So as an example of using the table for calculation:

if your foal is 3 months old and is 12.3H = 51inches, than 51/.77 = 66 inches (16.2H) on the low end and 51/.75 =68 inches (17 hands on the high end of estimation).

whereas if your foal is 12.3H or 51inches at 6 months, than 51/.86 =59 inches (14.3H) or 51/.83= 61 inches( 15.1H).

Gelding can affect the final height of the mature size, by adding several inches. We have seen several cases of foals that were gelded young in life that although they grew slowly, they continued to grow past 5 years of age.

Environmental conditions during key growth phases of the foal can dramatically change the final mature height. We often see cases of "stunting" of a foal where the foal inutero and/or after birth grew in an adverse environment. This may have been the result of "being a twin" in utero; lack of proper mare care nutrition during pregnancy where there was nothing to spare to give the foal; a lack of adequate worming program for the young horse; lack of needed diet nutrients needed by the nursing dam and young horse; lack of exercise to stimulate bone and muscle growth; illness or long-term injury, etc. It is sad when someone emails us about a 12H, two or three year old Arabian horse they have bought that have been told their horse should mature several hands taller by the seller. Yes, miracles do happen; but probably not as often as many people would like. It is true, that most horses exhibit a growth spurt when they go to a training barn. This phenomena often happens regardless of how old the horse is. Equus had an article several years ago stating that exercise could improve the height of a horse's withers by up to 2 inches, as the increased muscle acheived from the training appeared to lift the wither area upward. This combined with the fact that most Arabian horses that go to training arrive with very short trimmed barefeet, that after several months of being shod with pads and show shoes at some training barns, their feet are transformed into probably the highest heeled feet you will ever see on a horse, except perhaps for the highly padded shold long toes of the American Saddlebred show horse or Big Lick TN Walker. It is the equivalent of comparing the measurements of a lady wearing tennis shoes versus high heels.

1 or 2 inch mature height variance from parental size is normal. Typically a foal will mature somewhere in the range of it's parents heights assuming that their heights were not artifically enhanced or adversely affected by enviromental conditions. It is not unusal for a foal to grow 1 or 2 inches shorter than its parents or 1 or 2 inches taller. With optimum nutrition we have seen the average height of people inching forward with each generation, it is not unreasonable to expect the same thing to occur with horses.

One of the most interesting notes as far as breeding for size is in a study documented here http://www.reproduction-online.org/cgi/content/full/127/1/67?ck=nck which indicates that the size of the mare caring the foal can either enhance or adversely affect the final height of the foal . Real life examples are occurring today within the Arabian horse breeding world where embryo transfer into a recipient mare takes place. Placing an Arabian foal embryo into a huge draft horse size mare has resulted in foals growing over a hand taller than full siblings that did not have that benefit of size of dam. If the foal with the enhanced growth is a stallion, it has been said they do not pass on this enhanced size, but rather, sire a normal sized foal equivalent with the produce of their full male siblings. A mare that has received the enhanced size can be a different story, as the the greater size of the mare will allow her foals greater mature heights. But again, the stallions that are the result of this type of increase do not pass the height along, as this height enhancement is not genetic but is a result of environmental conditions.

We often see where the first foals of a mare are smaller than subsequent siblings as are the foals of older mares. We contribute this to the size of the mare's uterus as well as the environmental nourishing and conditions for the growing prenatal foal.

On the subject of height, it is amazing how often people do not know what a "hand" means. A hand is 4 inches. The increments after the decimal point indicate a number of inches such as 1, 2, or 3. There is no such thing as 14.5. 14.5 is 15.1H. I think sometimes people over state their horses measurements through ignorance by measuring a horse and stating that their horse is 15.3 meaning 15Hands and .3 of a hand, so they really mean about 15.1H. This can be a shocker if you are looking for a 16H horse and end up getting something closer to 15H.

"How to measure a horse's height" - here is another area where it is amazing how many horses are not even close in height to what their owner reports it is. It is hard for legitimate sellers measuring their horses with a stick and level on concrete at the actual withers to compete with people that guess or estimate with a tape or use a point half way up the neck of the horse that is so far removed from the withers, it is unimaginable. Now it is true, there are a lot of different conformation types when looking at withers. There are horses whose butts are higher than their withers, who will appear taller in the saddle riding next to a very high withered horse whose butt is several inches lower than their withers. Some horse's withers actually are half way up the horse's neck as in some Saddlebreds and new Egyptian Arabian horses. Physical mass gives the illusion of more height than a very light weight, ultra refined horse. It is amazing to see how much difference can result when someone tries to "eye ball" a height using a height/weight tape versus measuring the horse on level concrete and using a measuring stick with a level. Here are some links to articles about withers - http://en.wikipedia.org/wiki/Withers