Hair Growth Target Length
Calculator
Results
- Length still to gain (cm)
- 20
- Gross growth (cm per year)
- 15
- Net growth after trims (cm per year)
- 11
- Months needed
- 21.818181
Beauty and personal care results
| Length still to gain (cm) | 20 |
| Gross growth (cm per year) | 15 |
| Net growth after trims (cm per year) | 11 |
| Months needed | 21.818181 |
formula-map diagram
- Length still to gain (cm)
- 20
- Gross growth (cm per year)
- 15
- Net growth after trims (cm per year)
- 11
- Months needed
- 21.818181
Beauty and personal care relationship
Formula
months = (target − current) ÷ (monthly growth − monthly trim)= 20
Note
This is a simplified model: it applies the mixing ratio, dilution, application rate, growth rate or price you enter to plain arithmetic. Mixing figures use the ratio on your own product's label, because developer, powder and colour ratios differ from brand to brand. Dilutions use C1 x V1 = C2 x V2 and the standard conversion of 20 drops per millilitre; real droppers vary with the oil's viscosity and the dropper itself. Sunscreen amounts use the Du Bois body surface area formula with the application rate you enter, and the SPF figure uses the textbook approximation protection time = SPF x unprotected burn time, which is a simplification: sweat, water, towelling, UV index, altitude, reflection and your own skin all shorten it in practice. Hair, nail and hair-removal timings assume a constant linear growth rate, which varies by person, body area, age, season and health. Costs ignore taxes, sales and waste. Always patch test a new product on a small area first and follow the product label and any professional instructions. This is not dermatological, cosmetic or medical advice; see a dermatologist or a qualified professional for anything affecting your skin, scalp or health.
More in Beauty and personal care
See all →Frequently asked questions
What average hair growth rate does this calculator use?+
It typically uses the widely cited average of about 1.25 cm (roughly half an inch) of hair growth per month, or about 15 cm per year, though this is a population average and individual rates vary based on genetics, age, and health.
How is the time to reach a target length calculated?+
Time needed = (target length minus current length) divided by the average monthly growth rate, giving an estimated number of months (or years) to reach the desired length, assuming growth continues at a constant rate the whole time.
Why does the calculator's estimate not account for trims along the way?+
The calculation projects growth from the hair follicle only; it does not know about, and cannot account for, any trims or cuts you plan to have during that period, so if you get regular trims to maintain healthy ends, your net visible length increase will be slower than the pure growth-rate estimate.
Does hair growth rate really vary between individuals, or is it mostly the same for everyone?+
It varies meaningfully: factors including genetics, age, hormonal status, nutrition, and overall health can shift individual growth rates faster or slower than the population average, so the calculator's result should be treated as a reasonable estimate rather than a precise prediction for any one person.
Why might my hair seem to grow slower in winter than the calculator suggests?+
Some research suggests hair growth rate can fluctuate seasonally, often slightly slower in colder months and faster in warmer ones, alongside other factors like nutrient intake and scalp health, all of which mean an average annual rate smooths over real month-to-month variation.