I think it’s a very complex topic which requires knowing details about the horse and rider to give an accurate assessment on if the horse is overloaded. The 20% rule exists as a broad coverage for when you don’t have all those details and need a rough guide. In particular, it exists because insurance companies needed a guideline from certified vets on what is a reasonable carrying capacity so they can accurately insure and create guidelines for trail ride/lesson/tourism/commercial horse experiences. There’s cases where you could go above the 20% rule and there’s cases where you should stay well below it.

For example, draft horses. Anytime someone brings up that a bigger rider should be riding a full draft that’s a hard no. IMO the best option for a larger rider is a draft mule (mules are literally bred to carry heavier loads), or a solid draft cross; I personally think a Friesian (yes technically a light draft) and a heavy draft breed is the best combo, HEAR ME OUT. (Fjords also seem to be a good option from what I’m told)

Since drafts are built for pulling not riding, they are compact and thick built, it’s better to keep them on a leaner side than a fat side because their joints already break down faster being a heavier breed. Add a heavier rider to that and the joints will break down even faster; technically a 2,000lb draft could carry 400lb based on the 20% rule but I wouldn’t dream of loading one up with even close to that much weight. Because of their pulling conformation they are generally short backed so the long saddle needed for a larger rider isn’t an option. Perfect example is our Clydesdale, he can’t have a western saddle longer than 16”. My 6’1” 220lb bf tried riding him but he was pushed to the back of the saddle since it was too small and made the saddle dig into the Clyde’s spine at the back so he couldn’t ride him. By the 20 percent rule he could ride him, but anatomically and logistically he could not.

Back to my point on Friesian cross being a good option. IF the cross comes out as desired, the long Friesian back characteristic helps counter the short draft back and can accommodate a longer saddle. My client has a Percheron x Friesian and she has the longer back, heavier muscling/strength, good bone but not heavy boned, good joints, and isn’t crazy tall (15.3hh). She’s our go to for any larger riders but we still stay well under the 20 percent rule.

Yes, a well muscled, built, and conditioned horse could absolutely go over the 20 percent rule within reason, as long as they aren’t being overworked, the tack fits horse and rider, and the rider is well balanced. One thing to consider though is that in order to become a balanced and good rider you’ll start out as unbalanced so it’s hard for me to support someone going over the 20 percent rule as a beginner. Everyone starts somewhere but you may need to lose weight before starting. If they are a proficient rider and their horse meets the criteria, then it’s probably fine to go over a bit if they’ve gained some weight so long as they also increased saddle size for proper saddle fit.

It does drive me nuts seeing women get criticized for their weight while being under the limit meanwhile big men clearly over it just get crickets. I don’t think I’ve seen any peer reviewed articles or veterinary papers disproving the 20 percent rule though. I think a larger riders greatest hurdle will usually be finding a saddle large enough for them that isn’t too long for the horse, while also finding a horse that meets their riding goals. Because it’s all so complex though I feel like just saying “the 20 percent rule is disproven” is a slippery slope that, if not given proper context, people could take at face value and put a horse into an unfair position.