Phototrichogram: How Hair Growth Is Actually Measured
A phototrichogram is a method of measuring hair by photographing a small marked area of scalp twice, a couple of days apart, and comparing the two images. Hairs that have visibly lengthened between the two photographs are in the growing phase. Hairs that have not moved are resting. From that single comparison the method gives a hair count, a growth rate and the proportion of growing to resting hairs, which is far more information than the eye can collect.
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Key takeaways
- A phototrichogram photographs the same clipped patch of scalp twice, typically two to three days apart, and measures what changed.
- Because scalp hair grows roughly 1 cm a month, a two-day gap is enough for growing hairs to lengthen measurably and resting hairs not to.
- It reports hair density, growth rate and the growing-to-resting ratio, which is usually around 85 to 90 percent growing in an unaffected scalp.
- Trichoscopy looks at the scalp once, under magnification. A phototrichogram measures change over time. They answer different questions.
- It is a measurement tool used in clinics and research, not something Watermans offers. We sell cosmetic hair care only.
What is a phototrichogram?
A phototrichogram is a standardised way of counting and measuring hairs in a defined area of scalp using close-up photographs taken at two points in time. The technique was developed to put numbers on something notoriously hard to judge by eye, and it remains one of the few methods that can separate a change in how many hairs are growing from a change in how fast they are growing.
The principle rests on one piece of biology. Hair in the anagen (growing) phase extends at roughly 1 cm a month, around 0.3 to 0.4 mm a day. Hair in the telogen (resting) phase does not extend at all. Photograph the same clipped hairs 48 to 72 hours apart and the ones that have got longer are growing, while the ones that have not are resting. Everything else the method reports follows from that.
How does a phototrichogram work, step by step?
The procedure is short, painless and follows the same five stages wherever it is carried out. Its accuracy depends almost entirely on repeating the conditions exactly the second time, which is why the marked area matters more than the camera.
- A small area is selected and marked. Usually about 1 to 2 cm across, often at the vertex or the frontal scalp, and recorded so it can be found again precisely.
- The hair in that area is clipped very short. Clipping to roughly 1 mm gives every hair the same starting length, so any later difference is growth rather than history.
- A first magnified photograph is taken. This is the baseline image. Lighting, angle and distance are standardised.
- A second photograph is taken 48 to 72 hours later. Same spot, same settings. Some protocols dye the area first so fine, pale hairs show up against the skin.
- The two images are compared. Hairs are counted, lengths measured, and each hair classified as growing or resting according to whether it extended.
Why the hair has to be clipped
Clipping is the part people object to, and it is not optional. Without a common starting length there is no way to tell a hair that grew 0.7 mm from one that was simply longer to begin with. The clipped patch is small and grows back at the same rate as everything around it, so it is usually invisible within a few weeks.
What does a phototrichogram actually measure?
Four numbers come out of the comparison, and each answers a different question about what is happening on the scalp. Taken together they distinguish a scalp that is growing fewer hairs from one that is growing the same number more slowly, or more finely, which is a distinction no mirror will ever give you.
| Measurement | What it tells you | Typical unaffected range |
|---|---|---|
| Hair density | How many hairs occupy the measured area | Commonly reported around 200 hairs per cm² on the scalp, with wide variation by person and site |
| Growth rate | How far hairs extended between the two photographs | Roughly 0.3 to 0.4 mm per day, about 1 cm per month |
| Anagen to telogen ratio | What share of hairs are growing versus resting | Around 85 to 90 percent growing in an unaffected scalp |
| Hair shaft diameter | Whether strands are getting finer over time | Varies widely by hair type, so it is the trend that matters, not one figure |
The growing-to-resting ratio is the number most often quoted, because a shift towards resting is the signature of a shedding episode. A scalp where the resting share has climbed well above the usual band is behaving the way telogen effluvium behaves. A scalp where the count is steady but diameters are falling is the pattern of gradual thinning instead.
What is the difference between a phototrichogram and trichoscopy?
Trichoscopy examines the scalp once under magnification and looks at what is there right now: hair thickness variation, the state of the follicular openings, scaling, redness and the pattern of empty spaces. The timed method photographs the same spot twice and measures what changed in between. One captures a still, the other captures motion.
| Phototrichogram | Trichoscopy | |
|---|---|---|
| What it does | Measures change between two timed images | Examines scalp features in a single session |
| Needs clipping | Yes, to a common starting length | No |
| Visits needed | Two, 48 to 72 hours apart | One |
| Best at | Growth rate and the growing-to-resting ratio | Scalp condition and pattern recognition |
| Common use | Research and tracking change over months | Assessment in clinic |
Neither replaces the other, and neither is something you need before making sensible changes to a hair routine. If you simply want to know whether your hair is thinning, the six home checks you can do in a mirror will get you most of the way for nothing.
