Entry of the Day
Hair Loss
Male Pattern Baldness
Why male pattern baldness happens, the age it usually starts, how to tell it from a mature hairline or shedding, and what medicines and a transplant can and cannot do.
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Hair Library · The Hair Encyclopedia
Everything About Hair. Backed by Science.
Hair science in plain English: reference entries on hair biology, hair loss, scalp health, treatments, hair care and hair transplantation. Written from published research, linked to PubMed and medically reviewed.
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Today in the Hair Library
One entry, five quick facts and one picture. New every day.
Did You Know?
- The shaft is made in the bulb, pigment is added in the bulb, and when a follicle rests, the bulb is the part that shrinks.
- The Norwood scale sorts male pattern hair loss into seven stages.
- A scalp yeast infection usually means an overgrowth of, or reaction to, Malassezia, a yeast that lives on every scalp.
- Rosemary oil has not been shown to stop hair loss.
- A graft is a unit of skin with one to four hairs, not a single hair, so a number on its own can mislead.
From entries in the library
Just Added
Rice Water for HairPublished 31 Oct 2026 · Hair Care & Ingredients
PRP for Hair LossPublished 31 Oct 2026 · Hair Loss Treatments- Male Pattern BaldnessPublished 31 Oct 2026 · Hair Loss
How Often to Wash HairPublished 31 Oct 2026 · Hair Care & Ingredients
Hair Color GeneticsPublished 31 Oct 2026 · Hair Biology
Picture of the Day
From the entry: Crown Balding
Hair Science at a Glance
The basics every entry builds on, in one place.
Hair is a thread of keratin made by a living follicle in the skin. The shaft you see is no longer alive, so it cannot repair itself; growth, thickness and colour are decided in the follicle below. Each follicle runs its own cycle of growth, rest and shedding, so you lose some hairs every day but never all at once.
Every figure on this page comes from an entry in the library, where you will find the studies behind it. For the words themselves, see the hair glossary.
Hair Facts in Numbers
| Fact | The short answer | Full entry |
|---|---|---|
| What hair is made of | Mostly keratin, a tough structural protein, in three layers: the cuticle outside, the cortex in the middle and, in thicker hairs, a central medulla | |
| Hairs on an adult scalp | Roughly 80,000 to 120,000 terminal hairs, the thick pigmented hairs you can see and count | |
| How fast hair grows | About 0.3 to 0.4 mm a day: roughly 1 cm a month and about 12 cm a year | |
| The growth cycle | Anagen (growth) 2 to 6 years, catagen 1 to 2 weeks, telogen (rest) 2 to 4 months; 85% to 90% of scalp hairs are growing at any one time | |
| Normal daily shedding | 50 to 100 hairs a day, according to the American Academy of Dermatology | |
| Where colour comes from | Two melanin pigments made in the follicle: dark eumelanin and reddish pheomelanin | |
| Why hair turns gray | With age, the pigment cells in each follicle (melanocytes) make less melanin; genes set most of the timing | |
| Fine hair and thick hair | Soft vellus hair and thick terminal hair can grow from the same follicle, which can switch in both directions | |
| Hair types | Twelve patterns from 1A to 4C: straight, wavy, curly and coily, each split into A, B and C | |
| After a hair transplant | Transplanted hairs usually shed in the first weeks while the follicles stay alive under the skin; new growth begins around months 3 to 4 |
Start Here
Three reading paths through the encyclopedia. Start with the one that fits you.
Understanding your hair loss
For patients: the essentials, in plain words.
Clinical reference
For professionals: biology, evidence and complications.
Caring for your hair
For everyone: hair types, products and what the evidence says.
Browse by Category
From the biology of a single follicle to hair care, treatments and hair transplantation.
Hair Biology
9 entriesHow hair is built, how it grows and why it changes colour.
View all 9 entries →Hair Types & Texture
7 entriesFrom 1A straight to 4C coily, porosity and curl patterns.
View all 7 entries →Hair Loss
20 entriesPatterns, causes and stages of hair loss, including hair loss linked to hormones and medicines.
View all 20 entries →Scalp Conditions
6 entriesPain, bumps, pimples and infections of the scalp, and when to see a doctor.
View all 6 entries →Hair Loss Treatments
12 entriesWhat the research says about medicines and therapies for hair loss.
View all 12 entries →Hair Care & Ingredients
12 entriesOils, shampoos, serums, supplements and the ingredients inside them.
View all 12 entries →Hair Transplantation
29 entriesEvery technique, how grafts are planned, recovery and safety.
Techniques & Procedure 12
Grafts & Planning 6
Recovery & Aftercare 7
Complications & Safety 2
History & Records 1
A category opens once it has at least three entries. Next: Hair Genetics · Hair Technology · Hair Regeneration.
Questions People Ask About Hair
Each question has a full, referenced answer in the library.
