Hair Growth Calculator
See how long your hair will be after a given time, or how long it takes to reach a target length.
Last updated · View algorithm, constants and default inputs (JSON)
About this calculator
Hair growth varies widely with person, scalp, health, and time. This simple constant-rate projection is for planning only, not a promise or medical assessment.
Worked example
calculated with the default values below
Current hair length 12 in · Target hair length 24 in · Months to project 12 months · Growth rate 0.5 in/month · Monthly trim 0 in Net growth per month 0.50 in · Projected length 18.0 in · Projected length 45.7 cm · Months to reach your target 24.0 months · Target status Reachable at this model rate
General background references
Disclaimer
These calculators provide planning estimates only. Building codes, wiring regulations and safety requirements differ by country and change over time, and nothing here replaces a licensed professional or a permit. For structural, electrical or gas work, follow local code and have the installation inspected.
Implementation source
This record describes the implementation used by this page. It is not independent validation. External references, when available, support only the stated formula or definition; estimates and custom rules retain their own assumptions.
Formula and variables
L = L_0 + max(0, g − t)×m
- L_0
- Current length
- g
- Monthly growth
- t
- Monthly trim amount
- m
- Number of months
- L
- Projected length
Months to target is available only when the target is already met or net modeled growth is positive. If trims equal or exceed modeled growth before reaching the target, no finite time is reported.
Frequently asked questions
Why does a monthly trim barely change how long my hair gets?
The calculator subtracts your monthly trim from the growth rate to get net growth per month, then multiplies by the projected months. At the default 0.5 inch monthly growth, even a 0.25 inch trim only cuts net growth to 0.25 inch, so over 12 months the trim removes about 3 inches of potential length. Trims reduce the final length linearly and never stop you reaching the target, they only delay it.
What if trimming matches or exceeds the modeled growth rate?
The model reports that the target is not reached at the current rate and leaves months to target blank. It cannot predict biological changes; growth varies between people and over time.