A three-string bale of hay normally consists of 17 flakes weighing approximately 8 pounds each. Flakes are the way a bale splits, as detailed by Hay USA, which recommends feeding by weight rather than by flake.
Dr. Bray from Star Milling Co. emphasizes that feeding by weight and not flake is important to prevent overfeeding because bale sizes vary depending on hay type, maturity and cutting method as well as the geographical location in which it is harvested. To illustrate, he compares a 90-pound bale of grass hay to a 115-pound bale, each of which contains 16 flakes, but the weight difference is 5.6 pounds versus 7.2 pounds. He recommends feeding by flakes only when the exact weight is known.