Every number has a job. Here is the math.

What this model is and is not

QuoteCoat is an educational planning model for ordinary interior repainting. It does not inspect a surface, identify a coating, see repairs, price a contractor's actual overhead, or apply local tax, licensing, lead, asbestos, or disposal rules. A written quote after a site visit is different evidence.

The formulas below describe the calculator code in production as of July 25, 2026. No contractor, paint manufacturer, government agency, or paid expert has reviewed or endorsed QuoteCoat. The cited sources support specific reference points; the productivity, price, prep, texture, and markup ranges remain QuoteCoat modeling judgments.

1. Turn dimensions into paintable area

1. Gross wall area

   2 × (length + width) × height × room count. All dimensions are in feet and the result is square feet.

2. Openings

   We subtract 21 sq. ft. per door and 15 sq. ft. per window. Those are simple typical opening allowances, not published standards. The subtraction is capped at gross wall area so net wall area cannot become negative.

3. Ceiling and trim

   Ceiling area is length × width × room count. Trim is estimated as visible face area: 5.5-inch baseboard around the perimeter, plus 3.5-inch casing along 17 linear feet per door and 14 linear feet per window.

4. Selected and coated area

   Only checked surfaces (walls, ceiling, and trim) are added. Selected area is then multiplied by the chosen number of coats to produce coated surface area.

2. Estimate paint and primer

Sherwin-Williams says a gallon of paint typically covers about 350–400 sq. ft. and a gallon of primer about 200–300 sq. ft., while warning that method, texture, and surface condition change product requirements. QuoteCoat uses those published ranges as its smooth-surface baseline.

Texture factors are QuoteCoat assumptions. Raw paint need is coated area divided by 400 × texture factor at the low end and350 × texture factor at the high end, rounded to one decimal gallon. The purchase recommendation uses conservative coverage and rounds walls, ceilings, and trim up separately because they often need different products or colors.

Primer logic

Minimal prep assumes no primer. Standard prep spot-primes 20% of the selected, uncoated surface. Extensive prep assumes one full primer coat. Raw primer need uses the midpoint of the 200–300 sq. ft. range, adjusted for texture; whole gallons use the conservative 200 sq. ft. end. The exact product label always takes priority.

3. Build the materials range

These are editable market assumptions, not retailer quotes. Paint is modeled at $25–$40 per gallon for economy, $40–$65 for standard, and $65–$95 for premium. Primer is $25–$45 per gallon.

Whole-gallon paint, whole-gallon primer, and supplies are added, then multiplied by the selected regional factor. Low, typical, and high material values are rounded to the nearest $5.

4. Estimate single-painter-equivalent hours

Application time divides coated area by productivity. The ranges are 120–180 sq. ft./hour for walls, 90–140 for ceilings, and 25–45 for trim. These are QuoteCoat assumptions, not manufacturer production guarantees. A two-person crew may complete the calendar work faster while using a similar total number of labor hours.

Setup and cleanup add 0.75 + 0.5 × rooms low and1.25 + 0.75 × rooms high. Occupied rooms multiply the combined time by 1.10–1.25. Texture then applies the labor factor in the table above. The result rounds up to the next half-hour; the typical value is the midpoint of the rounded low and high, also rounded up to a half-hour.

5. Convert hours into a professional range

QuoteCoat uses billed labor of $45 low, $65 typical, and $90 high per hour. These are business billing assumptions, not take-home wages. The May 2025 BLS occupation profile for construction and maintenance painters is a useful wage benchmark, but a customer rate also has to carry payroll costs, non-billable time, insurance, tools, vehicles, estimating, administration, and scheduling risk.

For each band, the formula is(regionalized materials + labor hours × billed rate × regional factor) × markup. Markup factors are 1.12 low, 1.22 typical, and 1.35 high. Final professional values round to the nearest $25.

The regional factor defaults to 1.00 and is user-selected; QuoteCoat does not geolocate or automatically import a local index. BEA Regional Price Parities explain the underlying idea by comparing local price levels with the national level, but the calculator's factor is not an official BEA value or a painter-specific index.

6. Check a contractor quote

A quote below the modeled low is labeled below range; one from low through high is fair; one above high but no more than 125% of high is high; and one above 125% of high is very high. These labels describe a model comparison, not contractor quality, honesty, or value.

Effective price per square foot is quote total divided by selected, uncoated paintable surface area, not floor area and not coated area. Repairs, access, premium service, schedule, warranty, licensing, insurance, and a more complete scope can reasonably move a real bid outside the model.

Known exclusions and input limits

Primary sources

Source check date: July 25, 2026. Links go directly to the publisher, not to affiliate redirects.

• Sherwin-Williams paint calculator and coverage FAQSupports the 350–400 sq. ft. paint and 200–300 sq. ft. primer baseline; the page also warns that method and surface condition matter.
• U.S. BLS OEWS profile: Painters, Construction and Maintenance (47-2141)Current national, state, metro, industry, employment, and wage context. Employee wage is not contractor billing rate.
• U.S. BEA Regional Price ParitiesExplains comparisons of local price levels with the national level; QuoteCoat does not automatically import these values.
• U.S. EPA Renovation, Repair and Painting guidance for consumersLead-safe consumer guidance for work that may disturb paint in pre-1978 homes.
• U.S. EPA indoor-air and ventilation practices for remodelingAddresses ventilation, pollutant containment, occupant protection, and following product instructions.