Measure a rectangular building, add triangular gables, subtract standard openings, and adjust for surface texture. The result estimates finish paint, optional primer, supplies, labor, and cost scenarios.
Exterior calculator Content updated 2026-08-14Calculations run privately in your browser
Published scope: exterior-elevation geometry, triangular-gable arithmetic, opening deductions, gallon and container rounding, default example arithmetic, and limitations tied to the cited paint-manufacturer guidance.
The byline does not claim professional credentials, trade, finance, insurance, actuarial, underwriting, or claims experience, licensed review, or independent professional review for either author.
Enter exterior measurements
Example values are prefilled
How to use this exterior paint calculator
Estimate exterior wall and gable area, paint and primer containers, supplies, labor, waste, and project cost.
Formula
Gross wall area = 2 × (length + width) × wall height + gable count × gable base × gable height ÷ 2. Net modeled area subtracts standard openings and applies the texture factor. Product volume = net area × coats × (1 + allowance %) ÷ labeled coverage.
Quick example
Example: A 40 × 28 ft building with 10 ft walls, two 28 × 6 ft gables, three doors, and twelve windows has 1,285 sq ft after standard opening deductions. A texture factor can increase the modeled coating area before coats and waste are applied.
Accuracy and limitations
Complex footprints, dormers, soffits, fascia, trim, porches, shutters, masonry absorption, overspray, repairs, color change, and unusually sized openings require separate measurements. Never rely on a generic spread rate when a product label is available.
The rectangular model assumes two walls use the building length and two use the building width; measure additions, bays, dormers, and attached garages as separate shapes rather than stretching one dimension.
Measure gables vertically
Use the average vertical wall height only for the rectangular portion. For a triangular gable, measure the horizontal base and the vertical rise to the peak; the calculator applies one-half × base × height for each gable.
Use product-specific coverage
Replace the default coverage with the exact label or technical-data value for the chosen coating and substrate. Rough masonry, weathered wood, deep profiles, spraying, and a dramatic color change can reduce field coverage.
Check opening deductions
Deduct standard doors and windows only when their sizes reasonably match 21 and 15 square feet. Measure garage doors, window walls, and unusually large openings directly and adjust the modeled area outside this simplified count.
How the calculation works
Build gross elevation area
The calculator multiplies the building perimeter by average wall height, then adds the triangular area of every entered gable.
Model the surface actually coated
Standard opening deductions are removed and the selected texture factor increases the remaining area to represent additional surface demand.
Convert area to paint demand
Modeled area is multiplied by finish coats and the allowance percentage, then divided by labeled square feet per gallon.
Round to purchasable containers
The unrounded gallons are divided by the selected container size and rounded up, because a fraction of a sealed container cannot be purchased.
Worked calculation: A rectangular house is 40 ft long by 28 ft wide with 10 ft average walls, two 28 ft × 6 ft triangular gables, three standard doors, twelve standard windows, smooth siding, two finish coats, 10% allowance, and 350 sq ft/gal coverage.
Result: With one-gallon containers, round 8.08 gallons up to nine gallons. At the default $58 per gallon, finish paint costs $522 before primer, supplies, labor, taxes, and preparation.
Next step: Check the chosen coating's labeled spread rate on the actual siding, then separately measure trim, soffits, fascia, garage doors, and any nonstandard openings.
Interpret the result
Measured versus modeled area
Measured area reflects the simplified building geometry after opening deductions; modeled area includes the texture adjustment and should not be mistaken for a survey measurement.
Required gallons
This is the mathematical volume based on the entered spread rate. It is useful for comparing assumptions but is smaller than the rounded purchase quantity whenever a partial container remains.
Cost range
Product and labor totals use the editable prices and rates, not a contractor bid. Add access equipment, repairs, caulk, masking, trim coatings, taxes, and disposal when they are in scope.
Common mistakes to avoid
Using roof-slope length as gable height instead of the vertical rise from the gable base to the peak.
Counting two finish coats but entering a manufacturer spread rate that already describes coverage for the entire coating system.
Applying one smooth-siding coverage assumption to brick, stucco, shingles, and deeply textured siding.
Forgetting soffits, fascia, trim, shutters, porch ceilings, detached structures, and access or masking costs.
Deducting every small opening even though cutting, masking, and roller loss may offset the saved paint.
Decision checklist
Confirm which elevations, gables, doors, trim, and specialty surfaces are included in the painting scope.
Inspect for peeling paint, moisture, rot, chalking, mildew, failed caulk, and repairs before estimating labor.
Verify primer need, coverage, coat count, temperature range, recoat time, and application method on the selected product documents.
Price the actual container sizes available locally and retain compatible product for labeled storage and future touch-ups.
Obtain a site-specific professional quote when height, lead-safe work, difficult access, or extensive preparation affects the project.
Frequently asked questions
Should trim be included?
Measure trim, doors, shutters, and specialty coatings separately because their products, application methods, and coats often differ.
Should every window be deducted?
Standard deductions improve the estimate, but small openings may be offset by cutting and masking losses. Measure unusually large glass and doors directly.
Does rough masonry use more paint?
Often, but the product, porosity, profile, and application method control. Use a verified spread rate or a conservative editable texture factor.
How do I handle an L-shaped house or an addition?
Break the exterior into rectangles and triangles, calculate their areas separately, and combine them. The length-and-width model is intended for one rectangular building footprint.
Should I buy the exact calculated gallons?
Plan around purchasable container sizes and keep a reasonable amount for touch-ups. Confirm return and tint policies before deliberately overbuying custom-tinted paint.
Does the labor estimate include preparation and high access?
Not automatically. The editable labor rate must reflect scraping, repairs, masking, height, lift or scaffold access, application method, and local conditions.
Related field-planning guides
Use these workflows to verify measurements, product assumptions, purchasing quantities, and conditions before relying on the estimate.
