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Paint Coverage Calculator
Estimate paint needed after deducting openings and adding coats.
Paint Coverage Calculator: Estimate paint gallons from wall area after deducting openings, multiplying by the number of coats and allowing for application loss. Enter the selected coating's coverage per gallon for one coat on your surface. The page distinguishes the calculated amount from a purchase estimate rounded upwards to whole gallons. Runs 100% locally in your browser with zero server file uploads.
- Category
- Calculators
- Runs
- In your browser
- Cost
- Free · no sign-up
- Availability
- Ready to use
Gallons to buy (estimate)
2 gal
- Paintable area (estimate)
- 240 sq ft
- Calculated paint (estimate)
- 1.51 gal
Gallons = (wall area − openings) × coats ÷ coverage × (1 + waste)- Every result is an estimate; verify surface preparation, measurements, and local practices before ordering.
- Allow for the stated waste and product-coverage assumptions; openings are deducted only where requested.
- Compaction, settling, substrate condition, cuts, and installation method can change actual quantities.
Formula and worked example
Gallons = (wall area − openings) × coats ÷ coverage × (1 + waste)
A worked example with sample values:
| Inputs | |
|---|---|
| Wall length | 40 ft |
| Wall height | 8 ft |
| Openings | 80 sq ft |
| Coats | 2 coats |
| Product coverage | 350 sq ft/gal |
| Waste allowance | 10% |
| Results | |
|---|---|
| Paintable area | 240 sq ft |
| Calculated paint | 1.51 gal |
| Gallons to buy (estimate) | 2 gal |
Coverage formula with two coats
Let L be total wall length in feet, H height, O openings in ft², k whole coats, C ft² covered per gallon per coat and w loss allowance in percent. Paintable area A = L × H − O. Gallons G = A × k ÷ C × (1 + w ÷ 100); gallons to buy = ceiling(G).
A 40 ft wall run at 8 ft height gives 320 ft². Deducting 80 ft² leaves 240 ft². With two coats, illustrative coverage of 350 ft² per gallon and 10% loss, G = 240 × 2 ÷ 350 × 1.10 = 1.508571 gallons. The widget displays 1.51 calculated gallons and 2 gallons to buy. The example coverage is not a claim about any paint.
Surface condition and coverage units
Sherwin-Williams' poor-hiding guidance (https://www.sherwin-williams.com/home-builders/products/resources/sw-article-pro-transparent) identifies surface porosity and colour changes as factors in coating performance. Coverage, hiding and required preparation are separate considerations. Take application losses from your method and experience rather than increasing the number of coats to imitate wasted paint.
NIST's area conversion (https://www.nist.gov/pml/special-publication-811/nist-guide-si-appendix-b-conversion-factors/nist-guide-si-appendix-b8) gives 0.09290304 m² per ft². A label in m² per litre also needs a volume conversion: this page's gallon convention is the US liquid gallon, about 3.785412 litres. Do not treat an imperial gallon, litre and US gallon as interchangeable coverage units.
How to use it
- Enter measurements using the units shown beside each field.
- Choose the product coverage and allowances for your measured job.
- Calculate, then compare the rounded quantity with your layout and product label.
Privacy & limitations
Inputs and calculations stay in your browser. No values are uploaded.
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Frequently asked questions
Is paint coverage per coat or for the finished two-coat job?
Use coverage for one coat. The calculator multiplies paintable area by the number of coats, so a two-coat coverage figure would count the coats twice. If the label gives a range, choose a value appropriate to the substrate and application method; check the product label.
Can I include primer and topcoat in one calculation?
Calculate them separately when their coverage, surfaces or coat counts differ. A primer may cover areas the finish coat does not, and changing colour can affect the required coating system. One coverage field cannot describe several different products accurately.
Why does a small increase in area add a whole gallon?
The calculated gallons change continuously, but the purchase count rounds upwards to an integer. Actual container sizes can differ, so use the unrounded quantity when comparing packs. Openings must be smaller than gross wall area; subtract only surfaces that will remain unpainted.
Embed this tool
Free to use on any website. Paste the code where the tool should appear; the link under it lets readers open the full page.
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