Where 1,604 comes from
It is worth deriving once, because every coverage figure in the trade is this number divided by something. A US gallon is 231 cubic inches. Spread at one mil, which is a thousandth of an inch, it covers 231 ÷ 0.001 = 231,000 square inches. Divide by 144 and that is 1,604.17 square feet. So a product applied at 30 wet mils covers 1,604 ÷ 30 = 53.5 square feet per gallon, and no marketing figure can change that.
Run it the other way and it becomes an audit. Fifteen gallons — three five gallon pails — spread over 600 square feet in two coats has covered 1,200 square feet of coat area. Fifteen times 1,604 is 24,063 square feet at one mil, divided by 1,200 gives 20.05 mils a coat. Against a 30 mil target that is two thirds of the film, and the whole job is going to be a third light unless something changes.
| Line | Default job | Arithmetic |
|---|---|---|
| Material used | 15.00 gal | 3 pails × 5 gal |
| Coat area | 1,200 sq ft | 600 sq ft × 2 coats |
| Wet film per coat | 20.05 mils | 15 × 1,604 ÷ 1,200 |
| Total wet so far | 40.10 mils | 20.05 × 2 |
| Target total | 60.0 mils | 30 × 2 |
| Shortfall on the coated area | 19.90 mils | 60.0 − 40.10 |
| Material for the shortfall | 7.44 gal | 600 sq ft at 19.90 mils |
| Plus 5% loss | 7.81 gal | |
| Reinforced band | 1.77 gal | 45 sq ft at 60 mils, plus loss |
| Still needed | 9.58 gal | 2 pails |
The band line is the one that gets left out of orders. Ninety feet of 6 inch fabric is only 45 square feet, about 7 percent of the job, but at 60 wet mils it drinks 1.77 gallons — more than a tenth of everything used so far, on a strip you could roll out in ten minutes. Fabric that is not fully wetted out leaves dry threads that wick water straight through the membrane, so the band cannot be starved to make the numbers work.
Why the pail count and the gauge disagree
Three things eat film without anyone noticing. A porous or open substrate takes a real volume of the first coat into itself, and that material is gone as far as film build is concerned. A rolled coat thins at the edges of every pass and over every high spot, so the average can be right while the thin places are not. And an unmeasured job drifts toward whatever feels like enough on the roller, which is reliably less than the data sheet asked for, because the correct film for most of these products feels excessive.
None of that is fixed by ordering more material. It is fixed by measuring, which is why this page starts from what has already been used rather than from what should be. If you have not opened a pail yet, run the same arithmetic forwards: divide 1,604 by your target wet mils to get the honest coverage rate, then divide the area by that. The spread rate calculator handles the volume solids and thinning if the spec is written in dry film, and the film build page does the same for wood finishes, where the same physics produces a very different set of numbers.
Limits
The page has nothing to say about whether the film you achieved is acceptable. It does not know your product, your substrate, your climate or your specification, and the target on the form is a number you typed in. It also assumes the material went where you say it went — spillage, a pail left open, and material used on a stretch you did not measure all land in the same place, which is an apparently thick film on paper and a thin one on the wall.
For the geometry side of a wet-area job, the shower waterproofing take-off works out the area and the banding, and the below-grade take-off does the same for a foundation wall.
Questions people ask
How many square feet does a gallon of liquid waterproofing cover?
1,604 divided by the wet mils you are applying, and nothing else. At 30 wet mils that is 53.5 square feet per gallon; at 60 mils it is 26.7; at 15 mils it is 107. The 1,604 comes from a gallon being 231 cubic inches spread a thousandth of an inch thick. Any coverage figure quoted without a film thickness attached is meaningless, because it is one point on that curve and you have no way of knowing which.
How do I check the wet film thickness while I am working?
A notched wet film gauge pressed into the fresh coat reads it directly, and it takes seconds. Read it in several places rather than one, because film varies across a pass and the thin spot is the one that matters. The other check is the one this page does: measure the area you covered, count the material that went onto it, and see what film that implies. If the gauge and the pail count disagree, something is going somewhere you have not accounted for.
Why does the reinforcing fabric need so much extra material?
Because the fabric has to be fully saturated with no dry threads left, and that generally means a coat bedded underneath and a coat worked over the top. On the defaults here the band is 45 square feet against a 600 square foot job — about 7 percent of the area — but at 60 wet mils against 60 total in the field it takes 1.77 gallons, over a tenth of what has been used so far. Dry threads in a reinforced band wick water through the membrane, so this is not a place to economise.
Is wet film the same as dry film?
No, and mixing them up is a common way to end up with half the membrane you thought. Dry film is the wet film multiplied by the volume solids of the product. Something at 60 percent solids applied at 30 wet mils leaves 18 dry. Some data sheets specify wet, some specify dry, and some give both — check which yours is before entering a target here, and use the spread rate calculator to convert if it is written in dry film and you have thinned the product.
Can I just add another coat instead of measuring?
Sometimes, but coats are not a quantity. Two heavy coats and four thin ones can end up at very different films, and some products have a maximum film per coat as well as a minimum, above which they skin over and stay soft underneath. Others have a recoat window that closes. This page reports mils rather than coats for exactly that reason, and what your product allows is on its data sheet.