Importance Two components of social norms—descriptive (estimated prevalence) and injunctive (perceived acceptability)—can influence youth tobacco use.

Objective To investigate electronic cigarettes (e-cigarette) and cigarette descriptive norms and measure the associations between overestimation of e-cigarette and cigarette prevalence and tobacco-related attitudes and behaviours.

Design Cross-sectional.

Setting School-based, using paper-and-pencil questionnaires.

Participants US 6th-12th graders participating in the 2015 (n=17 711) and 2016 (n=20 675) National Youth Tobacco Survey.

Exposure Students estimated the percent of their grade-mates who they thought used e-cigarettes and cigarettes; the discordance between perceived versus grade-specific actual prevalence was used to categorise students as overestimating (1) neither product, (2) e-cigarettes only, (3) cigarettes only or (4) both products.

Outcomes Product-specific outcomes were curiosity and susceptibility (never users), as well as ever and current use (all students). Descriptive and multivariable logistic regression analyses were performed. Statistical significance was at P<0.05. Data were weighted to be nationally representative.

Results More students overestimated cigarette (74.0%) than e-cigarette prevalence (61.0%; P<0.05). However, the associations between e-cigarette-only overestimation and e-cigarette curiosity (adjusted OR (AOR)=3.29), susceptibility (AOR=2.59), ever use (AOR=5.86) and current use (AOR=8.15) were each significantly larger than the corresponding associations between cigarette-only overestimation and cigarette curiosity (AOR=1.50), susceptibility (AOR=1.54), ever use (AOR=2.04) and current use (AOR=2.52). Despite significant declines in actual e-cigarette use prevalence within each high school grade level during 2015–2016, perceived prevalence increased (11th and 12th grades) or remained unchanged (9th and 10th grades).

Conclusions Four of five US students overestimated peer e-cigarette or cigarette use. Counter-tobacco mass media messages can help denormalise tobacco use.