Driveway sealer coverage is quoted per pail rather than per gallon, which makes it easy to mis-estimate by a factor of five. A five gallon pail covers roughly 400 to 500 square feet per coat, so a gallon covers 80 to 100 — a long way from the 350 you are used to with paint.
Two coats is the normal specification
Almost every asphalt sealer is specified as two coats, and the calculator defaults to that. One coat looks black on the day and greys out within a season.
The second coat goes on perpendicular to the first. That is not cosmetic — it fills the striations the first squeegee pass left and gets the film thickness even.
The first coat has to be dry before the second, which usually means several hours in good conditions and sometimes overnight. Rushing this is what produces a soft film.
Surface condition changes the number a lot
A driveway that has been sealed before and is in good shape will hit the label coverage.
A driveway that has never been sealed, or that has gone grey and porous, will drink the first coat. Expect coverage on that first pass to be noticeably worse than the label — sometimes half.
Old, badly oxidised asphalt with loose aggregate on the surface is not a sealing job at all until it has been cleaned and the loose material removed. Sealer over loose stone bonds to the loose stone.
Cracks and holes come first
Sealer is a coating, not a filler. It bridges nothing. Cracks up to about half an inch get crack filler; anything wider or any actual pothole needs patching material.
All of that has to be done and cured before sealing, and it needs its own quantity estimate — crack filler is sold by the tube or the block and is entirely separate from the pail count here.
Sealing over an unfilled crack simply produces a sealed driveway with a crack in it, and water still gets into the base.
Timing and weather
Sealer needs warm, dry conditions and a clear forecast. Most products want surface temperatures above about fifty degrees and rising, with no rain for twenty-four hours and ideally longer.
Applying in the late afternoon means it will not cure before the temperature drops. Early on a warm day is right.
New asphalt should not be sealed for its first season — it needs months to cure and release its oils. Sealing it early traps them and leaves a permanently soft surface.
How often, and the honest answer
Every two to four years is the usual advice for asphalt, and it depends heavily on sun exposure and traffic.
It is possible to overdo this. Sealer applied annually builds a thick film that cracks in a web pattern independently of the asphalt underneath, which looks worse than no sealer at all.
If the surface is still black and the texture is closed, it does not need sealing yet.