Running out of paint mid-job means a second supply-house run and a color-match risk. Buying three extra gallons ties up cash and leaves half-used cans in the garage. Here is a straightforward way to estimate paint for a room, then check it with a calculator so you are not guessing from the ladder.
Paint quantity is wall (and ceiling) area divided by coverage per gallon, times the number of coats, plus a small waste factor. Write it once and reuse it on every bedroom, hallway, and accent wall.
Most interior latex lists 350–400 sq ft per gallon on a smooth, primed surface. Textured walls, bare drywall, deep color changes, or cheap paint that needs a third coat often cover closer to 250–300 sq ft. When the can says 400 and the wall is porous, believe the wall. Exterior masonry and rough cedar are a different game — check the product data sheet, not the living-room rule of thumb.
You do not need perfect architectural takeoffs for a bedroom. A standard interior door is about 21 sq ft (3×7). A typical double-hung window is often 12–20 sq ft. If you are unsure, measure width × height and round up. Subtracting openings keeps you from buying a full extra gallon “just in case” on every small room. Closets: include them if you are painting them; skip them if the client said walls only.
Room is 12 ft by 14 ft with 8 ft ceilings, one door (21 sq ft), and two windows (15 sq ft each).
Walls only, two coats, 350 sq ft/gal coverage, 10% waste:
365 × 2 ÷ 350 × 1.10 ≈ 2.3 gallons → buy 3 gallons (or two gallons plus a quart if your supplier can tint an exact match and you accept the risk of a second trip).
Walls and ceiling together with the same assumptions: (365 + 168) × 2 ÷ 350 × 1.10 ≈ 3.4 gallons → buy 4 gallons of wall paint, and treat ceiling paint as a separate SKU if the sheen or color differs. Flat ceiling white is usually not the same can as eggshell wall color.
New drywall, water stains, smoke, or a jump from dark to light almost always need primer. Count primer as its own coat in the math. PVA drywall primer on new board is cheap insurance against flashing. Shellac-based spot primer still wins on knots and water stains.
Flat or matte hides better on imperfect walls; eggshell or satin wipes cleaner in halls and kids’ rooms; semi-gloss on trim and wet areas. Sheen barely changes gallon math, but a bargain paint that needs three coats doubles your labor even when the can count looks fine. Trim and doors are closer to linear feet than wall gallons — a typical bedroom trim package is a quart to a half gallon of enamel, not another wall gallon.
Add 10% for normal cut-in and touch-ups. Use 15% for heavy texture, lots of windows, two-tone walls, or accent walls with sharp tape lines. Keep a labeled quart of the final color for repairs six months later; that is cheaper than a callback with a mismatched shelf can. If you are a pro quoting jobs, put leftover policy in the quote so the client knows whether unused gallons stay on site.
Material List Calculator Pro ($19) handles several rooms, adjustable waste, and your own supply-house prices in one shopping list. One-time purchase, no subscription.
Most interior latex covers about 350–400 sq ft per gallon for one coat on a smooth, primed surface. Rough or porous walls can drop that to 250–300 sq ft.
Usually yes for color changes or new drywall. Two coats means roughly double the paint, minus a little because the second coat often covers slightly better.
Only if you are painting it. Ceiling area is length × width. Walls are 2 × (length + width) × height, minus openings.
Add 10% for cut-in and touch-ups, or 15% if the walls are textured or you are cutting in a lot of trim.