This study examined satisfaction with ready-to-wear (RTW) clothing, item-and body-part–specific fit, and alteration behaviors among women in their twenties, and tested differences by body mass index (BMI) to inform pattern and product improvements. A questionnaire survey was conducted with 140 women aged 20–29 years. Descriptive statistics and one-way ANOVA with Duncan’s multiple range test were used. The most common monthly clothing expenditure was under KRW 100,000, and online shopping malls were the dominant purchase channel, reflecting the sample’s student-heavy profile and budget constraints. Pants and T-shirts were purchased most frequently. In body-shape perception, forward head/neck posture and rounded back/shoulders were commonly reported, and significant BMI-group differences appeared in perceived torso width, hip-to-waist proportion, thigh thickness, bust size, waist definition, buttock protrusion, slimness, and calf thickness. Overall RTW satisfaction was relatively high for design variety, color, and ease of garment care, whereas price fairness received the most negative evaluations; satisfaction with color and overall fit decreased as BMI increased. Fit assessments showed that jackets were generally acceptable but needed improvement in across-front width, sleeve-related dimensions, and garment length. Blouses and shirts showed broader fit issues, with significant BMI effects for neck circumference, across-front width, sleeve width, and wrist circumference; fit was poorer in the overweight-or-higher group. For bottoms, pants were rated as moderate for abdomen and hip circumferences, crotch length, leg length, and leg width; the underweight group reported lower fit for waist and abdomen circumferences and pants length. Skirts were mostly rated as moderate, with a difference in BMI only for waist circumference. Most participants reported no alterations; however, the underweight group most often took in the pants waist and length, suggesting a mismatch for slimmer consumers. These findings highlight the need to recalibrate ease allowances and grading rules for BMI-sensitive body parts, provide refined waist and length options for bottoms, and incorporate adjustability in sleeves and closures to reduce alterations and improve RTW satisfaction among young women.