what size needle???

what is the best size needle to use for subcut (under skin)?

im?

there are so many to choose from........everything from 18x1" to 20x 3/4"

Depends on what your giving.
Sub Q (under skin) or IM would determine length. For Sub Q I usually use a 3/4 inch. For IM I use 1 inch.
Gauge (the first number and what determines the diameter of needle) highly depends on what your giving. If giving Alpha 7 an 18 gauge is almost too small, as the vaccine doesn't flow very good through it, so I usually use 16 gauge.
If it is a med/vacc that has the consistancy more like water, use a smaller gauge which means a larger number. I would use a 18
If vaccinating cows around a chute, I would use a 14 X 1. Remember, the lower the number the larger the needle. The higher the number, the smaller the number.
I never go larger than 14, or smaller than 18.
So in summary, determine HOW you will give the meds, and WHAT you are giving, you will then know what needle to use.

Show Heifer is correct about the gauges but 5/8 is the best for SQ because that will not penatrate the muscle, Inch works just fine but 5/8 is what most vets will use.

for SQ on calves I like to use 18 gauge 1inch, at an angle and for IM on calves I us 1 1/2 x 18 gauge
for SQ on cows I use 16 x 1in and 16 x 1 1/2 for IM. cows tend to bend anything smaller than 16 to easy. IMO