What size bike helmet do I need?

Measure your head circumference about an inch above your eyebrows and match it to each brand's size chart — sizes are not interchangeable across brands. Reviewers across r/bikecommuting and r/cycling repeatedly emphasize that fit shape matters as much as circumference: Giro tends to fit rounder heads, Smith and Specialized tend to fit longer/oval heads, so trying before buying (or buying from a retailer with free returns) is the consensus advice.