Construction Tools
How many gallons do you actually need? Enter your room size and get an exact count for paint and primer. Brand-neutral. No upsell.
Last reviewed May 2026
Updated May 2026
Deducts 21 sq ft each
Deducts 15 sq ft each
Required for new drywall, bare wood, stained surfaces, or major color changes
Premium paint covers up to 400 sq ft/gal on smooth previously painted surfaces.
Quality multiplied by surface type gives your effective coverage rate.
Sensible default: modify inputs to match your room.
Amazon carries Rust-Oleum, Behr, Glidden, and specialty primers with next-day delivery on many SKUs. Shop paint at Amazon → FTC disclosure: if you click and buy, we may earn a commission at no cost to you.
The core math is straightforward. Gross wall area for a rectangular room is 2 x ceiling height x (room length + room width). Subtract 21 sq ft per standard door opening and 15 sq ft per standard window. That gives you net paintable area.
Multiply net area by the number of coats, divide by your effective coverage rate, and round up to the next whole gallon. Paint is sold in whole gallons: you cannot buy 2.3 gallons, so the ceiling function is correct here.
Coverage rates vary by paint quality and surface condition. This calculator applies a two-factor model: paint quality tier (premium / standard / budget) sets a base coverage rate, and surface type applies a reduction multiplier for porous or rough surfaces.
| Paint Quality | Smooth, Prev. Painted | New Drywall (primed) | Concrete / Masonry |
|---|---|---|---|
| Premium (SW Emerald, BM Aura) | ~400 sq ft/gal | ~320 sq ft/gal | ~240 sq ft/gal |
| Standard (SW SuperPaint, Behr Premium Plus) | ~350 sq ft/gal | ~280 sq ft/gal | ~200 sq ft/gal |
| Budget (Glidden Interior, store-brand) | ~300 sq ft/gal | ~240 sq ft/gal | ~175 sq ft/gal |
Sources: Sherwin-Williams product data sheets, Behr coverage chart, Glidden product specs.
Primer is not optional on new drywall. The paper face of fresh drywall absorbs paint aggressively and unevenly: one coat of paint over unprimed drywall will show lap marks, sheen variation, and bare spots called "holidays" that become visible as the light shifts through the day. One coat of drywall primer (PVA primer is the standard, about 300 sq ft per gallon) seals the paper and gives paint a uniform base to bite into.
Primer is also required when:
Self-priming paints do exist and can work on previously painted surfaces with minor color changes. Most professional painters still apply a dedicated primer coat on any of the situations above because it produces a better result with fewer total coats.
If your estimate is above 4 gallons of a single color, the 5-gallon bucket almost always wins on price. A 5-gallon bucket of interior latex paint in standard quality typically runs $150 to $175 at most home improvement stores (approximately $30 to $35 per gallon). Single gallons of the same paint run $40 to $50 each. The crossover math: 4 gallons at $45 each is $180 versus a 5-gallon bucket at $160. You save money and get better color consistency because all 5 gallons come from a single dye batch.
One note: not every color can be mixed into a 5-gallon bucket from every paint line. Deep bases for dark colors sometimes have mixing limitations. Ask your paint counter before assuming a 5-gallon order will work for your specific color.
The single biggest mistake homeowners make is using the coverage rate on the paint can without accounting for surface condition. A can that says "covers up to 400 sq ft per gallon" was tested on a smooth, previously painted surface in a controlled environment. New drywall, concrete block, rough plaster, and unpainted wood all absorb significantly more paint and will drive your coverage rate down 15 to 40 percent depending on the surface.
Contractors call the first coat on a porous surface the "filler coat": it goes into the surface rather than onto it. The second coat is where you start building actual coverage and color depth. This is why two coats is the professional standard even when one coat looks good in the afternoon light. The morning light will reveal the difference.
"New drywall will eat your paint. I always tell homeowners: budget for 2 gallons per room more than the calculator says if they skip primer. They never believe me until the second gallon runs out." (r/DIY thread on bedroom repaints)
"5-gallon buckets for anything over one room. Color matching between cans is not always perfect even with the same formula." (r/HomeImprovement)
"Two coats minimum. Period. One coat looks fine in fluorescent light. Natural light destroys it." (r/Painting)
If you just sized your drywall order, paint is the natural next calculation. FigureNerd's construction tools are built around this reality: when you are doing drywall, you can count on flooring, paint selections, trim, and carpet coming right behind it. These tools are designed to work together, in construction order, so you can size one job and immediately move to the next.
Flooring, trim, and carpet estimators are in development and will link from this page when ready.
Coverage per gallon depends on paint quality and surface type. Premium paints (Sherwin-Williams Emerald, Benjamin Moore Aura) cover approximately 400 sq ft per gallon on smooth previously painted surfaces. Standard mid-grade paints cover about 350 sq ft per gallon. Budget paints cover around 300 sq ft per gallon. New drywall and porous surfaces absorb more paint and reduce effective coverage by 10 to 25 percent. Textured or rough surfaces reduce coverage further. Always use the coverage rate on your specific paint can as the ground truth.
Primer is required on new drywall (the paper face absorbs paint aggressively and causes uneven sheen without primer), bare wood, stained surfaces, and when making a dramatic color change (dark to light or light to dark). Primer is optional on previously painted surfaces in good condition when the color change is minor. Self-priming paints claim to eliminate this step, but most contractors still apply a separate primer coat on new drywall and color changes over 2 tonal steps. Primer coverage rate is approximately 300 sq ft per gallon across surface types.
Two coats is the industry standard for most residential paint jobs. One coat may be acceptable when repainting an identical or very similar color on a smooth, well-prepared surface. Three coats are needed when covering a deep saturated color with a light color, painting over stains that bleed through, or when the surface texture is uneven and the first two coats reveal holidays (missed spots). Self-priming paint used as the first coat can reduce a 3-coat job to 2 coats in some situations.
A 5-gallon bucket of standard interior latex paint typically costs $150 to $175 versus $40 to $50 per gallon, which works out to $30 to $35 per effective gallon versus $40 to $50. The break-even is roughly 4 to 4.5 gallons. If your estimate exceeds 4 gallons of a single color, the 5-gallon bucket saves money and helps ensure color consistency (mixing a second can of the same color code can produce a visible batch difference). If you need less than 4 gallons, single gallons are the right call.
A standard interior door opening (3 ft wide by 7 ft tall) covers 21 sq ft of gross wall area. When calculating net wall area for paint, deduct 21 sq ft per door. This is the same deduction used for drywall estimating and is the standard recognized by most painting contractors and published estimating guides. For sliding doors or double doors, measure the actual opening and use that area.
A standard double-hung window (3 ft wide by 5 ft tall) covers 15 sq ft of gross wall area. Deduct 15 sq ft per standard window when calculating net paintable area. Oversized windows, picture windows, or bay windows should be measured individually. If you are painting window trim separately, that calculation is not included in wall paint estimates.
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