Interior & Exterior Paint Calculator
Calculate exact paint gallons and quarts for interior rooms, ceilings, and baseboard trim. Automatically handles 1 vs 2 coat coverage, surface porosity (smooth drywall vs unprimed plaster/stucco), and opening deductions with a complete materials order list.
Room Dimensions & Coats
Paint Order Takeoff
Room Elevation & Finish Sheen Guide
Visual finish zoning: Flat on ceilings (hides imperfections), Satin/Eggshell on walls (washable balance), and Semi-Gloss on trim (moisture & scuff resistant).
Paint Coverage Formulas & Spreading Rate Constants
Paint coverage is governed by dry film thickness (DFT) and wet film thickness (WFT) spreading rates. One gallon of standard latex paint contains $231 \text{ cubic inches}$ of liquid. At an industry standard spreading rate of $350 - 400 \text{ sq ft/gallon}$, each coat dries to a thickness of approximately $1.5 - 2.0 \text{ mils}$ ($0.0015''$).
A_{\text{walls}} = \Big[ 2 \times (L + W) \times H \Big] - \Big[ (21 \times N_{\text{doors}}) + (15 \times N_{\text{windows}}) \Big]
2. Wall Paint Gallons Needed:
G_{\text{walls}} = \left\lceil \frac{A_{\text{walls}} \times N_{\text{coats}}}{\text{Spreading Rate (e.g. 375 sq ft/gal)}} \right\rceil
3. Ceiling Paint Gallons:
G_{\text{ceiling}} = \left\lceil \frac{L \times W}{375} \right\rceil
4. Trim & Casing Linear Footage:
\text{LF}_{\text{trim}} = 2 \times (L + W) + (17 \times N_{\text{doors}}) + (16 \times N_{\text{windows}})
\text{Trim Coverage} \approx 1 \text{ quart per } 120 \text{ linear feet}
5 Critical Painting & Surface Prep Pitfalls
1. The "Paint + Primer in One" False Promise
"Paint and Primer" products are simply higher-solid paints, NOT true primers. Applying them directly over raw drywall mud or bare plaster causes "flashing" (dull spots where porous gypsum sucks moisture out of the paint). Raw drywall always requires a dedicated PVA drywall sealer.
2. Premature Second Coat Application
While latex paint feels dry to the touch in 1 hour, chemical coalescing takes at least 4 hours. Rolling a second coat too early re-wets the partially cured undercoat, pulling it off the wall on the roller cover and creating rough, permanent orange-peel lumps.
3. Wrong Roller Nap Thickness for Surface Texture
Using a 1/2" or 3/4" thick roller nap on smooth drywall leaves heavy stipple texture that ruins smooth finishes. Conversely, using a 3/8" short nap on textured walls or masonry skips valleys and leaves microscopic pinholes that expose bare primer.
4. Flat Sheen in Bathrooms and Kitchens
Flat paint has zero resin sheen barrier, making it porous and vulnerable to water absorption. When applied in high-humidity bathrooms, steam penetrates the paint film, nourishing mildew and causing surfactant leaching (brown oily streaks). Bathrooms require Satin or Semi-Gloss.
5. Leaving Painter's Tape on Too Long
Standard blue painter's tape cures and hardens over time. If left on baseboards or trim for more than 48 hours—or if removed after paint has completely dried into a rubbery plastic film—pulling the tape tears the cured wall paint right off the sheetrock. Remove tape while the final coat is still damp.