Paint Calculator
Estimate wall paint from equal wall dimensions, openings, coats, product coverage, reserve, and can size.
Content updated August 5, 2026
Equal rectangular walls only. Actual coverage varies with product, color change, substrate, texture, preparation, application method, overspray, and touch-up needs; follow the product data sheet.
Estimate paint for one or more equal rectangular walls. The calculator finds gross wall area, subtracts the combined area of doors and windows, multiplies by the number of coats, then applies an editable reserve.
Coverage and can size are explicit inputs because the product, substrate, texture, color change, preparation, and application method affect real coverage. The result reports exact gallons and litres before rounding only the purchasing count to whole cans.
The Paint formula
Paintable area = wall length × wall height × walls − openings · coverage area = paintable area × coats × (1 + reserve% ÷ 100) · gallons = coverage area ÷ entered ft²/galLitres = U.S. gallons × 3.785411784 · cans = ceiling(gallons ÷ entered can size in gallons).
Worked example
Four 12 ft × 8 ft walls have 384 ft² gross area. Subtracting 64 ft² of openings leaves 320 ft². Two coats plus 10% reserve cover 704 ft²; at 350 ft²/gal, that is about 2.011 gallons, or three whole 1-gallon cans.
Assumptions, rounding, and limitations
Assumptions
- Every counted wall has the same rectangular dimensions.
- The openings input uses the selected wall unit squared and is subtracted once from gross wall area.
- The whole-number coat count applies to all paintable wall area.
- Coverage and can size come from the applicable product and purchasing plan.
Rounding: Area and paint volume retain full floating-point precision. Displays use up to three decimals; only the whole-can result rounds upward.
Limitations
- Does not infer primer, ceiling, trim, cabinet, masonry, color-change, texture, preparation, overspray, transfer efficiency, touch-up, or product-specific safety requirements.
- Equal walls and a single coverage rate are modeled; calculate unlike surfaces or products separately and follow the product label and data sheet.
Sources
- NIST Guide to the SI, Appendix B.9: Conversion Factors — National Institute of Standards and Technology
- How to Use Paint Calculator — Sherwin-Williams
- BEHR PREMIUM PLUS Interior Flat Paint — Behr
Subtract openings before multiplying coats
Doors and windows remove paintable surface from the gross wall area. The subtraction happens once, then the remaining area is multiplied by the number of coats so an opening is not accidentally counted for every pass.
The reserve is an optional percentage of coated area. Keeping it separate from the product coverage prevents the calculator from silently embedding a waste factor or assuming that every surface needs the same contingency.
Coverage belongs to the paint and the surface
Sherwin-Williams describes about 350–400 square feet per gallon as typical while warning that application method and surface condition change requirements. A current Behr product page lists 250–400 square feet per gallon depending on application method and substrate porosity and excludes spray loss.
Use the data sheet or label for the exact product and project. Primer, ceilings, trim, cabinets, textured masonry, strong color changes, spray transfer loss, and touch-up material should be modeled separately when their coverage differs.
Frequently asked questions
▶How does the paint calculator subtract doors and windows?
Enter their combined area in the selected wall unit squared. The calculator converts and subtracts it from gross wall area before multiplying by coats and reserve.
▶Why is paint coverage editable?
Coverage varies by product, substrate, porosity, texture, preparation, color change, and application method. Use the value for the exact paint and conditions.
▶Does each coat use the full paintable area?
Yes. The model multiplies paintable area by the whole-number coat count, then applies the reserve percentage.
▶How are whole cans calculated?
Exact gallons are divided by the can size you enter, then rounded upward because a fraction of a can is not a purchasable unit.
▶Does this include primer, ceilings, trim, or spray loss?
Not automatically. Calculate surfaces or products with different coverage separately, and include a reserve or adjusted coverage only when supported by the product and application plan.
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Open calculator →Disclaimer: Paint Calculator results are estimates for general information and education only, and are not financial, tax, legal or medical advice. Verify important decisions with a qualified professional.