboku-chō period, and examples with blade lengths exceeding 1 meter still exist.

Example: The "Ōdachi" at Kibitsu Shrine in Okayama (blade length approx. 220cm)

Naginata

The naginata is a polearm with a curved blade approximately 1 to 2 shaku long mounted on a long shaft (2–3 meters). While not classified as a Japanese sword proper, it frequently appears in sword collecting contexts.

Summary—Culture Behind the Numbers

Blade length classification is not merely a numerical standard. Each category—tantō, wakizashi, katana, and tachi—reflects the combat styles, social institutions, and aesthetic values of its era.

The "2-shaku barrier" (the boundary between katana and wakizashi) is deeply tied to Edo-period sword-wearing culture, while the "1-shaku barrier" (between wakizashi and tantō) reflects functional distinctions as defensive weapons. Understanding these measurements is a direct path to comprehending samurai culture.