Paint estimating is one division problem — surface area divided by coverage rate, times the number of coats. The art is in the coverage rate, which swings wildly with the surface: a sealed wall stretches a gallon to 400 square feet, raw block barely gets 100. This guide runs from room area to film thickness and cost — distilling the 100 questions painters and homeowners ask most into one readable pass.
Every paint estimate is the gross surface area, minus the openings, times coats, divided by the coverage rate. Gross wall area is the room perimeter times ceiling height; a ceiling matches the floor footprint (length × width).
Subtract doors and windows because they're trim-painted separately — leaving them in just makes you over-buy wall paint. For odd rooms (cathedral or sloped ceilings), split the surfaces into rectangles and triangles, area each, and sum.
Perimeter = 54 ft × 8 ft = 432 sq ft gross. Subtract one door and one window (~36 sq ft) → ~396 sq ft net. Two coats = 792 sq ft ÷ 395 ≈ 2 gallons.
This is the variable that makes or breaks an estimate. The "1 gallon = 400 sq ft" figure only holds on smooth, sealed surfaces. Porous or textured surfaces have more actual area and soak up paint, dropping coverage fast:
| Surface | Coverage / gallon |
|---|---|
| Previously painted, non-porous | ~400 sq ft |
| Smooth, primed drywall | 350–400 sq ft |
| Fresh unprimed drywall | 250–300 sq ft |
| Unpainted plaster (primer coat) | ~250 sq ft |
| Textured stucco exterior | 200–250 sq ft |
| Brick / heavy masonry | 200–300 sq ft |
| Raw concrete block (CMU) | 100–150 sq ft |
| Block filler on CMU | 75–125 sq ft |
Rough profiles like stucco, brick, or deep orange-peel can take up to 20% more paint than a flat wall because the peaks and valleys add real surface area. Porosity is the other driver: raw drywall, fresh plaster, and bare brick act like sponges on the first coat, which is exactly why a primer (or block filler on CMU) pays for itself.
Most jobs are two coats. The big exception is deep, vibrant colors — bright reds, dark blues, intense yellows have less hiding pigment, so they need 3–4 coats or a tinted gray primer to block out the old color. A paint's contrast ratio measures that opacity: a low ratio means more coats to hide what's underneath.
Sheen also nudges the numbers. Glossier paints (semi-gloss, gloss) carry more resin, can be slightly less opaque on the first coat, and demand a smoother wall because they reflect light and reveal every bump — that's extra prep in your labor estimate. Flat finishes hide flaws and are the norm for ceilings (~350 sq ft/gal).
Walls and ceilings use the area math above; trim is its own quick calculation. Baseboards are the room perimeter (minus door widths) × ~0.33 ft (a 4″ board), but the fastest path is the linear-foot rule:
Other handy interior figures: a hallway ceiling (say 4 × 20 = 80 sq ft) at two coats is just 160 sq ft — a single quart does it (a quart covers ~85–100 sq ft). Window casings run ~10 sq ft of trim each. A small bathroom (6 × 8, ~190 sq ft net) is about 1 gallon of moisture-resistant paint over two coats.
Exterior walls follow the same perimeter × height logic, then add the gable triangles and subtract big openings. Exterior work carries a default 15% waste factor for wind overspray and texture.
A few exterior adjustments worth remembering: lap siding's overlapping edges add area, so multiply the flat area by 1.15; brick needs an elastomeric/masonry coating at ~200 sq ft/gal first coat, ~300 second; soffits and fascia are roofline length × overhang width at a 350 trim rate. For piece items, a single-car garage door is 8 × 7 = 56 sq ft and a double is 16 × 7 = 112 sq ft (both two coats); a shutter is ~12 sq ft × 2 sides = 24 sq ft; an entry door is ~20 sq ft per side. Louvered doors and shutters multiply the flat area by 2–2.5× for all those slats.
Primer is a separate line item with its own lower coverage (250–300 sq ft/gal) because it penetrates and seals raw surfaces. A 1,200 sq ft area of raw drywall at 300 sq ft/gal needs exactly 4 gallons of primer. You can skip a dedicated primer only over a clean coat of the same paint type, or with a quality paint-and-primer product on a smooth non-porous wall. Two specialty primers matter: block filler for CMU (a thick primer at just 75–125 sq ft/gal that fills the pores) and an oil-based stain-blocking primer for bare wood, which seals tannins that would otherwise bleed yellow through latex.
For spec and industrial work, coverage is governed by film thickness rather than a rule of thumb. A mil is one-thousandth of an inch. Wet film thickness (WFT) is the layer right after application; dry film thickness (DFT) is what's left after the solvent evaporates — the difference is the paint's volume solids.
So a paint at 40% solids applied at 4.0 mils wet dries to 1.6 mils; to hit a 2.0-mil DFT spec with a 50%-solids paint you apply 4.0 mils wet. This is the theoretical maximum — real coverage is always lower once you subtract application losses.
How you apply the paint changes how much you buy. Brushes and rollers transfer efficiently (about 5% waste). Airless sprayers run only 60–70% efficient — 30–40% is lost to overspray fog — so bump the waste multiplier from 1.10 up to 1.30–1.35 for spray work.
| Method | Multiplier |
|---|---|
| Brush & roller (interior) | 1.10 |
| Brush & roller (exterior) | 1.15 |
| Airless sprayer | 1.30–1.35 |
Two hidden losses with sprayers: a 50-ft hose holds up to a quart that's lost at cleanout, and a thick (3/4″ nap) roller cover soaks up more paint than a thin one. Factor both into larger jobs.
Paint comes in quarts, gallons, and 5-gallon buckets. A 5-gallon bucket covers 1,750–2,000 sq ft of smooth wall in one coat and lowers your unit cost. Key conversions: 1 gallon = 4 quarts = 3.785 liters, and a liter covers about 8–10 m². When buying multiple gallons of a tinted color, "box" them — mix all the cans together in one bucket — so minor batch-to-batch color differences disappear.
Total cost stacks paint, primer, and tint against local prices, plus tools and tax.
| Item | Typical figure |
|---|---|
| Premium interior paint | $45–$85 per gallon |
| Professional labor | $1.50–$3.50 per sq ft |
| Painter output (brush/roller) | 150–200 sq ft per hour |
| Painter output (sprayer) | 800–1,000 sq ft per hour |
| Recoat window | 2–4 hours between coats |
Don't forget accessory line items: tape, drop cloths, extension poles, trays, sandpaper, and roller covers. Latex dries to the touch in about an hour but wants 2–4 hours before a second coat, and a sealed can keeps up to ~10 years if it never freezes. Low/zero-VOC acrylics are now standard for interiors.
Manual math and apps differ because apps round up to whole gallons and bake in a waste buffer. Round up yourself — 4.2 gallons means buy 5 (or 4 gallons plus a quart). A few finishing notes:
Map wall lengths and heights with a laser or tape, subtract the real opening sizes, re-run the gallon count, and cross-check against the supplier invoice before you crack the first can.