Divide the roof area by the coverage rate on the pail, then multiply by the number of coats. At 75 square feet per gallon, a 2,000 square foot roof needs about 27 gallons per coat, or 54 gallons for the two coats most systems specify. A 5-gallon pail is the usual unit.
The Roof Coating Calculator returns the gallons of coating material needed for a two-coat application on your roof. Select coating type (elastomeric acrylic, silicone, or polyurethane) and enter your roof area; the tool returns gallons needed using typical coverage of 1 gallon per 80-100 sq ft for two coats. Coatings extend the life of an existing flat or low-slope roof by 10-20 years for a fraction of the cost of a tear-off. Pair with our Coating Guide for surface prep and application steps.
Roof coating is sold by the gallon and covers a stated number of square feet per gallon per coat. Coverage drops sharply on a rough or porous surface, and most systems need two coats.
| Roof area | At 100 sq ft/gal | At 75 sq ft/gal | At 50 sq ft/gal | Two coats at 75 |
|---|---|---|---|---|
| 1,000 sq ft | 10 gal | 14 gal | 20 gal | 27 gal |
| 1,500 sq ft | 15 gal | 20 gal | 30 gal | 40 gal |
| 2,000 sq ft | 20 gal | 27 gal | 40 gal | 54 gal |
| 3,000 sq ft | 30 gal | 40 gal | 60 gal | 80 gal |
| 4,000 sq ft | 40 gal | 54 gal | 80 gal | 107 gal |
A 5 gallon pail is the usual unit. Buy whole pails — coating does not keep well once opened and the colour can vary between batches.
All of it is four lines of arithmetic. These are the equations a contractor estimate is built from, and running them yourself is how you check one.
| What you want | Equation | Worked at 2,000 sq ft |
|---|---|---|
| Gallons per coat | roof area ÷ coverage rate | 2,000 ÷ 75 = 26.7 gal |
| Gallons total | gallons per coat × coats | 26.7 × 2 = 53.4 gal |
| Pails to buy | ceiling(total gallons ÷ 5) | ceiling(53.4 ÷ 5) = 11 pails |
| Dry film thickness | (gallons × 1,604 × solids%) ÷ area | the figure the warranty is written against |
| Material cost per sq ft | (pails × pail price) ÷ area | compare against a quoted rate |
| Coverage from a rate | gallons × coverage rate | a 5 gal pail at 75 = 375 sq ft per coat |
The 1,604 in the dry film thickness equation is the constant that converts gallons of a 100% solids material spread over a square foot into mils of thickness. Multiply by the solids percentage on the data sheet to get the real figure, because that is what the warranty specifies — not the number of coats.
| Roof area | At 100 sq ft/gal | At 75 sq ft/gal | At 50 sq ft/gal | 5 gal pails at 75 |
|---|---|---|---|---|
| 1,000 sq ft | 20 gal | 27 gal | 40 gal | 6 pails |
| 1,500 sq ft | 30 gal | 40 gal | 60 gal | 8 pails |
| 2,000 sq ft | 40 gal | 54 gal | 80 gal | 11 pails |
| 3,000 sq ft | 60 gal | 80 gal | 120 gal | 16 pails |
| 4,000 sq ft | 80 gal | 107 gal | 160 gal | 22 pails |
| 5,000 sq ft | 100 gal | 134 gal | 200 gal | 27 pails |
Coverage rate is the number that moves most. A smooth membrane can reach 100 square feet per gallon; a rough, weathered or granulated surface can halve it. Estimate an old roof at the low end and you will not be short.
| Coverage rate | Covers | Two coats |
|---|---|---|
| 50 sq ft/gal | 250 sq ft | 125 sq ft |
| 60 sq ft/gal | 300 sq ft | 150 sq ft |
| 75 sq ft/gal | 375 sq ft | 188 sq ft |
| 80 sq ft/gal | 400 sq ft | 200 sq ft |
| 100 sq ft/gal | 500 sq ft | 250 sq ft |
The rate is printed on the pail and it is per coat. Halve it for the two coats most systems specify, then apply the surface penalty for texture.
Divide the roof area by the coverage rate on the pail, then multiply by the number of coats. At 75 square feet per gallon a 2,000 square foot roof needs about 27 gallons per coat, or 54 for the two coats most systems specify — 11 five-gallon pails.
At 75 square feet per gallon, a 5 gallon pail covers 375 square feet in one coat, or about 188 square feet at the two coats most warranties require. The rate is printed on the pail and it is stated per coat.
Gallons per coat is roof area divided by the coverage rate. Total gallons is that times the number of coats. Pails is total gallons divided by five, rounded up. Dry film thickness is gallons times 1,604 times the solids percentage, divided by area.
Most systems specify two, applied in opposite directions, and the warranty is written against a total dry film thickness rather than a coat count. One heavy coat does not reach the same thickness as two thinner ones and does not cure the same way.
Substantially. A smooth membrane can reach 100 square feet per gallon while a rough, weathered or granulated surface can drop to 50 — double the material for the same roof. Estimate an old roof at the low end of the range.