How fast does hair grow?Hair BiologyHow many hairs are on the human head?Hair BiologyWhat causes gray hair?Hair BiologyDo eyebrows grow back?Hair BiologyIs a mature hairline normal or receding?Hair LossCan birth control cause hair loss?Hair LossDoes red light therapy work for hair loss?Hair Loss TreatmentsDoes hair growth serum work?Hair Care & IngredientsHow many grafts do I need?Hair TransplantationCan a hair transplant change hair texture?Hair TransplantationCan I go to work after a hair transplant?Hair TransplantationHow long do hair transplants last?Hair Transplantation
Glossary: Hair Terms Explained
Every term answered in a few sentences, with a link to the full entry when there is more to know.
What is the dermal papilla?
The dermal papilla is a cluster of cells at the centre of the hair bulb. It signals the bulb and helps decide how thick, how long and what colour each hair grows.
Full entry: Hair Bulb Anatomy →What is a graft?
A graft is a follicular unit taken from the donor area and placed in the thinning area. It holds one to four hairs, rarely more, so 2,000 grafts is roughly 3,600 to 4,600 hairs.
Full entry: How Many Grafts Do I Need? →What is shock loss?
Shock loss is a temporary shedding phase after a hair transplant, triggered by surgical stress to the scalp and typically starting 2 to 4 weeks after the procedure. The follicles remain healthy beneath the skin, and new growth resumes within 4 to 6 months.
Full entry: Shock Loss →What is the Norwood scale?
The Norwood scale is the classification most doctors use for male pattern hair loss. It describes the usual ways the loss moves, stage by stage, from Norwood 1 to Norwood 7.
Full entry: Norwood Scale: Stages 1–7 →Interactive Tools
Find your stage, your hair type and a typical graft range.
Research Digest
New studies, summarized in plain language, with a link to the original paper. Each summary is medically reviewed.
Complications in follicular unit excision hair transplantation: current evidence and practical approaches
Lists complete follicular unit extraction and avoiding transection, cutting through the follicle, as key preventive measures in FUE.
Romera de Blas C, Vega Díez D, Ricart Vayá JM, Gómez Zubiaur A · Cited in: Hair Bulb Anatomy
Dermal Papilla Cells: From Basic Research to Translational Applications
Describes the main parts of the hair bulb and states that the dermal papilla controls the growth, shape, size and colour of hair.
Zhang HL, Qiu XX, Liao XH · Cited in: Hair Bulb Anatomy
Frequently Asked Questions About Hair
Short answers to the questions people search most, each linked to the full entry.
What is the science of hair called?
Hair science is called trichology: the study of the hair and scalp, from how a hair is built to why it falls out. Our entries are written by Nazmi Güldeş, a certified trichologist, and most are medically reviewed by Dr. Tompi, a plastic surgeon. Each entry shows its byline and review date.
What is human hair made of?
Human hair is made mostly of keratin, a tough structural protein. The shaft has three layers: the cuticle of overlapping scales outside, the cortex in the middle, which gives strength and holds the colour, and in thicker hairs a central medulla. Our hair shaft anatomy entry explains each layer.
What are the stages of the hair growth cycle?
Anagen, catagen, telogen and exogen. On the scalp, anagen, the growth phase, lasts 2 to 6 years, catagen 1 to 2 weeks and telogen, the rest phase, 2 to 4 months; then the old hair is released and a new one starts in the same follicle. Our hair growth cycle entry covers each phase.
What are some interesting facts about human hair?
A healthy adult scalp carries roughly 80,000 to 120,000 terminal hairs, and each grows about 1 cm a month. Shedding 50 to 100 hairs a day is normal, according to the American Academy of Dermatology. The shaft you see is not alive, which is why split ends cannot repair themselves.
How fast does hair grow?
Scalp hair grows about 0.3 to 0.4 mm a day, which adds up to roughly 1 cm a month and about 12 cm a year. The speed varies with age, ancestry and hair colour, and a follicle only adds length during its growth phase, as our hair growth guide explains.
What are the different hair types?
The common chart has twelve hair types: four families, 1 straight, 2 wavy, 3 curly and 4 coily, each split into A, B and C by how tight the pattern is. Type 4 hair, especially 4B and 4C, is the most fragile. Our hair type chart shows all twelve.
What is proven to regrow hair?
For pattern hair loss, the medicines approved by the US FDA are topical minoxidil and, for men, oral finasteride 1 mg. Topical minoxidil is the only FDA-approved option for women, and its gain lasts only while you use it. A hair transplant works differently: it moves follicles that resist DHT from the back and sides of the head.
SourcesNumbered in every entry, linked to PubMed, with study type and published corrections shown.
Medical reviewEach entry is reviewed and dated.
Revision historyEvery change is recorded.
Sources & copyrightHow we use sources · Corrections · Editorial policy
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