Is the string test ( measuring from hair line to middle of knee) accurate? My colt is a yearling gelding (14 months) going by the measurement he’s going to be 15.3-16h. He is currently 13.1-13.2. 

His dam is 14.3 and his sire is 16.1

I am 63 years old and have had horses since age 5.  My grandfather taught me to string one.  In my lifetime, it's been pretty darn accurate.  The last horse I bought -- a 4 YO gelding stringed to 15.3.  He is now a 7YO and 15.3.    The mare before that stringed to 15.1 as a 3 YO and she is 15.2 as a 16 YO.  And she was barely 14 as a 3 YO!!!     I bought that little mare believing she would grow -- and she did!  :) The gelding was just a hair over 15 as a 4YO and I believed he would hit 15.3.  He did. 

Can I ask is there an age that you should wait to do this until? I tested my 3 month old colt and I pray its not right, but maybe its too early?  It said he'd be 12 hands. His mama is 13.3 and his daddy is 15.1. All his daddys other babies have outgrown him, whether he was bred to littler or bigger mares.

I just read something that said you get the most accurate result with the string test after 12 months of age. 

https://www.chronofhorse.com/forum/forum/discussion-forums/dressage/13975-string-test-how-accurate-have-you-found-it-to-be

Accurate, but you gotta wait till they are a year.  

Just remember when you're doing this that the horse grows from the bottom up. The pasterns are the first thing to stop growing. Then the cannon. Then the forearm. Etc... I would not expect an accurate measurement until the baby is at least 6 months old.  Also it can be off when a horse is out of balance. Meaning they have a longer cannon bone than is optimium.  If you want to prove it's accuracy, measure a bunch of adult horses. It's pretty darn accurate. Much closer than any other old wives tale that I've been told over the years. 

Can someone explain how to do this? And I have a 4yr old everyone keeps saying hes going to grow but I'm just not believing it! Is it too late to do this test on him? 

No. In fact it is more accurate the older they are. You measure from the coronet band to the middle of the knee.  That measurement will result in xx inches and xx partial inches.  14 inches and 1/2" = 14.2 hands.  15 inches and 3/4" = 15.3 hands etc...

This is the way I was taught in my youth to string test.. You place a string{ I always used baleing twine} to the area you see in the picture go to the point of the feltlock and then hold the string up to the elbow bone and then bring the string up from the feltlock but keep the string on the elbow and go up to the withers, and how much amount you have in the string after coming off the feltlock up to the winters will determing the height of your horse, I dont know if I explained that very well, ha ha ha ha..

And here is a picture of what Oregon is talking about,, You can google How to string test a horse and alot will come up, I think even videos...

This one never worked for me. At all.  

Ummm no offense but this one was way easier on my ol brain lol!!! Thank you so much!!! 

LOL, did you do the test? 

I'm going to do it tomorrow! I got my horse rode and bathed and then the sky opened up on us so I had to go inside!