The math is forgiving — pick a coverage number, divide your wall area by it, multiply by the number of coats. What ends up in the garage two years later, half-dried-out, is the second gallon nobody needed because the first one was Behr Marquee and actually covered in one coat. The trick to buying paint isn't the formula. It's knowing which paint covers in one coat, which one needs two, and what the can label is lying about.

The formula

Gallons = ceil( (wall_area − openings) / coverage × coats × 1.10 )

  wall_area   = perimeter × ceiling_height
  openings    = doors + windows (only subtract if they're trimmed —
                you'll cut around the trim either way)
  coverage    = 325 sf/gal premium, 275 mid-tier, 225 first-coat new drywall
  coats       = 1 (premium one-coat) or 2 (everything else)
  × 1.10      = 10% waste

The premium tier — what one-coat actually means

Three paints have credibly delivered one-coat coverage in independent and trade testing for over a decade:

• Benjamin Moore Aura — Color Lock technology, ~$80/gal. The pro consensus on saturated colors (deep red, navy, hunter green) where every other paint needs three coats minimum.
• Sherwin-Williams Emerald — ~$80/gal. Slightly easier to roll than Aura; near-identical coverage. The contractor pick when SW is the closer store.
• Behr Marquee — ~$50/gal at Home Depot. The DIY value champion. One-coat hide on the qualifying base/color combinations only (check the lid sticker — there are about 1,000 colors that qualify and several thousand that don't).

Everything cheaper than this is two coats minimum. Two coats of $35 paint is more expensive than one coat of $80 paint after you factor in the second day of taping, prep, and rolling.

The primer decision tree

• New drywall — always. Use a dedicated PVA primer (Zinsser Bullseye 1-2-3 or Sherwin-Williams ProBlock). Self-priming finish paint is a shortcut that costs you texture flatness.
• Going darker — no primer needed if your finish is Aura or Emerald.
• Going lighter (especially over red, navy, or chocolate) — tinted primer in a gray that's halfway to your finish color. Saves a coat, sometimes two.
• Stains, water rings, smoke, marker — Zinsser BIN (shellac) for water and tannin; Kilz Original (oil) for smoke and nicotine. Latex primer doesn't lock these in — they bleed through every coat above it.
• Glossy or oil-painted trim being recoated — sand to scuff, then INSL-X Stix or Zinsser Smart Prime as a bonding primer. Skip this and the new paint peels in strips.

Sheen — the room-by-room call

• Flat / matte — ceilings always; bedrooms and formal spaces. Hides patches and texture imperfections best. Almost impossible to clean — kid hands and matte don't mix.
• Eggshell — bedrooms, hallways, living rooms. The default modern wall sheen. Wipeable enough for normal life.
• Satin — kitchens, kids' rooms, bathrooms. Wipes clean. Shows roller stipple if you don't use a microfiber cover.
• Semi-gloss — trim, doors, cabinets, exterior trim. The traditional choice — durable, cleanable, signals "trim".
• High-gloss — front doors, accent furniture. Shows every prep flaw — sand the substrate to 220 minimum.

The Floetrol + Penetrol additive trick

Two additives that pros use and DIYers usually don't even know exist:

• Floetrol (latex paint) — extends open time. Brushmarks self-level instead of curing in place. Roller stipple flattens out. Mix 8–16 oz per gallon (more in hot, dry conditions). The fix for the "why does my paint look streaky" problem.
• Penetrol (oil-based paint) — same job, oil chemistry. Use on alkyd trim or BIN primer.
• XIM Latex Extender — alternative to Floetrol; some pros prefer it for its slightly less-tacky open time.

