What percentage of vitamin C should a serum have?

Most pure L-ascorbic acid formulas land between 10% and 20%, which is the range many dermatology-brand serums target for a balance of potency and tolerability. Higher concentrations like 23% (as in The Ordinary's suspension) can deliver more but are more prone to irritation and grittiness. Gentler derivatives like ascorbyl glucoside or THD ascorbate use lower effective concentrations and suit sensitive skin better.