Helmet weight varies by discipline: road helmets typically run 200–280g, aero helmets 250–350g, and MTB helmets 280–380g (full-face helmets are much heavier at 600–1000g). If you're a road rider, anything under 250g is considered lightweight.
Here's the thing—weight differences under 30g are virtually imperceptible while riding. That's less than the weight of a small energy gel. It's tempting to chase the lightest helmet available, but sacrificing coverage, ventilation, or safety features to save 20g simply isn't worth it. A well-fitting, protective helmet that you forget is on your head beats an ultralight one that compromises elsewhere.
Focus on fit and protection first, then consider weight as a tiebreaker among similar options. If you're comparing MTB helmets, remember to account for visor weight, which can add 20–40g depending on the design.