The Borg Rating of Perceived Exertion scale ranges from 6 to 20.
Developed by Swedish psychologist Gunnar Borg in the 1960s, the original scale measures how hard physical activity feels to the person performing it. A rating of 6 represents no exertion at all, while 20 represents maximal exertion. Intermediate labels run from extremely light and very light through moderate, hard, and very hard effort.
The unusual starting point was intentional: Borg designed the scale to roughly correspond to heart rate in a healthy young adult when the rating is multiplied by 10. Thus, a rating of 12 loosely corresponds to a heart rate near 120 beats per minute, though the relationship varies by age, fitness, medication, and health.
People often confuse the original 6–20 scale with Borg’s later CR10 scale, which uses a roughly 0–10 range and is frequently used for localized discomfort or breathlessness. The question refers specifically to the original Borg RPE scale, not those newer variants or modern fitness apps that use a 1–10 rating.