Abstract Background BMI underestimates and overestimates body fat in children from South Asian and Black ethnic groups, respectively. Methods We used cross-sectional NCMP data (2015–17) for 38 270 children in three inner-London local authorities: City & Hackney, Newham and Tower Hamlets (41% South Asian, 18.8% Black): 20 439 4–5 year-olds (48.9% girls) and 17 831 10–11 year-olds (49.1% girls). We estimated the proportion of parents who would have received different information about their child’s weight status, and the area-level prevalence of obesity—defined as ≥98th centile—had ethnic-specific BMI adjustments been employed in the English National Child Measurement Programme (NCMP). Results Had ethnic-specific adjustment been employed, 19.7% (3112/15 830) of parents of children from South Asian backgrounds would have been informed that their child was in a heavier weight category, and 19.1% (1381/7217) of parents of children from Black backgrounds would have been informed that their child was in a lighter weight category. Ethnic-specific adjustment increased obesity prevalence from 7.9% (95% CI: 7.6, 8.3) to 9.1% (8.7, 9.5) amongst 4–5 year-olds and from 17.5% (16.9, 18.1) to 18.8% (18.2, 19.4) amongst 10–11 year-olds. Conclusions Ethnic-specific adjustment in the NCMP would ensure equitable categorization of weight status, provide correct information to parents and support local service provision for families.