What is Thinning Ratio (カット率)?

Thinning ratio is determined by the interaction of three variables: tooth count, tooth spacing (gap width relative to tooth width), and tooth profile (the shape of each tooth's cutting edge and top surface). **Tooth count** is the most visible specification. Professional thinning shears range from 7 teeth (wide-gap texturizing shears with 50%+ removal) to 40+ teeth (fine-blending shears with 10-15% removal). However, tooth count alone does not determine the ratio because teeth of different widths and spacing create different gap ratios. **Tooth spacing** is measured as the ratio of gap width to tooth width. If the gap between teeth equals the tooth width, the theoretical removal is 50% (assuming hair falls evenly into gaps and teeth). If the gap is half the tooth width, theoretical removal is approximately 33%. Most thinning shears use a gap-to-tooth ratio between 0.3:1 and 1.5:1. **Tooth profile** significantly modifies the actual removal from the theoretical ratio. A V-shaped tooth notch captures hair at the base of the V and prevents it from escaping during closure — increasing actual removal. A U-shaped notch allows some hair to slide out during closure — decreasing actual removal. Some manufacturers use a proprietary "mushroom" tooth profile where the top of each tooth is wider than the base, which captures hair during the initial bite but releases it partially during final closure for a softer blending effect. The fundamental problem with thinning ratio as a metric is the absence of standardized testing. UTSUMI developed the concept but tests using their own proprietary method — specific hair samples, controlled cutting angles, and standardized technique. Other manufacturers report their own ratios using different testing conditions (or no testing at all, simply estimating based on tooth geometry). Real-world thinning ratios vary significantly based on factors outside the shear's design. Fine, slippery hair slides out of tooth gaps more easily than coarse hair, resulting in lower actual removal. Wet hair compresses between teeth differently than dry hair. The cutting angle (perpendicular vs. diagonal) changes how hair enters the tooth gaps. A shear that removes 30% of medium-density dry hair might remove only 20% of fine wet hair and 35% of coarse dry hair. Some brands have begun addressing this inconsistency. Hikari publishes thinning ratios tested on standardized synthetic hair samples at controlled humidity. UTSUMI provides ratio ranges rather than single numbers (e.g., "25-30%" rather than "28%") to acknowledge the variability. However, the majority of manufacturers still publish single-number ratios without disclosing their testing methodology, and cross-brand comparison remains unreliable. Not all "30% thinning shears" actually remove 30%. Independent testing by Japanese sharpeners has shown that real-world removal rates can deviate 5-10 percentage points from the manufacturer's stated ratio. For this reason, experienced stylists often test a new thinning shear on a mannequin before using it on clients, to calibrate their expectations against the tool's actual performance.