Both sides, every board
A fence has two faces — the calculation multiplies run × height × 2. Stain soaks in, so coverage runs about <strong>250 sq ft per gallon per coat on rough sawn boards</strong>, and rough wood is exactly what most fence boards are. A 100-foot, 6-foot privacy fence is 1,200 sq ft of surface: roughly 5 gallons for one coat.
Spray, then back-brush
A pump or airless sprayer puts stain on a fence five times faster than a brush; back-brushing each board works it into the grain. Stain both faces the same day — sealing one side and not the other traps moisture inside the board and warps it.