Hair Growth Calculator

How fast does hair grow?

According to the American Academy of Dermatology, hair on the scalp grows about half an inch a month, or about 6 inches a year. Individual rates vary with genetics, age and health – some people grow a little faster, others slower – so the calculator lets you change the rate if you have measured your own.

Measure your own growth rate

Note the length of a section of hair from a fixed point (for example, a strand at the crown measured to the tip), or watch how far coloured or treated hair grows out from the roots. After two or three months, divide the new growth by the number of months. Root regrowth on coloured hair is the easiest way to see it.

The hair growth cycle

Each hair follows its own cycle, so at any time most scalp hairs are growing and a smaller share are resting. The length of the anagen phase sets your maximum length: people with a long growth phase can grow hair to the waist or beyond, while others find their hair stops at a 'terminal length' well short of that. Shedding 50 to 100 hairs a day is normal.

Gross growth vs net growth

Gross growth is what the roots produce. Net growth is what you keep after trims and breakage. If your hair grows 0.5 in a month and you trim half an inch every three months, you lose about 0.17 in a month to trims, so net growth is about 0.33 in a month. Six inches would then take about 18 months rather than 12.

Hair length milestones

Milestones depend on your height and where you measure from, so measure yourself and enter your own numbers for the best estimate.

What affects hair growth

• Genetics – sets your growth rate and how long hairs stay in the growth phase.
• Age – growth often slows and hair becomes finer with age.
• Nutrition – very low protein, iron, zinc or calorie intake can slow growth or increase shedding.
• Hormones – pregnancy, postpartum changes, thyroid disease and menopause can all affect hair.
• Stress and illness – a shock, high fever or surgery can trigger telogen effluvium, extra shedding two to three months later.
• Hair care – heat, tight styles and chemical treatments do not change root growth but cause breakage that reduces net length.

Tips to keep more of your length

• Trim less often or take off less – 'dusting' just the split ends.
• Use heat protectant and lower temperatures on styling tools.
• Avoid very tight ponytails and braids, which can pull hair out at the root.
• Detangle gently from the ends upwards, ideally with conditioner in.
• Sleep on a smooth pillowcase or with hair loosely tied.
• Eat a balanced diet; supplements help only if you are deficient.

When to see a doctor

See a doctor or dermatologist if you notice sudden or patchy hair loss, much more shedding than usual, thinning on the crown or parting, or hair loss with other symptoms such as tiredness or weight change. Many causes – iron deficiency, thyroid problems, alopecia areata – are treatable.

Measuring your hair length accurately

Measure from the same point each time – most people use the front hairline or the crown – to the ends of a section that is not layered. Straighten curly or coily hair gently when measuring, because curls can shrink to well under their stretched length. Write the date and length down every month or two, and take a photo in the same position against a shirt or the back of a chair to see progress.

Hair growth during and after pregnancy

During pregnancy, many people find their hair looks thicker because fewer hairs shed. After the birth, the extra hairs move into the resting phase together and shed over the following months. The American Academy of Dermatology describes this postpartum shedding as normal and notes that it often peaks around four months after giving birth, with hair usually returning to its normal fullness within about a year. Growth rate itself is not usually affected.

Does hair grow faster in summer?

Some small studies suggest hair grows slightly faster or sheds slightly differently with the seasons, but the effect is small compared with differences between people. Keep the default rate unless you have measured your own over several months.

Why some hair seems to stop growing

If your hair never seems to get past a certain point, the growth phase may be ending before it reaches your goal, or breakage at the ends may be cancelling out new growth. Look at the ends: thin, split or uneven tips suggest breakage, which gentler care and small trims can improve. Hair that is even and healthy but will not lengthen is more likely a genetic terminal length.

Sources

1. American Academy of Dermatology – Hair loss: who gets and causes; hair care tips — aad.org