The way I view it now after a ton of research is the norwood level depends on volume of hair loss.
Norwood 0 is a real thing. It's this
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Most people aren't even born with such a hairline.
Norwood 1 is average hairline as a child that can extend into teens and young adults.
Norwood 2 has a variety of forms. It can be a mature hairline or mild recession.
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The big picture is no one will say you are balding.
Norwood 3 is the first sign of balding due to sheer volume of recession and/or thinning at the vertex. Tom hanks is a sample most would call a norwood 2.5 mainly because it isn't far back. Using the ears can be a decent rule of thumb.. HOWEVER, head shape can be deceptive when doing this.
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As you can see even after the results his hairline does still dip toward the ears. The important thing is the recession on the interior part of the temples.
Norwood 4 is severe hairline recession mixed with clear loss at vertex/crown.
Normally both temple recession merge like here
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This is usually paired with crown balding.
Norwood 5 is very sparse hair in front and large balding in the back and a bridge of hair at midscalp.
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Norwood 6 is sparse hair on scalp, hair on sides is about 3 inches high. A lot of men stop here for awhile.
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Prince Williams is a prime example and for the life of me I have no clue why he isn't getting a hair transplant. Norwood 6 can be salvaged with a really good surgeon that I'm sure the royal family could afford. Also note the height of the hair on the sides.
But some men as they get older the donor area starts to die and you fall to
Norwood 7. Balding dips into the donor region and sides of your head are 2 inches high.
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Notice how there is another inch down the sides. Most balding men ultimately end up in stage 6 or 7 before stabilizing. Another subgroup is the A class, which is for anterior. These balding patterns are horrible in the early stages but ironically they are the best long term. A norwood 5A can get full recovery with the right surgeon.
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All of this is going to vary based on head shape, progression, etc. But the key concept is volume of loss.
To some, I am a norwood 2, to others I'm a norwood 3. I choose 2.5 to give everyone an idea of the level.
I'd say once you hit 2.5 you notice the hair loss happening and other people don't notice it until you hit 3 or 3.5 if you are lucky. Unfortunately once you hit that point a handful of women will turn you down.