Traditionally, Japanese swords are measured in shaku, sun and bu.
The 'shaku-system' works - just like the metric system - with decimals. The main difference is that basic unit is of one is 3.03 instead.
This gives us the following conversion chart.
|1 shaku||30.3 cm|
|1 sun||3.03 cm|
|1 bu||0.303 cm (3.03 mm)|
The nagasa is the most important specification of a sword(blade). This is considered the length, despite it's not the full length of the sword.
You can calculate the nagasa by measuring the distance from the mune-machi (back of the blade which sits inside the habaki) to the tip of the kissaki.
The amount in centimeters is divided by 30.3 to get the Japanese measurements. Here is a small conversion chart of the most common sword lengths:
|69.69 cm||2.3 shaku|
|71.2 cm||2.35 shaku|
|72.72 cm||2.4 shaku|
|74.24 cm||2.45 shaku|
|75.75 cm||2.5 shaku|
|77.27 cm||2.55 shaku|
|78.78 cm||2.6 shaku|