Brushes and rollers — the picks that actually matter

• Wooster Shortcut 2.5" angled sash — the "cuts in better than every other brush" pick on PaintTalk and the BBR (BuilderCalc-listed) brand-pick database. Stubby handle, no fatigue.
• Purdy XL Glide 2.5" angle — the alternative; slightly stiffer, longer handle. Equal favorite in trim work.
• Microfiber 3/8" nap roller — finest finish on smooth walls. Wooster Pro/Doo-Z or Purdy White Dove. Avoid generic woven covers — they shed lint into every coat.
• 9" frame, not 18" — for DIY rooms. The 18" saves time only if you have a sprayer back-rolling team. Solo, you'll smear edges.
• Wooster Sherlock GT or Mr. Long Arm — extension pole. Cuts the ladder time in half.

Worked example: 12 × 14 bedroom, 8-ft ceiling, two coats Behr Marquee

Wall area:    2 × (12 + 14) × 8 = 416 sf
Openings:     1 door (20 sf) + 1 window (12 sf) = 32 sf
Net area:     416 − 32 = 384 sf

Walls (Marquee, 325 sf/gal × 2 coats × 1.10):
  384 / 325 × 2 × 1.10 = 2.6 → buy 3 gal
  (or 1 gal + 1 quart if you use a tinted primer for coat 1)

Ceiling (12 × 14 = 168 sf, 1 coat flat ceiling paint):
  168 / 325 × 1.10 = 0.57 → buy 1 gal

Trim (semi-gloss, ~3× door + 2× window + baseboard, ~150 sf):
  150 / 200 × 2 × 1.10 = 1.65 → buy 2 quarts

Brushes:
  - 1 Wooster Shortcut 2.5" angled (cut-in)
  - 1 Purdy XL Glide 2" (trim)
  - 2 microfiber 3/8" nap rollers (one for primer, one for finish)
  - 1 frame, 1 tray + liners, 1 paint pole

Tape + plastic:
  - Frog Tape Yellow (delicate surface) — fresh-painted trim or wallpaper.
  - Frog Tape Green (multi-surface) — everything else.
  - 2 mil plastic for floors. 9 × 12 drop cloths > plastic for safety.

Box your paint

Two gallons of the same color from the same store on the same day will be slightly different shades. Pour every gallon into a 5-gal bucket and stir. Now your finish is one continuous color top to bottom; if you don't box, you'll see a hairline shade change halfway down the wall under raking light, forever.

The holiday hunt

After your final coat, hold a flashlight or headlamp flat against the wall and walk the perimeter at sunset. Every missed spot — "holiday" in painter slang — shows as a stripe. Touch up wet edge with a damp roller. Skip this step and you'll find them on the first sunny morning instead, after the trim is back on.

Common mistakes

• Painting raw drywall without primer. The paper drinks coat 1, you waste a gallon, the wall still looks splotchy.
• Cheap brushes. The brushmarks are the brush — Wooster or Purdy, no exceptions.
• Skipping the wet edge. Stop in the middle of a wall and the lap mark is permanent.
• Not boxing the paint. Subtle shade-change stripes you can't fix without redoing the wall.
• Painting at 50°F and 80% humidity. Latex needs 50°F minimum, ideally 60–80°F, under 60% RH. Otherwise it crawls and the second coat won't stick.
• Skipping the tinted primer over a saturated color. Three coats minimum to hide red without it. One with.

Run the math on your phone

The BuilderCalc app paint calculator does the gallons formula for any room shape, with separate ceiling and trim line items, and a coverage tier you can dial up or down based on the paint you're actually using. The Paint a room project guide walks the prep, primer, cut-in, roll, and holiday-check steps in order, and the Paint & Finishes reference covers sheen rules, primer decisions, and the brand picks above. Free, offline, on the App Store and Google Play.

Related

• BuilderCalc — DIY project guide + 14 calculators
• How much drywall do I need? Sheets, mud, screws

Note: Coverage values are pulled from manufacturer TDS sheets and field-trade consensus on PaintTalk and Fine Homebuilding. Your wall texture, paint batch, and applicator skill will move the number — buy the extra gallon, return it sealed if you don't open it.