October 2, 2026

If you have shopped for Japanese knives, you may have noticed something curious about their sizes.

Yanagiba often appear as:

240mm.
270mm.
300mm.
330mm.

Deba may appear as:

150mm.
165mm.
180mm.

To an overseas buyer, these can look like rather arbitrary numbers.

Why 270mm instead of 275mm?

Why does another size jump neatly to 300mm?

And why do so many traditional Japanese knives seem to move in increments of roughly 30mm?

The answer takes us beyond millimeters.

Behind many of these familiar knife sizes is an older Japanese system of measurement—one that predates the metric system by centuries.

It is called shakkanhō (尺貫法).

And once you understand it, some of the numbers on a Japanese knife specification sheet begin to look very different.

One of the traditional Japanese units of length is the shaku (尺).

One shaku is approximately 30.3 centimeters.

A shaku is divided into ten sun (寸), making one sun approximately 3.03 centimeters, or 30.3 millimeters.

That immediately reveals something familiar:

8 sun ≈ 242mm
9 sun ≈ 273mm
10 sun = 1 shaku ≈ 303mm

Now look again at familiar Yanagiba sizes:

240mm
270mm
300mm

The pattern no longer seems so arbitrary.

What an international customer sees as a 270mm Yanagiba sits very naturally within a traditional dimension of roughly nine sun.

A 300mm knife corresponds approximately to one shaku.

In modern commerce, these knives are normally presented in millimeters. But the rhythm of their sizing can still reflect an older way of measuring.

The metric number is modern.

The proportions behind it can be much older.

Japan used forms of the shakkanhō measurement system for many centuries.

Sakai City notes that Japan's historical measurement system was influenced by systems originating in China, and that standardized weights and measures were already incorporated into the Taihō Code in 701.

The shaku itself also had a physical, human connection: historical explanations associate it with the span of an outstretched hand.

Over centuries, measurement standards changed and became standardized, but units such as shaku and sun became deeply embedded in Japanese working vocabulary.

They were used in fields such as architecture, woodworking, textiles and traditional crafts.

Japan eventually transitioned officially to the metric system. General metrication took effect in 1959, while the remaining exception for land and buildings ended in 1966.

Today, Japan's Measurement Act does not permit traditional shakkanhō units to be used as legal units for transactions or certification where statutory units are required.

And yet the old vocabulary did not simply disappear from workshops and traditional practices.

That distinction is important.

Japan buys and sells in millimeters and centimeters.

But people can still think in sun and shaku.

Imagine that a particular form has been made for generations.

A craftsman learns not simply that an object should be “273 millimeters long,” but how its length relates to its width, thickness, curvature, balance and intended use.

Over time, measurements become part of the vocabulary of making.

Changing the label on a dimension is easy.

Changing the accumulated working language surrounding it is something else.

This phenomenon is not uniquely Japanese.

A carpenter in North America may instinctively think in feet and inches even when working from a drawing that includes metric measurements.

A chef may describe a pan as 12 inches without considering why that particular unit feels natural.

A British person may know their height in feet and inches while buying food by grams.

Measurement is mathematical.

But familiarity with measurement is cultural.

For traditional Japanese crafts, shaku and sun can function in much the same way: not necessarily as the legal units used to sell an object, but as part of an inherited language of dimensions and proportions.

There is another reason this story matters.

When we shop online, we encounter knives as numbers.

Blade length: 270mm.
Blade height: 32mm.
Thickness: 3.5mm.
Weight: 210g.

These measurements are extremely useful. At KIREAJI, we provide detailed dimensions precisely because customers purchasing from another country cannot hold the knife before choosing it.

But a knife was never created to exist on a specification sheet.

It exists between a hand, a cutting board and food.

A longer Yanagiba allows a chef to make a long slicing stroke through fish.

A Deba must combine appropriate length, thickness and weight for the work it is expected to perform.

A Gyuto must feel manageable in relation to the cook and the working space.

The important point is not that shakkanhō somehow magically produces better proportions.

It does not.

Rather, traditional dimensions remind us that the sizes of tools often develop through repeated human use.

Long before someone clicked “270mm” in an online filter, generations of cooks and craftspeople were already working out what different lengths allowed a knife to do.

There is also a practical lesson here for anyone buying a handcrafted Japanese knife.

A size name should not always be interpreted as an exact physical measurement of every finished knife.

Traditional knives are handmade objects. Depending on the maker, knife type, manufacturing process and the points between which a measurement is taken, the actual finished dimensions can vary.

This is especially important because “blade length” itself is not always measured identically by every maker or retailer.

A nominal 270mm knife therefore belongs to a size category, but the individual knife in front of you may measure somewhat differently.

For an overseas customer, the best approach is simple:

Use “240mm,” “270mm,” or “300mm” to understand the general size class—but check the actual dimensions provided for the individual knife whenever they are available.

The traditional size tells you where the knife belongs.

The actual measurement tells you what is in your hands.