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Is a phototrichogram measuring hair loss, hair shedding or hair breakage?
A phototrichogram measures hair loss and hair shedding, and is largely blind to hair breakage. Hair loss shows up as falling density and shrinking shaft diameter, because fewer and finer hairs occupy the area. Hair shedding shows up as a rising share of resting hairs, because more follicles have released their hair and paused.
Hair breakage is different in kind. The strand snaps somewhere along its length while the follicle carries on producing normally, so density and the growing-to-resting ratio can both look entirely healthy while your hair feels thinner and refuses to gain length. Since the method measures hairs at the scalp rather than at the ends, breakage happening 15 cm away is simply outside its field of view. Checking the end of a lost hair for a pale bulb tells you more about breakage in two seconds than any imaging will.
Who uses a phototrichogram, and where would you have one in the UK?
Research teams use it most, because it produces objective numbers that can be compared across groups and across months. Beyond research, some trichology practices and private hair clinics offer it as part of an assessment. It is not a routine NHS investigation, and a GP looking into hair loss will usually reach for a history and blood tests instead.
The Institute of Trichologists maintains a register of qualified UK practitioners, which is the sensible starting point if you want measurement of this kind. Watermans does not run clinics, does not offer consultations or assessments, and has no involvement in any of this. We are a cosmetic hair care company, and measurement sits entirely outside what we do.
How should you read phototrichogram results?
Read the direction of travel rather than any single figure, because the normal range for every one of these numbers is wide. One person can sit at 180 hairs per cm² with entirely healthy hair while another sits at 250, so your own previous measurement is a far better comparison than a population average. The comparison only holds if the same site was used both times.
- Resting share well above the usual band. Consistent with an active shedding episode, which typically follows a trigger by two to three months.
- Density steady, diameters falling. The signature of gradual thinning rather than a sudden shed.
- Density falling in one region only. Suggests a patterned rather than a diffuse process, which is why the site is always recorded.
- Everything stable across six months. Genuinely reassuring, and the most common result for people who were worried by a bad month.
One measurement means very little
A measurement taken once gives you a number with nothing to compare it against. Its value is entirely in repetition, and hair cycles slowly enough that useful intervals are three to six months, not weeks. Anywhere selling a single scan as an answer is selling the scan, not the information.
What are the limits of a phototrichogram?
Four limitations matter in practice, and all of them come from the same source: the method is exquisitely sensitive to whether conditions were repeated exactly. Change the site, the lighting, the magnification or the clipping length and the second set of numbers is not comparable with the first.
- It samples a tiny area. One or two square centimetres stands in for a whole scalp, so site selection drives the result.
- It requires clipping. Some people will not accept a shaved patch, particularly in a visible area.
- Manual counting varies. Automated systems reduce this but introduce their own errors on pale or very fine hair, which is why dyeing is sometimes used.
- It says nothing about cause. Numbers describe what is happening, never why. That question still needs a GP.
How can you track hair growth at home without a phototrichogram?
Three free methods capture most of the useful signal, and consistency beats sophistication every time. The single biggest improvement anyone can make to home tracking is fixing the conditions, because almost all apparent month-to-month change is really a change in lighting or hair position.
- The same photograph, monthly. Same room, same daylight, same parting, hair dry and unstyled, phone at the same distance. Put a reminder in your calendar.
- The parting width check. Part your hair in the usual place and photograph straight down. A widening gap over months is more reliable than any feeling about volume.
- The count in a set period. Collect shed hairs for the same 24 hours on the same day each week. Around 50 to 100 a day is normal, and your own trend is what counts.
For a fuller method, including how to avoid the most common tracking mistakes, see the guide to measuring hair growth properly. If you want to understand what your numbers are describing, how long the growing phase lasts and what shortens it covers the biology behind them.
When should you see a GP rather than pay for measurement?
