Quick answer
Time = (target length − current length + trimmed) ÷ monthly growth rate. Growing from 10 cm to 40 cm at 1.25 cm a month takes 24 months, or about two years.
Enter the current and target length, the monthly growth rate and any amount you trim. The calculator returns months, years and weeks. It estimates how long hair growth to a chosen length will take.
Related tools: Exponential Growth Calculator: Exponential growth over time at a constant percentage rate. BMI Calculator: Find your Body Mass Index (BMI) and weight status.
How to use the Hair Growth Calculator
- Enter current length (0 or more).
- Enter target length (0 or more).
- Choose length unit: Centimetres and Inches.
- Enter growth rate per month (same unit) (between 0.01 and 10).
- Enter trimmed off, if any (0 or more). It starts at 0.
- Press Calculate to see the result, and use Reset to start again.
| Input | What to enter | Allowed values |
|---|---|---|
| Current length | A number | ≥ 0 |
| Target length | A number | ≥ 0 |
| Length unit | Choose one: Centimetres; Inches | Starts at Centimetres |
| Growth rate per month (same unit) | A number | ≥ 0.01, ≤ 10 |
| Trimmed off, if any | A number | ≥ 0, starts at 0 |
Formula and method
Formula
months = (target − current + trimmed) ÷ monthly growth rate
IEEE-754 double precision; shown to at most 6 decimal places.
Hair growth differs between people and over time (published individual scalp rates span roughly 0.6 to 3.4 cm per month, with about 1 cm per month commonly quoted), so you enter the rate that suits you. The rate and all lengths use the same unit. The result is a simple linear projection.
Worked examples
These examples are verified: each one is an automated test case that the live calculator must reproduce exactly.
Example 1
Inputs: Current length = 10; Target length = 40; Length unit = Centimetres; Growth rate per month (same unit) = 1.25; Trimmed off, if any = 0.
| Result | Value |
|---|---|
| Time needed | 24 months |
| In years | 2 |
| In weeks | 104.4 |
| Growth to add | 30 cm |
Example 2
Inputs: Current length = 5; Target length = 20; Length unit = Inches; Growth rate per month (same unit) = 0.5; Trimmed off, if any = 2.
| Result | Value |
|---|---|
| Time needed | 34 months |
| In years | 2.83 |
| In weeks | 147.8 |
| Growth to add | 17 in |
Example 3
Inputs: Current length = 20; Target length = 20; Length unit = Centimetres; Growth rate per month (same unit) = 1; Trimmed off, if any = 0.
| Result | Value |
|---|---|
| Time needed | 0 months |
| Note | Your hair is already at or beyond the target |
The first line is the headline result (time needed). Below it the hair Growth Calculator lists in years, in weeks and growth to add, which describe the same case from other angles. IEEE-754 double precision; shown to at most 6 decimal places.
A typical rate
Scalp hair grows on average about 1 to 1.5 cm per month, or around 12 to 15 cm a year, so 1.25 cm is a middle value. Growth varies with age, genetics and health, and it slows with age.
Trims
Every trim pushes the date later. If you plan to cut 1 cm every few months, add the total to the 'trimmed' input. Regular trims prevent breakage and split ends but do not change the speed of growth.
How the result changes with current length
Holding every other input at the first worked example, this table changes only current length. It shows how sensitive the answer is to that one value; each row was produced by running this calculator.
| Current length | Time needed | In years | In weeks |
|---|---|---|---|
| 5 | 28 months | 2.33 | 121.7 |
| 10 | 24 months | 2 | 104.4 |
| 20 | 16 months | 1.33 | 69.6 |
What happens with invalid input
These messages are produced by the calculator itself, so you know what to fix.
- Current length: left empty shows "Enter current length." beside that field and no result.
- Growth rate per month (same unit): entering "0" shows "Growth rate per month (same unit) must be at least 0.01." beside that field and no result.
Choosing between the Hair Growth Calculator and similar calculators
Each row shows what a related calculator on this site asks for and what it returns, so you can pick the one that matches your question.
| Calculator | You enter | It returns |
|---|---|---|
| [Baby Growth Percentile Calculator](/health/baby-growth-percentile-calculator/) | sex and age | Weight-for-age percentile |
| [Exponential Growth Calculator](/math/exponential-growth/) | initial amount, growth rate per period and periods | Final amount |
| [BMI Calculator](/health/bmi-calculator/) | units, weight and height | Your BMI |
| [Calorie Calculator](/health/calorie-calculator/) | units, sex and weight | Maintenance calories |
| [BMR Calculator](/health/bmr-calculator/) | units, sex and weight | Basal metabolic rate |
Assumptions and limitations
- If the target is already reached the answer is zero
- Growth is assumed constant
This calculator gives general information based on published formulas. It is not medical advice, and it cannot account for your health history. Talk to a qualified clinician about decisions that affect your health.
If a required value is empty, not a number, or outside its allowed range, the form marks that field with a message and shows no result instead of a misleading number. The automated checks deliberately try invalid entries for current length and growth rate per month (same unit) to confirm this.
Frequently asked questions
Does trimming make hair grow faster?
No; growth happens at the root.
How long to grow 30 cm?
About 24 months at 1.25 cm/month.
What if my rate is slower?
Use your own measured rate.
What does the Hair Growth Calculator calculate?
How long it takes hair to reach a target length at a growth rate you choose. It returns Time needed, In years, In weeks and Growth to add.
What formula does the Hair Growth Calculator use?
months = (target − current + trimmed) ÷ monthly growth rate. Hair growth differs between people and over time (published individual scalp rates span roughly 0.6 to 3.4 cm per month, with about 1 cm per month commonly quoted), so you enter the rate that suits you.
What do I need to enter in the hair Growth Calculator?
You enter Current length, Target length, Length unit, Growth rate per month (same unit) and Trimmed off, if any.
How is the result rounded or limited?
IEEE-754 double precision; shown to at most 6 decimal places. If the target is already reached the answer is zero.
What are the limits of this calculator?
If the target is already reached the answer is zero. Growth is assumed constant.
What does the “Length unit” option change?
It sets how the hair Growth is worked out. The choices are Centimetres and Inches.
What is an example of a hair Growth result?
With current length 10, target length 40, length unit Centimetres, growth rate per month (same unit) 1.25 and trimmed off, if any 0, the time needed is 24 months.
Where does the method come from?
The method follows: Linear growth model; the rate is a user input, because no single authoritative rate applies to everyone.
Can I rely on this calculator for a decision?
This calculator gives general information based on published formulas. It is not medical advice, and it cannot account for your health history. Talk to a qualified clinician about decisions that affect your health. For this calculator in particular: If the target is already reached the answer is zero.
Related calculators
- Exponential Growth Calculator — Exponential growth over time at a constant percentage rate.
- BMI Calculator — Find your Body Mass Index (BMI) and weight status.
- Calorie Calculator — Daily calories to maintain, lose or gain weight.
- BMR Calculator — Estimate basal metabolic rate with the Mifflin–St Jeor and revised Harris–Benedict equations.
Sources
- Linear growth model; the rate is a user input, because no single authoritative rate applies to everyone
Content last reviewed 2026-11-04. Formulas and worked examples are checked by automated tests against independently computed values.