The angle at which you sharpen a katana can vary depending on personal preference, the intended use of the sword, and the specific techniques employed. However, a commonly recommended angle range for sharpening a katana is between 10 and 20 degrees per side. This means that the total included angle (the angle between the two sides of the blade) would be between 20 and 40 degrees.

It's important to note that the angle should be consistent along the entire length of the blade to ensure an even and uniform edge. Consistency in sharpening angle helps maintain balance, cutting performance, and durability of the sword.

When sharpening a katana, it is crucial to have a proper understanding of the blade's geometry, be skilled in using the sharpening tools, and follow safe sharpening practices. If you are not confident in your sharpening abilities, it is recommended to seek assistance from an experienced swordsmith or professional sharpener to avoid unintentional damage to the blade.

Additionally, it's worth noting that the specific angle used for sharpening a katana may differ depending on the intended cutting tasks, the blade's hardness and thickness, and the preferences of the individual or school of swordsmanship. Therefore, it is advisable to consult with an expert or refer to specific guidelines provided by experienced practitioners or instructors for sharpening techniques related to your particular katana.