Maybe this will help you out. I found it a couple of days ago on the Tractor Supply website. *NEEDLE SIZE Needles are measured two ways. Length and Gauge (diameter). When determining the correct gauge, remember that the higher the number, the smaller the diameter. Example: an 18 gauge needle is smaller than a 16 gauge needle. *NEEDLE SIZE RECOMMENDATIONS Longer needles are used for intramuscular injections (1-1/2") and shorter needles (1/2" -1") for subQ injections. Cattle, Hogs, Sheep - generally an 18 or 16 gauge with a 1 or 1-1/2 inch length. Sows, Boars - usually given I.M., an 18 or 16 gauge with a 1-1/2 inch length. Small Pigs- 18 gauge x 1/2 inch. Dogs, Cats - 20 or 22 gauge with a 1/2 or 1 inch length. CHorses - 18 gauge x 1 inch.