Hair growth calculator
Hair at 10 cm that grows 1.25 cm a month reaches 17.5 cm after 6 months. Enter your measured length, the months and your own average monthly growth, in centimetres or inches, to plan a grow-out with a dated target. Free in your browser.
- Free
- Runs in your browser
- No sign-up
- Your data stays on your device
Run — free
Worked example
- Initial length
- 10
- Months
- 6
- Growth rate
- 1.25
- Added length
- 7.5
- Projected length
- 17.5
- Precision
- 10
This hair growth calculator estimates how long hair will be after a given number of months, starting from a measured initial length and a constant average monthly growth rate. The model is linear: projected length equals initial length plus months times the monthly rate, so the same units you type for length and rate come back on the projected figure — centimetres with centimetres per month, inches with inches per month. Typical human scalp hair grows near one and a quarter centimetres, or about half an inch, each month, but you can enter any average you have measured on your own head or in a salon log. Negative months and a negative growth rate are rejected as invalid input because elapsed time cannot run backwards and this estimator does not model shrinkage or breakage as a negative rate.
What the linear hair-growth model means and what comes back
Human scalp hair does not grow in a perfectly straight line, but over months the most useful planning number is still a constant average monthly increment. This calculator treats that increment as a rate you supply, then applies the closed identity projected length equals initial length plus months times growth rate. If you start at ten centimetres and grow at one and a quarter centimetres per month for six months, the added length is seven and a half centimetres and the projected length is seventeen and a half.
The same arithmetic works in inches: eight inches plus twelve months at half an inch per month projects to fourteen inches. Zero months returns the initial length unchanged; a zero growth rate returns the initial length as well, which is the honest result when you want a no-growth baseline.
What you can do with it
Personal grow-out planning
Project how long your hair will be in three, six, or twelve months from a tape-measure start and the monthly average you have logged.
Salon and barber grow-out quotes
Give a client a dated length target from a measured starting length and a realistic monthly rate instead of a generic calendar promise.
FAQ
What units should I use for length and growth rate?
Any consistent pair. Centimetres with centimetres per month, or inches with inches per month. The engine does not convert units; it returns the projection in the same length unit you typed.
What happens if months or the growth rate is negative?
Elapsed time cannot run backwards, and this estimator does not model shrinkage as a negative monthly rate. Failed validation is not billed.
What monthly growth rate should I enter?
Use a rate you have measured. A common scalp average is about 1.25 cm per month (about 0.5 in per month), but age, location on the scalp, and care change that figure.
Can this calculator tell me if a hair treatment is working?
No. It is a planner, not a diagnosis: it shows how long your hair would be if the average growth you typed held for the months you typed. Nothing is taken from a medical table.
How long will my hair be after a year?
Add twelve months of your monthly growth to today's length. Starting at 8 inches and growing half an inch a month, you would reach 14 inches after 12 months. A zero rate keeps today's length as a baseline.
Related tools
For developers — API access
Everything on this page is available programmatically. This section is for teams who want to wire it into their own systems; everyone else can just use the tool above.
API endpoint
Prefer to automate it? One authenticated POST creates the task; the result comes back by webhook or a signed link. The same capability also runs here on the web, by email and from Telegram — and soon from our app too.
Call it from your stack
curl -X POST https://api.kit.forhosting.com/life/hair-growth \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"initial_length":10,"months":6,"growth_rate":1.25}'const res = await fetch("https://api.kit.forhosting.com/life/hair-growth", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"initial_length": 10,
"months": 6,
"growth_rate": 1.25
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/life/hair-growth",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"initial_length": 10,
"months": 6,
"growth_rate": 1.25
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/life/hair-growth", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"initial_length":10,"months":6,"growth_rate":1.25}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"initial_length":10,"months":6,"growth_rate":1.25}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/life/hair-growth", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)Example request
{
"initial_length": 10,
"months": 6,
"growth_rate": 1.25
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "life.hair_growth",
"status": "queued",
"_links": {
"result": "/tasks/tsk_…/result"
}
}The API is asynchronous: the call returns a task_id immediately and the result arrives by webhook. Polling is capped at 1 req/s per task.
Pricing
Published price — no tokens, no invented credits. A failed task is never charged.
Errors
| HTTP | Code | Meaning |
|---|---|---|
401 | unauthorized | Missing or invalid API key. |
402 | insufficient_balance | Your balance doesn't cover the task price. |
404 | unknown_type | That task type doesn't exist. |
429 | rate_limited | Too many requests. Use the webhook instead of polling. |
Technical notes
The same deterministic arithmetic runs free in the browser and through the API at $0.002 per successful call, with no network, no sampling, and no clock.
What the linear hair-growth model means and what comes back
The response echoes the three inputs you sent, reports added_length as the product of months and rate, reports projected_length as the sum, restates the formula as a short string you can show next to the answer, and includes the rounding precision used. Nothing is fetched from a medical table and nothing is sampled: the engine either multiplies and adds finite numbers or rejects the request. That makes the page a planner, not a diagnosis — it will not tell you whether a treatment is working, only how long hair would be if the average you typed held for the months you typed.
Input contract, units, rounding, and error behaviour
A negative months value is rejected because elapsed time cannot go backwards. Failed validation is not billed. POST a JSON body with three required finite numbers. initial_length is the current hair length in any consistent length unit; aliases length, current_length, start_length, initial, and start are accepted so spreadsheets and short forms can reuse the same contract. months is elapsed time in months, including fractional months such as one and a half; aliases month, n, and period are accepted. growth_rate is the average monthly increment in the same length unit per month — one point two five for centimetres per month, or zero point five for inches per month — with aliases rate, monthly_growth, monthly_rate, and growth. An optional precision field, with aliases decimals and dp, chooses how many decimal places to expose, defaulting to ten and capped at fifteen so JSON goldens stay stable. Each field may arrive as a JSON number or as a numeric string; non-finite values, empty strings, and missing required fields fail with invalid_input naming the field. A negative initial_length is rejected because a current length cannot sit below zero. A negative growth_rate is rejected because this estimator does not treat breakage or a trim as a negative monthly rate — subtract a cut after the projection, or enter a measured remaining length as a new initial.
Where a hair-length projection earns its keep
People growing out a cut want a date they can believe: how long until a bob reaches the shoulders, how many months until a fade grows into a style that can be tied back, how far a child's hair might reach before a school photo. Stylists and barbers use the same arithmetic when they quote a grow-out plan, because a measured starting length plus a client-specific monthly average is clearer than a generic twelve-month promise. Keep requests small, pin the rounded projected_length in tests, and you get reproducible grow-out numbers for journals, salon software, and edge workers without standing up a medical model of the follicle cycle.
Integration use cases
Deterministic product fixtures
Seed unit tests, journals, and grow-out timelines with stable projected_length values that never depend on a clock or a random draw.
Technical questions
How does the hair growth calculator estimate length?
It applies projected_length = initial_length + months × growth_rate. Enter current length, months, and your average monthly rate in matching units; the API returns added length and the projected figure.
What does the hair growth calculator cost?
The browser tool is free. API requests use prepaid KIT balance at $0.002 per successful call; invalid input is not billed.