What is the 3-finger rule for sunscreen?
The 3-finger rule is a simple way to measure the right amount of sunscreen for the face and neck. Squeeze sunscreen along the length of your index, middle, and ring fingers. That quantity, roughly equivalent to a quarter teaspoon or 1 to 1.5 ml, is what most sunscreen studies use as the standard application amount for the face.
SPF ratings are tested in labs using 2 mg of sunscreen per square centimeter of skin. In practice, most people apply far less than this, which means real-world protection is often lower than the SPF number suggests. The 3-finger rule is a practical shorthand to get closer to that tested amount without needing a measuring tool. Apply it evenly across the forehead, cheeks, nose, chin, and neck. Do not forget the ears and the back of the neck if they are exposed.