What this tool does and when to use it
This calculator turns your growth speed into a date: it takes your monthly growth, subtracts the length your trims take off, and divides the gap to your goal by what is left. Use it to plan growing out a bob, reaching bra strap or waist length, or to see how much a trim schedule is really costing you, in inches or centimeters.
How to use the hair growth calculator
- Measure your hair now. Measure a dry section at the back from the scalp to the tips, in inches or centimeters.
- Set your goal length. Enter the length you want, for example 16 to 18 inches for bra strap length.
- Pick your growth speed. Choose slow, average or fast. Average is about half an inch a month.
- Add your trim routine. Enter how often you trim and how much comes off, because trims slow your net progress.
- Read your timeline. See how many months it will take, the date you will get there and a month by month timeline.
How fast does hair grow?
Scalp hair grows about half an inch, or 1.27 centimeters, a month on average. That adds up to roughly six inches, or 15 centimeters, a year. Some people grow closer to a third of an inch a month and others nearer two thirds, depending on genetics, age, hormones and overall health. Growth is usually fastest from your teens to your thirties and slows a little after that.
Hair grows in cycles. Each strand spends two to seven years in the growing phase, then rests and sheds. The length of your growing phase sets your terminal length, the longest your hair can reach before it naturally falls out. That is why some people can grow hair to the waist and others find it stalls around the shoulders.
How much does hair grow in a week, a month and a year?
These figures are the calculator's three growth speeds spread over time, before any trims.
| Time | Slow grower | Average | Fast grower |
|---|---|---|---|
| 1 day | 0.3 mm | 0.4 mm | 0.5 mm |
| 1 week | 2 mm (0.08 in) | 3 mm (0.12 in) | 3.5 mm (0.14 in) |
| 2 weeks | 4 mm (0.16 in) | 6 mm (0.23 in) | 7 mm (0.28 in) |
| 1 month | 0.9 cm (0.35 in) | 1.3 cm (0.5 in) | 1.5 cm (0.6 in) |
| 3 months | 2.7 cm (1 in) | 3.8 cm (1.5 in) | 4.6 cm (1.8 in) |
| 6 months | 5.3 cm (2.1 in) | 7.6 cm (3 in) | 9.1 cm (3.6 in) |
| 1 year | 10.7 cm (4.2 in) | 15.2 cm (6 in) | 18.3 cm (7.2 in) |
Growth that small is hard to see day to day, which is why hair can feel stuck. Compare a photo taken against the same ruler or the same spot on your back every eight weeks instead.
How the calculator works
- Growth per month. Pick slow, average or fast. If you have measured your own growth over a few months, choose the closest option.
- Trim loss. We spread each trim across the weeks between cuts. A quarter inch every 12 weeks costs you about 0.08 inches a month.
- Net growth. Growth minus trim loss is the real gain each month.
- Time to goal. The gap between your current and goal length divided by net growth gives the months, and we turn that into a target date and a timeline.
Worked example
Your hair is 10 inches long and you want 18 inches. At the average growth of 0.5 inches a month, with a 0.25 inch trim every 12 weeks, the trims cost about 0.08 inches a month, so your net gain is about 0.41 inches a month. Growing 8 inches at that pace takes about 19 and a half months, after roughly 7 trims.
Common hair length milestones
| Length name | Approximate length | From a chin bob at average growth |
|---|---|---|
| Chin length | 8 to 10 in (20 to 25 cm) | Starting point |
| Shoulder length | 12 in (30 cm) | About 6 months |
| Armpit length | 14 in (35 cm) | About 10 months |
| Bra strap length | 16 to 18 in (40 to 45 cm) | About 1 to 1.5 years |
| Waist length | 22 to 26 in (55 to 65 cm) | About 2 to 3 years |
Tips to keep the length you grow
You can't change your natural growth speed much, but you can stop losing length to breakage. Keep heat styling to a minimum, detangle gently from the ends up, sleep on a silk or satin pillowcase and use a leave in conditioner on the lengths. Small, regular trims of an eighth to a quarter inch remove split ends before they travel up the strand, so they save length in the long run.
Looking for styles to grow into? Browse our long hairstyles guide, long layered cuts and soft waves for long hair, and use the face shape finder to pick a length that flatters you.
