Learning about the "sun" measurement system

Measurements and Japanese Knives

With Japanese knives, blade lengths are generally measured in millimetres (mm) and occasionally inches (in). Millimetres are a fairly precise measurement for Japanese knives, allowing you to dial in how long you want your blade to be for purchasing or custom orders. Sizes like Small, Medium and Large that you see in items like clothing aren’t nearly specific enough for knives – especially for professional kitchens and a chef’s personal preferences!

It’s very easy to accidentally purchase a knife that is too large for the home, or too small for the restaurant. We have general recommendations on sizes that you can see in our knife guide, but everyone is different.

Traditional Japanese measurements - the sun unit

Of course, knives were not always measured in the metric or imperial systems, as they were introduced very late into Japan's history. So what did they use before this?

In countries that had affiliations with China in the past such as Japan and Korea, Chinese traditional measurements were often employed, especially in the way of traditional crafts such as knife making. You see those measurements still used today!

You may come across the 寸 (sun - pronounced IPA /sun/, sounds slightly like a short version of the word “soon”) kanji in more traditional places and crafts in Japan. But why is this? What is that measurement system, and more importantly how does it convert to the blade length of a knife? This blog will briefly explain the measurement structure of a sun unit. It’s thanks to this measurement system that Japanese knives are the lengths they are today!

The length of a sun unit

Firstly, let’s translate precisely what a 寸 or sun is in modern measurements. From the late 1800s, 1 sun has been standardized as 30.3mm – it doesn’t quite fit into the metric measurement system (which would be 30mm), much like how 1 inch in the imperial measurement converts to 25.4mm in metric (not quite 25mm). 1 sun can be a different length in certain crafts, but for the sake of knifemaking 1 sun is still 30.3mm.

This measurement system is part of a larger series of Japanese traditional measurements, referred to as shakkanhou (尺貫法).

That said, Japan uses the metric system now and has done so in a more compulsory manner since the late 1950s (you do still see the imperial system in some situations). So, why is this measurement still important then? Let’s dive a little deeper.

How the sun causes blade length variance

Japanese knives have a blade length measured in mm as we already stated, but you may notice these mm lengths are generally (not always, like in the santoku knife example below) in 30mm increments. That’s because these knives were traditionally measured and made using the sun unit. For example, a blacksmith may make deba knives of a 4, 5, 6 and 7 sun blade length. This is approximately 120, 150, 180 and 210mm lengths respectively. With longer blades that are 300mm and above like yanagiba knives, you may also see them being referred to as one shaku (尺) as one shaku is ten sun long. So, this would make a 330mm blade a 1 shaku 1 sun blade (1尺1寸 or just 尺一/shakuichi). It's a measurement you still see in protected Japanese traditional crafts, and Sakai knifemaking is one of those according to the Ministry of Economy, Trade and Industry! In knifemaking, we use a combination of measurements to respect that tradition but also make things accessible in the current day and age.

Let’s focus on that 120mm blade length, which converts to roughly 4 sun. 4 sun is actually 121.2mm – it’s slightly off, with 1.2mm unaccounted for. Before sun were standardized, they were quite flexible and variations could be much larger! This can also be seen with just how many different sun measurement styles there are. For example, the sun used by Japanese tailors is 25% longer than the one we use.

Some of the situations where handcrafted knives do not measure perfectly to the millimetre can be due to these variances. You can remember 1 sun as 30mm for the sake of ease still, but for longer blade lengths you may see a bit more difference. Do remember also, these blades are made by hand! As with most things that are done by handwork, expect some uniqueness – which can include the length from time to time.

Remember, a sun unit for knifemaking is 30.33mm long!

With pressed knives this variance is not something that will really occur, this is more related to handcrafted knives. In fact, if it is slightly off being perfect that might be a sign that it is indeed handmade! But of course, craftsmen aim to get to as close to the correct blade length as possible – and very, very often do. They are masters at their crafts, after all. While most do, again not all blades correspond to a sun measurement. Some may have half sun lengths, such as 5.5 sun in the case of a 165mm deba knife (which we sell in our White Steel #1 Montanren range). Sometimes you'll also get knives that do not adhere to sun measurements at all!

In our store, while we measure everything with millimetre lengths, we also have sun rulers available, so you can see precisely what length the blade is via the traditional Japanese measurement system. We use these rulers in the store ourselves every day. Next time you go to a knife store (like us!) keep an eye out. If you see a ruler that doesn’t quite make sense to you, it’s probably a sun ruler!

Let's quickly recap the main points:

    • - Japan used to measure things using sun, and there’s different sun units for different crafts

    • - 1 sun for knifemaking is 30.3 millimeters, 1 shaku is 10 sun

    • - Knives are still handmade, so sometimes there is variance in blade length

    • - Some knives use half sun measurements too, like deba knives

    - Sun is why knives are generally in either 15 or 30mm increments - but not always

If you have further questions about the measurement, or want to make sure the blade length of a knife is precise to what you need - please contact us. We're always happy to help, especially when it comes to discussing traditional knifemaking concepts.