The standard: 15–20% of the price of the cut. Same range most people use for a restaurant server or a hairdresser — barbering is a service industry, and tipping is the norm, not the exception.
On a $50 haircut, that works out to $7.50–$10. Most people round up rather than do the math exactly — a flat $10 on a $50 cut is a completely normal, appreciated tip.
Cash is preferred, even if you pay the base price by card or app. A cash tip goes directly to the barber, same day, without waiting on a payout or losing a cut to processing fees. If you don't have cash on hand, saying so and asking about a digital tip option is fine too — most barbers have one.
When to tip more: your first visit with a new barber, a more involved cut (intricate fades, design work, longer sessions), a holiday, or if someone worked you in on short notice. Twenty-five percent or more is common in any of those cases.
When it's okay to tip less or skip it: if the barber owns the shop and sets their own prices, some people tip less or not at all, on the logic that the owner already keeps the full price. In practice, most regulars still tip owner-operators something — it's optional there in a way it isn't for an employee, not a hard rule against it.
For an at-home visit, none of this changes — it works the same as tipping at a shop, just handed over in person once the cut is done. Since payment already happens on the spot, cash tips are simple: no jar to find, no line to wait in.