Concentrate, mixed material, and which one the coverage applies to
Sealer arrives as a concentrate and goes down as a mix. The coverage figure on a data sheet almost always refers to the mixed material, so working backwards has two steps and people routinely do one. Add 25 percent water and 3 lb of sand to a gallon of concentrate and you have roughly 1.39 gallons of mix, which means 620 gallons of mixed material is 447 gallons of concentrate, not 620. Order against the wrong one of those and you are out by nearly two drums on a small lot.
The sand is easy to overlook in the volume because it is quoted by weight. Dry masonry sand runs somewhere near 22 lb to the gallon, so 3 lb adds about 0.135 gallons, and on a large order that is real material. The field is there so you can weigh a measured gallon of the sand you are actually using rather than accept a figure.
The crack arithmetic, and why a tenth of a pound a foot is the number to remember
A crack half an inch wide filled half an inch deep is a quarter of a square inch of section, which over a foot is 3 cubic inches, or 0.0017 cubic feet. At 68 lb per cubic foot that is 0.118 lb per foot before waste. Nine hundred feet of it is about 118 lb, or four 30 lb boxes with the waste allowance. That is a small enough number that people buy two boxes and stop halfway.
An overband changes it substantially. A 3 in band a tenth of an inch thick adds 0.0021 cubic feet per foot, which is more than the crack itself, so banding roughly doubles the sealant. Whether to band is a decision about the surface and about how the lot is used, and it is worth making before the order rather than after the kettle is hot.
Measuring cracks is the part nobody does
A wheel and half an hour gets a real number. Without one, estimates on the same lot from two experienced people routinely differ by a factor of two, because long thin things are hard to eyeball and because it is easy to forget the seams that run the length of the lot behind the parked cars. Measure the width too: a routed crack is wider than the crack was, and the routing decision changes the material by more than the waste allowance does.
Where sealcoating stops being the answer
Sealer is a surface treatment. It slows oxidation and it makes the lot look new, and it does not carry load, does not bridge a moving crack for long, and does not repair a base failure. A lot with block cracking a few inches across has failed below the surface, and sealing it produces a black lot with the same failure under it. Nothing on this page can tell the difference, because the difference is visible on the ground and not in a square footage.
Questions people ask
How much sealcoat do I need for a parking lot?
Net area times coats divided by the coverage of the mixed material, then divided again by how much mix one gallon of concentrate makes. For 30,000 sq ft in two coats at 70 sq ft a gallon that is 857 gallons of mix, and with 25 percent water and 3 lb of sand it comes back to about 618 gallons of concentrate. The coverage figure is where all the uncertainty is.
How much does the sand and water change the order?
A lot. Water at 25 percent and sand at 3 lb a gallon together take one gallon of concentrate to about 1.39 gallons of mixed material, so almost 30 percent of the mixed volume is not sealer. Working the order from mixed gallons instead of concentrate gallons overbuys by roughly that margin.
How much crack filler per linear foot?
Width times depth times the density. A half inch by half inch section at 68 lb per cubic foot is about 0.118 lb a foot before waste, so a thousand feet is around 130 lb with a ten percent allowance. An overband adds more than the crack itself does, so it is worth deciding before you order.
Should I put an overband over the crack?
That is a judgement about the surface and how the lot is used, not a calculation. What the page can tell you is the cost: a 3 in band a tenth thick roughly doubles the sealant against a flush fill on the same crack. Banded work is also more visible and more likely to be tracked in summer heat.
Can I sealcoat over alligator cracking?
You can put sealer on it and it will be black for a while. Interlocking cracking a few inches across means the failure is below the surface, so a surface treatment does not address it and the pattern comes back through. That is a patching or milling conversation. This page works quantities and cannot see your surface.