Enter your room dimensions and get an exact shopping list — one trip, not four.
Based on the dimensions on the left — edit them to match your project and this updates as you type.
Everything you need — one trip
3 gal covers your surface area with your chosen coat count
Tip: Buy the same batch number (lot) to avoid sheen variation between cans.
Seals the surface for better color coverage and adhesion
Tip: Tint your primer to match the topcoat — reduces coats needed.
3/8" nap for smooth/eggshell walls. Use 1/2" for textured walls.
Tip: Don't wash rollers between coats — wrap in plastic wrap overnight.
Standard 9" frame fits all standard covers
Liners make cleanup fast — swap instead of wash
For cutting in at edges, corners, and trim
Tip: Cut in one wall at a time, then roll before it dries for seamless blending.
Blue tape for trim, outlets, and ceiling lines
Tip: Remove tape while paint is still slightly wet for clean lines.
Canvas stays put and won't slip like plastic sheeting
Saves your back on ceilings and high walls — don't skip this
Tip: Attach roller to pole at a slight downward angle to avoid splattering.
Check if you need wood filler or spackle for nail holes, a putty knife, and a sanding block. Most forgotten trip #2 causes.
• Wall area = 2 × (length + width) × ceiling height
• Ceiling area = length × width
• Door deductions: 20 sq ft each · Window deductions: 15 sq ft each
• Paint coverage: 400 sq ft per gallon · Primer: 300 sq ft per gallon
• Results rounded up to nearest whole gallon
Sherwin-Williams states that a gallon covers about 350 to 400 square feet, and adds its own caveat: “depending on wall texture and desired coverage, having a little more than a gallon on hand may be best.” That range is measured on smooth, primed, previously painted drywall. Very little in a real house is all three of those things at once.
Texture is what eats the difference. Light orange peel — the finish on most drywall hung since the 1980s — pulls coverage toward the bottom of that range, because the surface has more actual area than its length times its height. Knockdown, stucco and bare brick take it lower again. Bare drywall is a separate problem: the paper face and the joint compound drink the first coat at different rates, which is why an unprimed wall can look patchy after two coats and fine after three.
This calculator works from 400 — the optimistic end of that range — and rounds every result up to a whole gallon. On smooth, primed drywall that is right. On anything textured, add a gallon to whatever it gives you, and treat the leftover as the touch-up can you will want in two years when a chair back scuffs the paint.
Skipping primer is the usual way a two-coat job becomes a four-coat one. Priming a wall that did not need it is the usual way to buy a gallon you never open. Prime when one of these is true:
A wall already painted a similar color in flat or eggshell needs none of this. Wash it, patch it, put the paint on.
Walls only, 8 ft ceilings, one standard door (20 sq ft) and the windows noted (15 sq ft each), at 400 square feet per gallon — the same figures the calculator above uses. Double the gallons for two coats. The calculator adds the ceiling on top of this by default, which is another length × width, so its total for the same room will read higher than the row here.
| Room | Wall area | One coat |
|---|---|---|
| Bathroom, 5 × 8, one window | 173 sq ft | 0.4 gal |
| Bedroom, 10 × 12, two windows | 302 sq ft | 0.8 gal |
| Bedroom, 12 × 14, two windows | 366 sq ft | 0.9 gal |
| Living room, 14 × 20, three windows | 479 sq ft | 1.2 gal |
| Open plan, 20 × 30, four windows | 720 sq ft | 1.8 gal |
Round up to the next whole gallon rather than buying a quart to make up a shortfall. A quart costs more than half a gallon does, and it is tinted separately, so it is the batch most likely to read slightly different on the wall.
A 12 × 12 ft room with 8 ft ceilings has 384 square feet of wall before you subtract anything. Take out a standard door (20 sq ft) and two windows (15 sq ft each) and you are at 334. At 400 square feet per gallon that is a little over three quarters of a gallon per coat, so two gallons covers two coats with enough left for touch-ups. Buy the second gallon at the same time as the first — paint is tinted in batches, and a gallon mixed three weeks later can be visibly off against the first.
Subtract them once the openings pass roughly a tenth of the wall area, which in practice means any room with more than one window. Below that, leaving them in gives you a margin you will use on touch-ups. A standard 3 ft by 6 ft 8 in interior door is 20 square feet, and a typical double-hung window about 15 — the figures this calculator deducts.
Only on a small room. A gallon covers 350 to 400 square feet in one coat on smooth, primed, previously painted drywall — so it covers 175 to 200 square feet in two. That is a bathroom or a small hallway. For a bedroom, plan on two gallons.
The ceiling is length × width, so a 12 × 12 room adds 144 square feet — well under half a gallon per coat. Ceilings are usually painted flat white in a dedicated ceiling paint, which is formulated thicker to cut spatter and to hide the roller lap marks that show badly on a large uninterrupted surface.
3/8 inch for smooth drywall and plaster, 1/2 inch for light orange-peel texture, and 3/4 inch or more for knockdown, stucco or brick. Nap that is too short leaves the valleys of a texture unpainted and reads as thin coverage, which is the usual reason a wall seems to need a third coat.
It needs more coats rather than more square footage per coat, and the two amount to the same trip back to the store. Saturated reds, deep blues and true yellows carry less opaque pigment and commonly take three coats over a light wall. A grey-tinted primer underneath cuts that to two more often than a white primer does.
Written and maintained by the BuildGuiders team. The 350–400 sq ft per gallon figure is Sherwin-Williams' own published spread rate, on its paint calculator page. Product recommendations are researched and compared against manufacturer specifications and verified buyer feedback — we do not lab-test paint. One thing we could not pin down: no manufacturer publishes a spread rate for knockdown or stucco, so “lower again” is deliberately vague rather than a number we would have had to invent.