Roof Coating Calculator
A roof coating calculator divides roof area by 100 sq ft per gallon, so a 30 × 20 ft roof needs 6 gallons for a first coat and costs $270 at $45 per gallon.
Roof coating calculator formula
A roof coating calculator divides the roof area by how much one gallon covers. Gallons = roof area ÷ 100. The tool uses 100 sq ft per gallon for a first coat. Cost = gallons × price per gallon, with $45 as the starting price. The tool does not round to whole gallons, so you can see the exact amount.
The formula in plain words
Multiply length by width to get the roof area in square feet. Divide by 100, the area one gallon covers in this tool. The result is the gallons for one coat. If the label asks for two coats, double it.
Worked examples
These match the calculator exactly.
- Small flat roof: 30 ft × 20 ft, $45 per gallon
- Area: 600 sq ft
- Gallons: 600 ÷ 100 = 6.00
- Cost: 6 × $45 = $270
- Larger roof: 40 ft × 25 ft
- Area: 1,000 sq ft
- Gallons: 10.00
- Cost: 10 × $45 = $450
- The same 30 × 20 ft footprint with a 4 in 12 pitch
- Slope factor: 1.054, so the roof area is 600 × 1.054 = 632.5 sq ft
- Enter 632.5 as the area, for example length 632.5 and width 1
- Gallons: about 6.3, or roughly $285 at $45
Coatings are often sold in 5 gallon pails, so example 1 means 2 pails with some left over.
Slope factors
| Pitch | Slope factor |
|---|---|
| Flat | 1.000 |
| 4 in 12 | 1.054 |
| 6 in 12 | 1.118 |
| 8 in 12 | 1.202 |
| 12 in 12 | 1.414 |
Multiply the footprint by the factor to get the real roof area. The roof pitch calculator works out the pitch from a rise and run.
Coverage depends on the product
The 100 sq ft per gallon figure is a general first coat rate. Real coverage depends on the coating type, how rough the surface is, and the thickness the maker specifies. A porous or aged roof soaks up more. Read the label and use its number. If the label says 80 sq ft per gallon, divide the area by 80 yourself.
Roof coating calculations by thickness (mils)
Coating makers often specify a dry film thickness in mils (thousandths of an inch) instead of square feet per gallon. The standard equation converts that into gallons.
One US gallon holds 231 cubic inches. Spread over 1,604 square feet, that is a layer 1 mil (0.001 in) thick: 231 ÷ (0.001 × 144) = 1,604. Only the solids stay on the roof once the coating dries, so divide by the volume solids from the product data sheet.
Gallons = roof area (sq ft) × dry mils ÷ (1,604 × volume solids as a decimal)
Example: a 2,000 sq ft roof, 20 dry mils, coating with 60% volume solids. 2,000 × 20 = 40,000. 1,604 × 0.60 = 962.4. 40,000 ÷ 962.4 = 41.6 gallons, before any allowance for waste, texture or overspray.
Wet film thickness works the same way: wet mils = dry mils ÷ volume solids. At 60% solids, 20 dry mils means applying about 33 wet mils.
Preparing the roof
Coatings stick to clean, dry roofs. Wash off dirt and loose material, repair cracks and open seams, and let the roof dry. Follow the manufacturer on primers and on temperature limits. Many products should not be applied if rain or very hot or cold weather is coming. A coating does not fix a roof that is failing or holding water, so have a roofer inspect it first.
How to measure
Measure the length and width of each roof section and multiply. Add the sections. Include parapets and curbs if they will be coated. For a gable or hip roof, find the footprint and multiply it by the slope factor. Skylights and vents that will not be coated can be subtracted.
Budgeting the whole job
The coating is only one line on the bill. Count primer if the maker requires it, seam tape or mesh for repairs, rollers and brushes, and cleaner for prep. If you hire the work, ask whether the quote covers prep and both coats. A simple check is to divide the quote by the roof area. A 600 sq ft roof quoted at $1,800 is $3 per sq ft, which you can compare with the coating cost of $270 to see how much goes to labor and prep.
Weather and timing
Coatings cure by drying or reacting, and weather affects both. Most makers set a temperature range and say how long the roof must stay dry afterward. Plan a window of several dry days, and start early enough that the surface is not damp from dew. Read the label for the time between coats, and for how long before the roof can take foot traffic.
Common mistakes
- Using the footprint for a sloped roof.
- Forgetting the second coat.
- Using a generic coverage rate instead of the label.
- Skipping repairs before coating.
- Working without fall protection.
Related pages
For roll roofing, see the rolled roofing calculator. For metal panels, use the metal roofing calculator.
Frequently asked questions
How much roof coating do I need?
Divide the roof area in square feet by the coverage per gallon. The tool uses 100 sq ft per gallon for a first coat, so a 600 sq ft roof needs 6 gallons. Check your product's label, since coverage varies by brand and surface.
Do I need a second coat?
Many products call for two coats. Treat the answer as one coat and multiply by the number of coats the label requires. Two coats on the 600 sq ft roof would be 12 gallons.
How do I measure a sloped roof?
Multiply the footprint by a slope factor. A 4 in 12 pitch is about 1.054, 6 in 12 is about 1.118 and 8 in 12 is about 1.202. A flat roof has a factor of 1.
What roof types can be coated?
Coatings are mostly used on low slope and flat roofs, metal roofs and some membranes. Check that your product is made for your roof surface, and ask the maker how to prepare it.
Is roof work dangerous?
Yes. Falls from roofs are a serious risk. Use proper fall protection, work on dry days, and consider hiring a roofing contractor.
Estimates only. Verify quantities with your supplier before purchasing.