Coverage is the single most common pre-purchase question, so here are three concrete jobs at the stated 55 sq ft per bottle. A 6 ft × 4 ft garden gate, both faces plus the frame, comes to roughly 55–60 sq ft — one bottle, with a little in reserve for hinges and the latch.
A single-axle box trailer — frame rails, crossmembers, tongue, fenders and the underside of the deck — works out around 110–130 sq ft once you account for two coats on the rails that take the most spray from the road. That is the 3-bottle Project Pack, which is also the point where shipping becomes free.
A pickup chassis with surface rust along both rails, the crossmembers and the inside of the arches lands between 200 and 260 sq ft with a second coat on the worst sections. The 5-bottle Workshop Pack covers it and leaves you a spare bottle, which is usually the right call: running out halfway under a vehicle is a far more expensive mistake than one extra bottle.
If your job is not on this list, put the measurements into the calculator below — it applies the same 55 sq ft per bottle figure and rounds to whole bottles.