The proper way to measure concentrates is by weight so an old kitchen scale is handy. You’ll probably use an old coffee can or other container to scoop your feed. Weigh the feed in the scoop and note how much feed is in the container. That way you won’t have to weigh the feed each time. Also, be aware that all feed does not weigh the same. Your coffee can of oats will be much lighter than a pelleted concentrate.
There are no absolute standards for feeding a horse. Monitoring your horse’s condition and energy levels is the only way to tell if your horse needs more or less concentrates or hay. If your horse is too energetic, you’ll want to decrease the amount of energy producing feed. If your horse is loosing weight or tires easily in work and is eating free choice quality hay or pasture you’ll need to increase the horse’s feed concentrates.
The bulk of your horse’s diet should be either hay or grass. If you are going to add grain or concentrates to your horse’s diet you can replace up to 40% of its fodder with the concentrate. In some special cases horses are fed only 1% of their body weight in hay. But feeding high concentrate to fodder ratios can cause boredom (fence chewing, weaving, cribbing), colic, or other problems.