Aims: To test whether reducing the standard serving size of alcoholic beverages would reduce voluntary alcohol consumption in a laboratory (study 1) and a real‐world drinking environment (study 2). Additionally, we modelled the potential public health benefit of reducing the standard serving size of on‐trade alcoholic beverages in the United Kingdom.

Setting: A semi‐naturalistic laboratory (study 1), a bar in Liverpool, UK (study 2). Participants were students and university staff members (study 1: n = 114, mean age = 24.8 years, 74.6% female), residents from local community (study 2: n = 164, mean age = 34.9 years, 57.3% female).

Interventions and comparators: In study 1, participants were assigned randomly to receive standard or reduced serving sizes (by 25%) of alcohol during a laboratory drinking session. In study 2, customers at a bar were served alcohol in either standard or reduced serving sizes (by 28.6–33.3%). Outcome measures were units of alcohol consumed within 1 hour (study 1) and up to 3 hours (study 2). Serving size condition was the primary predictor.

Findings: In study 1, a 25% reduction in alcohol serving size led to a 20.7–22.3% reduction in alcohol consumption. In study 2, a 28.6–33.3% reduction in alcohol serving size led to a 32.4–39.6% reduction in alcohol consumption. Modelling results indicated that decreasing the serving size of on‐trade alcoholic beverages by 25% could reduce the number of alcohol‐related hospital admissions and deaths per year in the United Kingdom by 4.4–10.5% and 5.6–13.2%, respectively.

Conclusions: Reducing the serving size of alcoholic beverages in the United Kingdom appears to lead to a reduction in alcohol consumption within a single drinking occasion.