A vellus-to-terminal hair ratio exceeding 20% is one of the most reliable diagnostic markers for androgenetic alopecia (AGA). Tracking this ratio over time gives you a quantitative severity score that clinical assessments alone often miss.
What Are Vellus and Terminal Hairs?
Every hair follicle on your scalp produces one of two types of hair at any given time. Terminal hairs are thick, pigmented, and deeply rooted. Vellus hairs are thin, short, often colorless, and sit closer to the skin surface.
In a healthy adult scalp, terminal hairs make up roughly 90-95% of all follicles. The remaining 5-10% are naturally vellus or transitional. When androgenetic alopecia begins, DHT (dihydrotestosterone) causes terminal follicles to shrink through a process called miniaturization, gradually converting them into vellus-producing follicles.
This conversion does not happen overnight. A single follicle may take 2-5 years to fully miniaturize. That slow progression is exactly why ratio tracking is so valuable: it catches the shift before you notice visible thinning in the mirror.
Why the Ratio Matters for AGA Diagnosis
Dermatologists and trichologists use the vellus-to-terminal ratio as a primary diagnostic tool during trichoscopy (scalp microscopy). Here is how the ratio maps to AGA severity:
| Vellus-to-Terminal Ratio | AGA Severity | Clinical Interpretation |
|---|---|---|
| Below 10% | Normal / No AGA | Healthy follicle distribution |
| 10-20% | Early / Subclinical | Miniaturization beginning, not yet visible |
| 20-30% | Mild AGA | Noticeable thinning on close inspection |
| 30-40% | Moderate AGA | Visible density loss, Norwood 3-4 range |
| 40-50% | Advanced AGA | Significant thinning, Norwood 4-5 range |
| Above 50% | Severe AGA | Major density loss, Norwood 6-7 range |
The 20% threshold is particularly important. Research published in the Journal of the American Academy of Dermatology established that a vellus-to-terminal ratio above 20% in the frontal or vertex scalp reliably distinguishes AGA from normal age-related hair changes.
How Miniaturization Progresses by Norwood Stage
The vellus-to-terminal ratio maps closely to the Norwood scale used for classifying male pattern hair loss. As miniaturization increases, more grafts would be needed if surgical restoration becomes a goal.
| Norwood Stage | Typical Vellus Ratio | Grafts Needed for Restoration |
|---|---|---|
| Stage 2 | 15-25% | 800-1,500 |
| Stage 3 | 25-35% | 1,500-2,200 |
| Stage 3V | 30-40% | 2,000-2,800 |
| Stage 4 | 35-45% | 2,500-3,500 |
| Stage 5 | 40-55% | 3,000-4,500 |
| Stage 6 | 50-65% | 4,000-6,000 |
| Stage 7 | 60-80% | 5,500-7,500 |
These numbers highlight why early detection through ratio tracking matters. At Norwood 2, when miniaturization is just beginning, treatment can halt or reverse the process. By Norwood 5-6, many follicles have permanently lost their ability to produce terminal hair.
Clinical Trichoscopy: The Gold Standard
Trichoscopy uses a dermatoscope at 10-70x magnification to examine individual follicles. A trained trichologist counts terminal and vellus hairs in a standardized area (usually 1 cm square) and calculates the ratio.
The advantages of clinical trichoscopy include:
- Direct visualization of hair shaft diameter
- Identification of follicular unit composition (single vs. multi-hair units)
- Detection of perifollicular signs like brown dots or yellow dots
- Precise measurement of hair shaft thickness in micrometers
The limitations are equally clear. Trichoscopy requires a clinic visit costing $100-300 per session. Most patients get assessed once, receive a diagnosis, and rarely return for follow-up ratio measurements. That creates a data gap between appointments that can span months or years.
AI-Based Ratio Estimation Between Clinic Visits
myhairline.ai provides a way to estimate hair caliber variation from regular photos taken at home. The AI analyzes density patterns across different scalp zones and tracks changes between sessions. While this does not replace the precision of clinical trichoscopy, it fills the gap between appointments.
Here is how the tracking process works:
- Baseline photo: Take your first photo under consistent lighting
- Zone mapping: The AI identifies frontal, temporal, mid-scalp, and vertex zones
- Density scoring: Each zone receives a density estimate based on visible hair distribution
- Change detection: Subsequent photos are compared against your baseline
- Trend reporting: The system flags zones where density is decreasing, suggesting ongoing miniaturization
The key advantage is frequency. A clinical trichoscopy every 6-12 months gives you 1-2 data points per year. Home-based AI tracking every 2-4 weeks gives you 13-26 data points in the same period.
Treatment Response Monitoring
Tracking the vellus-to-terminal ratio is most powerful when you are actively treating hair loss. Without objective data, you are left guessing whether your treatment is working.
Finasteride Response Tracking
Finasteride blocks 5-alpha reductase, reducing DHT levels by approximately 70%. In responders, this slows or reverses miniaturization.
- Expected timeline: 3-6 months for initial response
- Efficacy rate: 80-90% halt further loss, 65% experience regrowth
- Ratio improvement in responders: 8-15% decrease in vellus proportion over 12 months
- Side effect rate: 2-4% of users experience sexual side effects
Tracking tip: take photos every 2 weeks for the first 6 months. Some users experience a shedding phase at weeks 4-8 as miniaturized hairs are pushed out by new terminal growth. Without tracking data, this shedding can cause unnecessary panic.
Minoxidil Response Tracking
Minoxidil works differently from finasteride. It extends the growth (anagen) phase of the hair cycle and increases follicular blood flow.
