Work out how many gallons of roof coating you need from your roof size, the coating’s volume solids and the dry thickness your warranty or data sheet asks for. The calculator also shows the number of coats, pails and primer.
Coverage is set by dry film thickness and volume solids. The formula is shown under “How we calculate”.
Enter your roof size and coating. Volume solids and target mils come from your product’s data sheet.
A roof coating is sold by the gallon but judged by its dry film thickness, measured in mils (thousandths of an inch). Part of every gallon is water or solvent that evaporates as the coating cures. What stays on the roof is the solids. So the number that matters on a data sheet is volume solids, the percentage of the wet coating that remains as dry film.
One U.S. gallon is 231 cubic inches. Spread one mil (0.001 in) thick, that covers 1,604 sq ft. That fixed fact, plus your product’s volume solids, gives the theoretical coverage for any thickness:
This is why a high-solids silicone and a water-based acrylic need very different amounts for the same roof. The two data sheets we checked put it like this:
| Product (data sheet) | Volume solids | Published rate | Dry film |
|---|---|---|---|
| Sikalastic-515 AC (acrylic) | more than 52% | 2 coats × 1.5 gal / 100 sq ft (10-year system) | 26 mils |
| Sikalastic-515 AC (acrylic) | more than 52% | 2 × 1.5 + 1 × 1.0 gal / 100 sq ft (15-year system) | 34 mils |
| Thermo-Sil Pro 9600 (silicone) | 97% | at least 1.5 gal / 100 sq ft, single pass | 22 mils |
You can check the formula against these sheets. The 10-year acrylic system is 3.0 gal per 100 sq ft, which is 48 wet mils. At 54% solids that leaves 26 dry mils, and the sheet only promises “more than 52%”. The silicone’s 1.5 gal is 24 wet mils, and at 97% solids that leaves about 23 dry mils, just over its 22-mil minimum. The small gap is the allowance manufacturers build in for real roofs.
Every number above comes from the steps below — the same formula and constants the calculator runs, so you can check it yourself.
| Coating type | Volume solids | Default dry mils | Max per coat |
|---|---|---|---|
| Acrylic (water-based) | 52% | 26 | 2.0 gal / 100 sq ft (32 wet mils) |
| High-solids silicone | 97% | 22 | 2.5 gal / 100 sq ft |
| Other | you enter | you enter | 2.0 gal / 100 sq ft (our assumption) |
The coverage formula is unit arithmetic. The coating defaults come from the manufacturer data sheets below, which we opened and checked in October 2026.
Last reviewed: October 2026.
A coating restores and protects a roof that is still sound. It does not replace one that has failed. The acrylic data sheet we checked says the applicator must confirm the roof is in sound condition with proper slope to drain, complete a moisture scan, and remove and replace all wet insulation before coating. Seams and flashings that have failed must be repaired first.
| Check | Why it matters |
|---|---|
| Ponding water | Neither data sheet supports coating where water stands longer than 72 hours |
| Wet insulation | Coating over saturated insulation traps the moisture; the Sika sheet requires wet insulation be removed and replaced |
| Adhesion | Both sheets call for an adhesion test before coating (Sika cites ASTM D903) |
| Asphalt surfaces | Mod-bit, BUR and cap sheets can bleed through light coatings; a bleed-blocking primer is used |
| Temperature & weather | The acrylic needs 50°F or warmer and dry weather to cure; silicone cures slower in cool, humid weather |
Want to know what a coating job costs installed? Use our roof coating cost calculator. For the material in a new single-ply roof, see the EPDM & TPO roll calculator.
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It depends on the product’s volume solids and the dry thickness you need. A water-based acrylic at about 52% solids needs around 3 gallons per 100 sq ft for 26 dry mils. A high-solids silicone at 97% needs about 1.5 gallons per 100 sq ft for 22 dry mils.
It is the share of the wet coating, by volume, that stays on the roof as dry film after the water or solvent evaporates. A 52% solids coating applied 50 mils wet dries to about 26 mils.
At 1.5 gallons per 100 sq ft, a 5-gallon pail covers about 330 sq ft in one coat. At 3 gallons per 100 sq ft, it covers about 165 sq ft for the full system. Enter your roof in the calculator for an exact count.
Real roofs absorb material, have rough surfaces and lose some to overspray. The acrylic data sheet we checked warns that rough or granulated surfaces may need more than its listed rate. The extra percentage is yours to set; 10% is our default for a smooth roof.
Not as a fix. The acrylic and silicone data sheets we checked both say the product is not intended for water standing longer than 72 hours. Correct the drainage first.
Each product has a maximum per coat. The acrylic we checked allows up to 2 gallons per 100 sq ft per coat, so its 3-gallon system goes on in two coats. The silicone allows up to 2.5 gallons in a single pass.