Hamilton-Norwood scale
The Hamilton-Norwood scale is the most widely used way to describe the stage of male pattern hair loss, running from Type I (little or no recession of the hairline) to Type VII (only a horseshoe of hair left at the sides and back).
Full definition
The scale began with James Hamilton, who published a classification of male patterned hair loss in 1951. In 1975 Dr O'Tar Norwood, a dermatologist and hair transplant surgeon, revised it after studying hair loss in 1,000 men, and the combined Hamilton-Norwood scale is now the most widely used classification for men (Gupta and Mysore, 2016). Norwood observed that thinning usually starts at the temples and at the crown, then slowly spreads across the top of the scalp. At Type I there is minimal or no recession. At Type II there are triangular, usually symmetrical areas of recession at the temples. Type III is the earliest stage Norwood counted as baldness, with deep recession at the temples. By Type VII only a narrow horseshoe-shaped band of hair remains on the sides and back. Norwood also described a "Type A" variant, in which the front hairline moves steadily backwards without leaving an island of hair at the front and without a separate bald patch on the crown. Clinicians use the scale to describe and track hair loss over time, for example when you are checking whether treatment is working or planning treatment around a hair transplant. It describes men; female pattern hair loss uses the Ludwig and Sinclair scales. If you are noticing a receding hairline or a thinning crown, the stage matters less than an accurate diagnosis, which is what a doctor's assessment is for.
What it does not mean
A Norwood stage is a description, not a forecast. It does not tell you how fast your hair loss will progress, whether a treatment will help you, or how much it affects you. It also does not measure overall thinning or density, and the review authors note that every classification system has limitations. A stage number is not a diagnosis: other conditions can thin the hairline or crown, so the cause still needs a clinical assessment.
UK regulatory source
No UK regulator defines this scale; it is a clinical classification. Norwood OT. Male pattern baldness: classification and incidence. South Med J 1975 (PMID 1188424); Hamilton JB. Patterned loss of hair in man: types and incidence. Ann N Y Acad Sci 1951 (PMID 14819896); Gupta M, Mysore V. Classifications of patterned hair loss: a review. J Cutan Aesthet Surg 2016