- Expected timeline: 4-6 months for visible results
- Efficacy rate: 40-60% experience moderate regrowth
- Common initial response: Increased shedding at weeks 2-6 (a positive sign)
The vellus-to-terminal ratio responds to minoxidil more slowly than to finasteride. Minoxidil primarily thickens existing miniaturized hairs rather than blocking the cause of miniaturization. Track for at least 6 months before evaluating response.
PRP Therapy Tracking
Platelet-Rich Plasma (PRP) therapy delivers concentrated growth factors to the scalp.
- Cost: $500-$2,000 per session
- Sessions needed: 3-4 initial sessions, then every 3-6 months
- Reported density improvement: 30-40% in clinical studies
PRP is one of the treatments where tracking provides the most value. At $500-2,000 per session, knowing whether the treatment actually improves your ratio helps you make informed decisions about continuing.
How to Build a Vellus-to-Terminal Tracking Protocol
A structured tracking protocol gives you the cleanest data. Here is the approach that yields the most consistent results.
Photo Consistency Rules
- Same lighting position for every photo
- Same time of day (hair appearance changes with oiliness through the day)
- Same camera distance (use a phone mount or selfie stick for repeatability)
- Dry hair, unstyled, no products applied
- No hat or headwear for at least 30 minutes before the photo
Tracking Schedule
| Phase | Frequency | Duration |
|---|---|---|
| Baseline establishment | Weekly | First 4 weeks |
| Active treatment start | Every 2 weeks | Months 1-6 |
| Ongoing monitoring | Monthly | Month 6 onward |
| Post-treatment evaluation | Monthly | After stopping treatment |
What to Document Alongside Photos
Your ratio does not exist in isolation. These factors influence your results and should be logged with each tracking session:
- Current medications and dosages
- Treatment adherence (missed doses)
- Sleep quality and stress levels
- Diet changes or supplement additions
- Seasonal notes (hair shedding patterns shift seasonally)
Interpreting Your Ratio Trends
Not every density fluctuation means your treatment is failing or succeeding. Here is how to read your data:
Positive signals (treatment working):
- Gradual increase in density scores over 3+ months
- Initial shedding followed by density stabilization
- Improved density consistency across zones
Neutral signals (hold course):
- Stable readings with minor fluctuations (under 5% variance)
- Seasonal shedding patterns (fall shedding is normal)
- Less than 3 months of data collected
Warning signals (reassess treatment):
- Consistent density decrease over 3+ consecutive readings
- Accelerating decline in a specific zone
- No improvement after 9-12 months of treatment
The Role of Ethnicity in Baseline Ratios
Baseline follicular density varies significantly by ethnicity, which affects how the vellus-to-terminal ratio presents clinically.
| Ethnicity | Avg Follicular Units per cm2 | Baseline Density |
|---|---|---|
| Caucasian | 200 | High unit density, finer hair shafts |
| African | 150 | Lower unit density, thicker individual shafts |
| Asian | 170 | Moderate density, thickest individual shafts |
| Hispanic | 170 | Moderate density, variable shaft thickness |
| Middle Eastern | 180 | Moderate-high density, thick shafts |
These differences mean that a 20% vellus ratio looks different on different scalps. A Caucasian scalp with 200 follicular units per cm2 can lose more units before thinning becomes visible compared to an African scalp with 150 units per cm2.
When to Escalate to Clinical Trichoscopy
AI tracking is a monitoring tool, not a replacement for clinical diagnosis. You should schedule a trichoscopy appointment when:
- Your tracking shows consistent density decline over 3+ months despite treatment
- You are considering starting or changing medication
- Your AI density score drops more than 10% from baseline
- You are evaluating surgical options (transplant planning requires exact graft counts)
For surgical planning specifically, a dermatologist needs to assess donor area density. Safe extraction limits (roughly 45% of donor follicles) and graft survival rates (90-95% for FUE) require clinical measurement that photo-based tracking cannot provide.
Combining Ratio Data with Treatment Costs
Understanding your ratio trend helps you budget for treatment. Here is how common treatments compare on a cost-per-improvement basis:
| Treatment | Annual Cost | Expected Ratio Improvement | Cost per 1% Improvement |
|---|---|---|---|
| Finasteride (generic) | $60-120 | 8-15% in responders | $4-15 |
| Minoxidil (5% topical) | $100-200 | 5-10% in responders | $10-40 |
| PRP (3-4 sessions) | $1,500-8,000 | 10-15% | $100-800 |
| LLLT (home device) | $200-800 (one-time) | 3-8% | $25-267 |
These numbers make the case for tracking clearly. If finasteride costs $60-120 per year and produces measurable ratio improvement, the data confirms you should continue. If PRP at $2,000 per session shows no ratio change after 3 sessions, the data tells you to reassess.
Start Tracking Your Ratio Today
The vellus-to-terminal hair ratio is the single most informative metric for understanding androgenetic alopecia progression. Whether you are newly diagnosed or years into treatment, building a longitudinal record of your ratio changes gives you and your dermatologist the data needed to make informed decisions.
Upload your first photo at myhairline.ai/analyze to establish your baseline density scores. Consistent tracking over time will reveal whether your current approach is working or whether adjustments are needed.
Medical disclaimer: This article is for informational purposes only and does not constitute medical advice. The vellus-to-terminal ratio information presented here is based on published research but should not replace consultation with a board-certified dermatologist or trichologist. Always consult a healthcare professional before starting or changing any hair loss treatment.