Donor density determines the ceiling of what a hair transplant can achieve at any Norwood stage. Every graft transplanted to a bald area comes from the donor zone on the back and sides of your head, and this supply is finite. Planning your transplant around donor capacity is what separates a result that lasts from one that runs out of options.
Understanding Donor Density
Donor density is measured in follicular units (FU) per square centimeter. Each follicular unit contains 1 to 4 individual hair shafts. The density varies significantly by ethnicity and genetics.
Density by Ethnicity
| Ethnicity | Average Density (FU/cm2) | Hairs Per FU | Effective Coverage |
|---|---|---|---|
| Caucasian | 170 to 230 | 2.0 to 2.5 | Highest per graft |
| African | 120 to 180 | 1.5 to 2.0 | Curl creates volume illusion |
| Asian | 140 to 200 | 1.5 to 2.0 | Thicker individual shafts |
Effective coverage depends not only on follicular unit density but also on hair caliber (thickness), color contrast with skin, and curl pattern. Dark hair on light skin creates more contrast per graft, while curly hair provides more volume per follicle.
Safe Extraction Limits
To avoid visible thinning in the donor area, surgeons limit extraction to 25 to 30% of the total follicular units. Exceeding this creates a "moth-eaten" appearance in the donor zone.
| Donor Area Size | Density | Total FU Available | Safe Extraction (25-30%) |
|---|---|---|---|
| 200 cm2 | 170 FU/cm2 | 34,000 | 8,500 to 10,200 |
| 200 cm2 | 200 FU/cm2 | 40,000 | 10,000 to 12,000 |
| 250 cm2 | 230 FU/cm2 | 57,500 | 14,375 to 17,250 |
| 200 cm2 | 120 FU/cm2 | 24,000 | 6,000 to 7,200 |
In practice, lifetime graft availability typically falls between 4,000 and 8,000 grafts for most men, accounting for the usable safe donor zone.
Matching Donor Supply to Norwood Demand
The Supply-Demand Table
| Norwood Stage | Grafts Required | Minimum Donor Density Needed | Feasibility |
|---|---|---|---|
| Norwood 2 | 800 to 1,500 | Any (120+ FU/cm2) | Easily achievable |
| Norwood 3 | 1,500 to 2,200 | Any (120+ FU/cm2) | Easily achievable |
| Norwood 3V | 2,000 to 2,800 | 140+ FU/cm2 | Achievable for most |
| Norwood 4 | 2,500 to 3,500 | 150+ FU/cm2 | Achievable, plan for future |
| Norwood 5 | 3,000 to 4,500 | 160+ FU/cm2 | Achievable in 1 to 2 sessions |
| Norwood 6 | 4,000 to 6,000 | 170+ FU/cm2 | Requires 2+ sessions |
| Norwood 7 | 5,500 to 7,500 | 180+ FU/cm2 | Challenging, full coverage unlikely |
When Demand Exceeds Supply
At Norwood 6 and 7, graft demand often approaches or exceeds the lifetime donor supply. When this happens, surgeons must prioritize zones:
- Frontal hairline (highest priority): Creates facial framing, most visible impact
- Mid-scalp (medium priority): Connects hairline to existing hair
- Crown (lowest priority): Least visible in daily interactions, highest graft consumption for coverage
How to Assess Your Donor Density
Professional Assessment
A transplant surgeon uses a densitometer (a handheld magnification device) to count follicular units in multiple sample areas of your donor zone. The process takes 10 to 15 minutes and provides precise measurements.
Key measurements the surgeon should provide:
- FU/cm2 at the occipital midline (center back)
- FU/cm2 at each parietal zone (sides)
- Average hairs per follicular unit
- Hair caliber in microns (thicker hair provides better coverage)
- Total usable donor area in cm2
Self-Assessment Method
While not as precise as a professional evaluation, you can estimate your donor density:
- Part your hair at the back of your head
- Photograph a 1cm x 1cm area with a macro lens or magnifying glass
- Count the visible follicular unit groupings
- Compare to ethnicity benchmarks above
AI-Assisted Estimation
Get your free AI Norwood assessment to analyze your current stage and identify the zones requiring grafts. This helps estimate demand, which you can then compare against your donor supply during a surgical consultation.
Strategic Planning by Norwood Stage
Norwood 3 to 4 (Young Patients)
If you are under 30 and at Norwood 3 to 4, conservative planning is essential. You may progress to Norwood 5 or 6 over the next 15 to 20 years, and every graft used now is unavailable later.
Strategy: Use 1,500 to 2,500 grafts for the frontal zone. Reserve 3,000 to 5,000 grafts for future sessions. Stay on finasteride (80-90% halt further loss) to slow progression.
Norwood 4 to 5 (Established Pattern)
At 35 to 45 with a stable pattern, more aggressive planning is appropriate.
Strategy: Use 3,000 to 4,000 grafts across frontal and mid-scalp zones. Plan one follow-up session of 1,500 to 2,500 grafts for crown reinforcement 12 months later.
Norwood 6 to 7 (Advanced Loss)
Donor supply is the primary constraint. Full coverage is rarely possible.
Strategy: Prioritize 2,500 to 3,000 grafts for a natural frontal hairline. Consider SMP for the crown. Use body hair transplant (BHT) from chest or beard as supplemental donor if scalp supply is exhausted.
Supplemental Donor Sources
When scalp donor supply is insufficient, some surgeons harvest grafts from alternative sites:
| Source | Density | Hair Quality | Best For |
|---|---|---|---|
| Beard | High | Thick, single-hair FUs | Crown fill, density boost |
| Chest | Moderate | Fine, variable growth | Crown and mid-scalp |
| Leg/arm | Low | Very fine | Not recommended |
Body hair transplant survival rates are generally lower (70 to 85%) compared to scalp donor (90 to 95%), and the hair characteristics differ. These are best used as supplements, not primary sources.
FAQ
How many grafts can the donor area provide?
The average donor area is 200 to 250 cm2. With safe extraction limits of 25 to 30% of follicles, Caucasian donors typically yield 6,000 to 8,000 lifetime grafts, African donors 4,000 to 6,000, and Asian donors 5,000 to 7,000. These limits determine how many sessions and total coverage are possible.
What is a good donor density for a hair transplant?
A donor density above 170 FU per cm2 is considered good for transplant candidacy. Above 200 FU/cm2 is excellent and allows for aggressive coverage plans. Below 140 FU/cm2 may limit the total grafts available and require more conservative planning.
Can you run out of donor hair?
Yes. The donor area has a finite supply of follicles. Overharvesting (extracting more than 25 to 30% of follicles) creates visible thinning in the donor zone. This is why multi-session planning and conservative extraction are critical, especially for younger patients who may need future procedures.