How can I estimate square footage of fencing for stain?
Sprayers make the job much quicker and easier, however there will be a lot more waste. Take the fence height X the length X two (sides) and you will have an apx of the square feet.  Example  4ft high X 100 feet long X 2 sides = 800 square feet. 800/250= 3.2 gals.

Realistically, if you are careful and try to hit the pickets and not waste as much on the spaces, you may use a slight bit less. Keep in mind, if you are using a stock color, you can return unused/unopened product for a refund at most stores.