There are a few methods to use to calculate how much weight a horse can carry.
One is simplistic: A rough guide to how much weight a horse can carry is to take the horse's weight (in lbs) divide by six --this equals the total weight the horse can carry, including tack.
Or the 20% Rule: 1000 lb horse can carry 200 pounds.
Another formula is based on measuring the bone:
When people speak of a horse's "bone", they are referring to a measurement taken at the circumference of the foreleg, just below the knee. Horses have very slim legs, and a great deal of weight and mass to carry, so the quality of their support structures is very important.
The foreleg circumference measurement can help determine the horse's ability to carry weight. A light-boned or refined horse will be limited in weight carrying capacity. A regular-sized horse with "good bone" can measure 21.75cm (8 1/2 inches) or more.
The maximum weight a horse can carry safely varies according to the breed of the horse and its usage. There is no absolute rule about how much weight a horse can carry, but generally speaking the lighter-framed the horse the less he can carry.
Native ponies are very strong and even some of the individuals in the small breeds are capable of carrying adults without difficulty. It is much easier for a horse to carry a fit, well-balanced rider, than an unfit, overweight unbalanced rider, even if they both weigh the same.
Quality of bone is something else, and more variable. Some horses that have had poor nutrition, over-fed, or over-stressed at a young age may have poor quality bone; in this case, even a good external measurement may be inaccurate in regard to weight carrying ability. Some of the smaller breeds are known for the quality of their bones: Arabians, for instance.
So "bone" and "quality" are not such simple concepts or easy to measure!
A study done of the Tevis horses and rider weight is available on Susan Garlinghouse's website. The study concluded that condition score is more important than rider weight compared to the weight of the horse. It's also important
as to how much the horse weighs and how much the rider weighs as the combined weight is what the horse carries. The more weight the horse carries, the more stress and strain, simple physics. The more weight the horse carries, the more potential lameness. Long, slow, steady conditioning is more important for the heavy-weight rider, as well as cover on the horse's back and ribs. For more details on the study, see Susan's website.
There probably is no strict weight ratio, and it may be that as horses get taller that the soundness, weight
carrying ability, coordination, and athleticism all tend to go down. The length of the back also
plays into the variables
of the amount of weight a horse can carry: "the shorter the back, the more weight the animal
will be able to pack and the longer time it will be able to stand being
ridden before fatigue sets in."
The length of the back may not be the most important variable; most likely the width of the
back is more important. Broader loins will have a greater the weight-carrying
ability regardless of the height of the horse.
The loin (coupling) should be well-muscled and strong as opposed to being
long, weak and poorly muscled. The loin is the pivot point of the horse's
back and is the area between the last rib and the croup. Short, muscular
loins are needed to carry power from the hind legs forward.
Another consideration is the "flexibility" of the back. Flexibility can be mistaken
as a positive attribute, but it may be indicative of a weak back!
There are a couple of different ways to measure the loin. One is to measure the heart girth and then take the loin/groin measurement. The closer these figures are, the better, assuming that the heart girth itself is substantial.
Or you can measure the loin area right on top. The width of the loin ends where the ribs start to curve downwards. Well-sprung ribs should help contribute to a wider loin area.
Before buying a riding horse, you'll want to check the loin width, and also check to see if the loin muscles are strong. This is done by feel. If the loin has been stretched, the muscles will feel stringy with no substance. If you haven't guessed, this is not good, especially for a riding horse, and particularly for a large rider. Unfortunately, horses ridden with stiff backs tend to have weak loins. Riding in the "lounge chair" position contributes to this weakening.
Below is a picture marked as to where to measure for the loin.
The pictures below are of two 16 year old, chestnut mares, both are of similar height, bone, and body condition. Which one has a better weight-carrying ability?
My thoughts on picking horses for large people and keeping those horses sound
1. Pick the horse with the soundest conformation you reasonably can. No horse
has absolutely perfect conformation, but generally speaking, avoid the very
longest backs and legs that aren't pretty straight and on "each corner."
2. Look for a horse with "substance." Two good indicators of strength are wide
loins and big cannon bones. The horse's height has little if anything to do
with his weight carrying ability - in fact, being too tall (as in draft horses)
works against him.
3. Keep the weight of the tack to the minimum that will sufficiently do the
job - avoid the heaviest saddles.
4. Make sure the saddle fits, and buy a saddle with the largest area of contact
with the horse for increased weight distribution. Of course, buying a saddle
with extended panels won't help, if it pokes into the loins, or make the saddle
bridge. With many Icelandics, saddle fit may involve some slight compromises,
so probably best for the heavy rider to select a horse with as few saddle
restraints as possible. For instance, even though short backs are generally
stronger, they also limit the number of saddles that can be considered,
especially on a small breed.
5. Be sure to pad and place the saddle appropriately - not on the shoulder nor
on the loins, and so that the spine is protected. An off-center saddle is
probably even more uncomfortable to the horse if the rider is heavy or
6. Keep the horse in shape, particularly his back. My vet said having even a
smaller person ride the horse between the large person's rides could build and
maintain back strength.
7. Watch for the horse to show signs of hollowing his back and get to work on
the cause ASAP.
8. The large rider needs to be particularly conscious of his/her balance when
riding and the longer the ride, the more important that becomes. Riding
position is also important. Overall fitness helps the rider's balance.
9. Avoid riding gaits that have suspension phases (trot, canter, even one-foot
support tolt) for very long periods. When you need to trot, be able to post
the trot in good rhythm with the horse, and learn to do a two-point seat to help
balance your weight for the horse's comfort when trotting or cantering. If you
are heavy, have the horse walk as much as you can, unless you and the horse are
both really fit and strong.
10. As much as possible, avoid riding in less than desirable footing - mud,
freshly plowed fields, deep sand, asphalt, ice, really hard soil, etc. If you
need to cross an area with poor footing, walk, or even get off and lead your
horse, if it's a long stretch and/or the horse needs a break.
11. Give the horse some breaks on long rides, and get off for a while. If your
riding partner is smaller, consider switching horses at some point. Of course,
that may create new problems if the saddles aren't exchangeable from horse to
horse, or from rider to rider, or if the smaller person's horse is much less fit
or is very small-boned. (Nothing is easy!)
It's probably impossible to hit all these items right on the money, and there
are examples of durable and sound horses that have not had the best
conformation. The worst scenario I can think of is having a heavy,
out-of-balance rider who rides only once every few months get on an out-of-shape
horse for a long ride on hard (or sandy or muddy) hilly terrain in a 50-pound
western saddle that pinches and bridges - cantering the whole way! If the horse
is conformationally weak, it's even worse. The shorter the ride, probably the
less critical it is that every item be met. I think it might be a good idea if
the horse or rider is weak in one of these areas, to try to hit the other items
as close to the mark as possible. I'm sure there are other things that I've
I've accumulated these thoughts from the recent "Beasts of Burden" article in
"The Horse" magazine, Dr. Deb Bennett's Conformation Analysis books, various
other articles in "Equus" and "The Horse" that I've read over the years, and
conversations with vets, trainers and saddle fitters.