Hair Growth Calculator (time to reach a goal length, with trims)

Understanding the hair growth calculator — concept and principle

This calculator estimates how long it will take your hair to reach a goal length, taking into account how much you cut off at regular trims. It also gives the approximate date you will get there and the length you can expect after one year.

Growth minus trims: with the default inputs the result is 16.9 months (15 ÷ 0.89 · about Apr 2028).

How it is calculated

Net growth per month = growth rate − trim length ÷ months between trims

Months between trims = weeks × 7 ÷ 30.4375

Months to goal = (goal length − current length) ÷ net growth per month

Goal date = start date + months × 30.4375 days

Length after one year = current length + net growth × 12

Set weeks between trims to 0 if you do not plan to trim.
30.4375 days is the average month (365.25 ÷ 12).

The principle

Scalp hair grows from follicles at a fairly steady rate, on average about 1.25 cm (half an inch) a month, or roughly 15 cm a year. Regular trims take some of that growth away, so what matters is the net gain per month. Dividing the length still needed by the net gain gives the time. The rate differs between people and changes with age, hormones, nutrition and health, so a measured personal rate gives a better answer than the average.

Worked example

Default inputs: current 20 cm, goal 35 cm, growth 1.25 cm/month, 1 cm cut every 12 weeks, start 10 Nov 2026.

Months between trims = 12 × 7 ÷ 30.4375 = 2.760.
Net growth = 1.25 − 1 ÷ 2.760 = 1.25 − 0.362 = 0.89 cm/month.
Growth needed = 35 − 20 = 15 cm.
Months = 15 ÷ 0.8877 = 16.9 months, reached around 8 Apr 2028.
Trims along the way = 16.9 × 30.4375 ÷ 84 → 6; length after 1 year = 20 + 0.8877 × 12 = 30.7 cm.

Each point is the calculator re-run with only “Length cut at each trim (cm)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Length cut at each trim (cm)	Time to reach the goal (months)
0.25	12.9
0.5	14
0.75	15.3
1	16.9
1.25	18.8
1.5	21.2
2	28.6

How to read the result

If the time is longer than you expected, the trim schedule is usually the easiest lever: smaller or less frequent trims raise net growth. Each hair also has a growth phase of a few years, after which it sheds, so everyone has a natural maximum length, and long ends wear and break.

Common mistakes

Assuming hair grows much faster than average because of a product or supplement.
Measuring from different points on the head each time.
Ignoring breakage, which acts like an extra trim.

Keep in mind: This is an estimate. Sudden hair loss, patchy loss or very slow growth can have medical causes and is worth checking with a doctor or dermatologist.

Frequently asked questions

How fast does hair grow?

About 1.25 cm (half an inch) a month on average, roughly 15 cm a year.

Do trims make hair grow faster?

No. Trimming does not change growth at the root, though it can reduce split ends and breakage.

How can I find my own growth rate?

Measure the same section from the same point once a month for two or three months and average the change.

Sources: American Academy of Dermatology Association, hair growth information (about ½ inch a month)

Checked 9 Nov 2026. Average growth of about ½ inch a month (about 6 inches a year) as stated by the American Academy of Dermatology Association and dermatologist Dr. Hadley King, quoted in USA TODAY, “How fast does hair grow?” (syndicated on AOL); month length 30.4375 days (365.25 ÷ 12).

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