High moisture expansion of feedstuffs is a risk factor for esophageal obstruction or gastric distension in horses. This study screened common horse feed ingredients and mixed feeds for their water-holding capacity (WHC) and nutrient composition to determine their water expansion capacities. Twenty dry feed ingredients and 21 pelleted compound feeds were analyzed. The WHC was measured for four different incubation times, that is, 1, 5, and 60 minutes, and 24 hours. Based on the WHC, a traffic light system was established with cereal grains being in green area (WHC <1 g water/g feed), and dry beet pulp pellets, partially extracted linseed and dried carrots in the red area (WHC of >2.5 g/g). Alfalfa meal, wheat bran, apple pulp, and mixed pulp of apples and pears, linseed meal, and sunflower meal were defined in the yellow to orange area (WHC of 1–2.5 g/g). In compound feeds, WHC of 0.91 to 1.56 g/g were found. The WHC of feed ingredients was found to be positively correlated to the neutral detergent fiber (r = 0.64, P < .001) and ash content (r = 0.56, P < .01). Overall, the study classified horse feedstuffs according to their WHC from safe (green) and risky (red) in terms of water expansion capacity. The WHC of the first minute provides a feasible tool for assessing the amount of soaking water and expansion of single and compound feed ingredients before feeding them to horses.