I am not sure but here is a tip. When you shed a hair with a bulb attached, stretch it and measure it. I think I have about 2.5 inches of new growth now. About 1 month ago I was bad and had a blowout and my hair was straight. My hair measured 22 inches then. I measured it about a week ago and it is about 20 to 21 inches. This is without any heat or any attempt to straighten the hair. Due to my estimate of my new growth it appears that holding your hair tight and measuring the new growth with a ruler methodolgy may work. Good luck. Just pay attention to where the new growth ends it can be difficult to tell the difference between the relaxed and natural hair when you pull it tight.