Angle of a cutting edge

Edge Angle and its impact on Sharpness and Hardness

The angle of a knife's cutting edge plays a critical role in both its sharpness and how long it stays sharp. Comparing two blades—one with a narrow edge angle (B1) and the other with a wider angle (A1)—the blade with the narrower angle (B1) will slice through food more easily, which is perceived as sharper.

Additionally, even when the edge becomes slightly worn (as shown by the red line in fig. A2 / B2), the thinner, more acute angle of blade B2 will still retain its cutting ability. In contrast, blade A2, with its wider edge, will feel noticeably duller. This means that a narrower edge angle not only provides greater initial sharpness but also maintains that sharpness for a longer period.

Graphic of knife blade with wide angle on cutting edge vs narrow acute edge angle

Graphics with thickness behind edge of knife blade wide edge angle vs narrow acute edge angle after edge is rounded and worn

Looking at the narrow edge in fig. B1, it’s also easy to understand you need a harder material to hold the edge without rolling or deforming.

This leads to two key insights:

The harder the knife, the more acute an angle it can hold.
The more acute the edge angle, the sharper the knife will be, and the longer it will retain that sharpness.

However, the sensitivity of a thinner blade geometry, as explained in the chapter Blade, also applies to the cutting edge. The thinner the edge, the more delicate and prone it is to damage.

However, what was explained in chapter "Blade" about the sensitivity of a thinner blade geometry, naturally applies to the cutting edge as well. The thinner it is, the more sensitive it will be.

Western angle

In Chapter 2, we also explored the differences between Western and Japanese blade geometries. In line with the philosophy of creating sturdy knives from softer steel, Western (German) blades are typically sharpened at a wider angle of 30-35° (included angle, 15-17.5° per side). This results in knives that are less sharp initially and dull more quickly, but are much more resistant to damage or chipping when subjected to rough handling. TWestern knives are generally sharpened symmetrically (fig C), meaning the edge has the same angle on both sides.

Japanese angle and asymmetric edges

In line with the Japanese philosophy of thinness, Japanese knives are sharpened to more acute edge angles, typically around 20° (10° per side). This makes Japanese edges sharper and helps them stay sharp longer. However, this fine edge is more vulnerable to chipping if subjected to abuse.

Japanese knives are usually sharpened asymmetrically (70/30), meaning the angles are different on each side. The left side has a wider angle (shorter bevel), while the right side has a narrower angle (wider bevel). For instance, a 70/30 edge on a knife with a total angle of 20° would have a 14° angle on the left and a 6° angle on the right.

What effect does an asymmetric edge have? In theory, an asymmetric edge (fig D) causes the knife to steer to the right when cutting. This happens because the wider angle on the left creates more resistance, pushing the blade to the right and making it harder to achieve a straight cut.

So why do Japanese knives often have asymmetric edges? One common theory is that the asymmetry compensates for the asymmetric blade geometry found in many traditional Japanese knives. For example, if the blade is thinner on the left side, it would naturally steer to the left when cutting. The wider angle on the left counteracts this by pushing the blade to the right, balancing the cut and helping to maintain a straight line.

However, many modern Japanese knives sold internationally are made with multi-layered, symmetrical blade geometry, ensuring the core steel stays centered. In such cases, an asymmetric edge doesn't offer any practical advantage.

It's also worth noting that in terms of cutting performance—both initial sharpness and edge retention—there’s no significant difference between symmetric and asymmetric edges. Therefore, the symmetry of the edge is not a marker of quality and can be adjusted to the user’s preference during sharpening.