Buy the right number of cans
The deck floor is the easy part. Railings, spindles, and stair risers hide a surprising amount of surface area, and porous old wood drinks stain. This accounts for all of it.
Total run of railing. Leave 0 if none.
Estimating stain honestly
Why railings blow the estimate
A deck floor is a flat rectangle, but a railing is posts, two rails, and a row of spindles, each with four sides. Measured properly, a run of railing carries several square feet of surface per linear foot — often as much paint area as a good chunk of the floor. Ignoring it is the single most common reason people run out of stain mid-project.
Porosity changes everything
Coverage rates on the can assume a cooperative surface. New, smooth, or dense hardwood sits near 300 sq ft per gallon; grey, weathered, or rough-sawn softwood can drop to 150 as the thirsty grain pulls stain in. When unsure, plan for the lower rate and return the unopened extra.
One coat or two?
Most penetrating oil stains are formulated to saturate in a single coat, and a second coat can sit on top and later peel. Semi-transparent and solid film-forming stains usually want two thinner coats — and the second always covers more area than the first, since the wood is no longer bare. The coat options here reflect that.
Buy a little extra
Dye-lot color can vary between cans bought later, so it's worth buying your whole job at once and boxing (mixing) multiple cans together for a uniform color. A small leftover is cheap insurance against a visible seam.