US: 38-40 to United Kingdom Size 34-36
Cross-market men’s sleepwear conversion mapping 38-40 specifications into 34-36 for lounge pants, pajama sets, and robes.

Analyzing Sleepwear Size Equivalence Across Markets
US: 38-40: Corresponds to a relaxed chest range of 98–102 (38.6–40.2 ), elastic waist range of 84–90 (33.1–35.4 ), and fit classification Relaxed / Comfort Fit.
United Kingdom Size 34-36: Corresponds to a relaxed chest range of 90–94 (35.4–37 ), elastic waist range of 76–82 (29.9–32.3 ), and fit classification Relaxed / Comfort Fit.