See a GP first if hair is coming out in clumps, if patches have appeared suddenly, if the scalp is sore, scaly or inflamed, or if hair changes arrive with tiredness, heavy periods or weight change. A GP can check iron and thyroid levels, which no photograph can, and that is the information that actually changes what you do next. If your question is about products or a routine rather than a medical one, you can talk to Gail Waterman, our co-founder, as an AI avatar on the Watermans homepage.
Measurement is worth paying for when you already know what you are dealing with and want to follow it objectively over months. It is a poor first step when the question is still what is going on, because a number without a cause leaves you exactly where you started, only poorer.
Which hair care products are worth tracking this way?
Cosmetic products change how hair looks and how much of it survives everyday handling, and those are the things worth photographing over months. Watermans products are cosmetic and are not medicines, so nothing here acts on the growing-to-resting ratio a phototrichogram reports. What a monthly photo can honestly show is whether your parting looks fuller and whether you are keeping more length. To track whether any of this is changing over time, our guide to hair density measurement sets out a home photo method that is actually repeatable.
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A sulfate free daily shampoo with biotin, caffeine, rosemary, niacinamide and hydrolysed lupine protein. The measurable change to look for is fewer short broken pieces and a parting that photographs fuller, not a different hair count.
Grow More® Leave-In Scalp Elixir £25.50
An overnight leave-on scalp elixir for lower density areas at the parting and crown. Apply to a dry scalp at night. Give any cosmetic routine three months before judging it, because hair cycles far more slowly than expectations do. For the measurements you can reasonably take at home instead, see our guide to HRT hair biomarkers.
What are the most frequently asked questions about the phototrichogram?
Does a phototrichogram hurt?
No. The procedure involves clipping a small patch of hair short and photographing it twice, and nothing penetrates the skin. Some protocols apply a temporary dye so pale hairs show up against the scalp, which can leave a faint mark for a day or two.
How long does the procedure take?
Each session takes roughly 15 to 30 minutes, and there are two of them 48 to 72 hours apart. The measurement itself is the comparison between the two images, so a single visit cannot produce a result.
Will I have a visible bald patch afterwards?
A small clipped area, usually 1 to 2 cm across, stays short for a few weeks. Scalp hair grows roughly 1 cm a month, and the site is normally chosen where surrounding hair covers it. Ask where the site will be before agreeing.
Is a phototrichogram available on the NHS?
It is not a routine NHS investigation. A GP looking into hair loss will usually take a history, examine the scalp and arrange blood tests such as iron and thyroid function, because those change what happens next in a way a photograph does not.
How often should the measurement be repeated?
Every three to six months at the earliest. Hair cycles slowly, the growing phase can run for years, and repeating at shorter intervals mostly measures noise. Anything promising meaningful change in four weeks is overselling.
Can the measurement tell me why my hair is thinning?
No. It describes what is happening, never the cause. Falling density and finer shafts look much the same whether the driver is hormonal, nutritional or something else, which is why blood tests and a proper history still matter more.
Is trichoscopy or a phototrichogram better?
Neither is better, they answer different questions. Trichoscopy is a single magnified look at scalp features and needs no clipping. The timed method measures change between two timed images. Assessment often uses trichoscopy, and tracking over time uses the other.
Can I do this measurement at home?
Not properly. The method depends on standardised magnification, lighting, site marking and clipping length, and home photographs cannot hold those constant. A fixed monthly photograph of the same parting in the same light is the realistic home alternative.
What is the bottom line on the phototrichogram?
A phototrichogram turns two timed photographs into hair density, growth rate and the growing-to-resting ratio, and it is the most objective way to follow a scalp over months. Its strength is comparison, so a single reading is close to useless while a series taken at the same site tells a real story.
For most people it is not the first step. Work out whether you are dealing with loss, shedding or breakage, see a GP if anything about the pattern is sudden, patchy or painful, and start a monthly photograph of the same parting in the same light. That costs nothing and answers the question most people are actually asking.
While you are tracking, the cosmetic side is worth getting right so that damage is not confusing your own results. A hair growth shampoo that reduces breakage means the hair you photograph next month is hair you kept, rather than hair that snapped between the two pictures.
Sources & references
- StatPearls, Physiology: Hair
- StatPearls, Anatomy: Hair Follicle
- DermNet, Trichoscopy
- DermNet, Hair loss (alopecia) overview
- DermNet, Telogen effluvium
- DermNet, Female pattern hair loss
- NHS, Hair loss
- The Institute of Trichologists
Watermans products are 100% cosmetic and do not treat medical hair loss. Individual results may vary.
