Paint Calculator
Calculate exactly how many gallons of paint you need. Enter room dimensions (length, width, and ceiling height), subtract doors and windows, and get coverage based on the industry-standard rate of 350 sq ft per gallon for interior latex paint. Works for any brand — Sherwin-Williams, Benjamin Moore, Behr, and more. Your result includes a supplies checklist with quantities sized to your room.
Room Dimensions
Enter room length and width to see your estimate.
What You Actually Need to Paint a Room — Supplies, Specs, and Where the 350 Rule Bends
The seven supplies that earn their place — and the boxed kits that don't
The checklist the calculator builds has seven items on it, and each one does a job nothing else on the list can: a roller frame with covers lays paint on the open wall, an angled sash brush cuts the edges the roller can't reach, a tray holds the working paint, tape protects baseboards and casings, a drop cloth catches what falls, an extension pole reaches the ceiling line, and spackle with a sanding sponge fixes the wall before any of that starts. Nothing on the list is padding, and two of the quantities — tape rolls and drop cloths — are computed from your room's dimensions rather than guessed.
What's deliberately not on the list is the shrink-wrapped “room painting kit.” The price looks right until the knit cover sheds lint into your first coat and the plastic tray cracks at the second pour. Buying the pieces separately costs roughly $60 to $90 from zero, and the difference shows up on the wall: a woven 9-inch roller cover holds its fibers, a metal tray outlives the project, and the frame, brush, pole, and canvas cloth are one-time buys that survive years of repaints. The per-room consumables come down to tape, tray liners, and roller covers — everything else amortizes to almost nothing by your second room.
Roller nap, brush angle, tape width — the three specs that decide the finish
Nap is the fiber length on the roller cover, and it should match the surface. Use 3/8-inch on smooth drywall and plaster — the standard interior wall — where it holds enough paint to cover without stippling the finish. Move up to 1/2-inch on light texture like orange peel, and 3/4-inch on heavy texture like knockdown or masonry, where the longer fibers push paint into the low spots a short nap skips over. Going the other way, a 1/4-inch nap or a foam roller gives the flattest finish on doors, cabinets, and trim. Longer nap always carries more paint, but it leaves more texture behind — pick by surface, not by “more is better.”
The brush that matters is a 2.5-inch angled sash brush with synthetic filament (natural bristle goes limp in water-based paint). The angle is the point: it lets the bristle tips ride a straight line where wall meets ceiling, which is what cutting in is. For tape, 1.88-inch is the width that protects a baseboard from roller splatter; the narrower 1.41-inch rolls save a little money on door casings. One caveat that saves a repaint: if the surface you're taping was painted less than 30 days ago, use delicate-surface tape, because standard tape can lift paint that hasn't fully cured. Pull the tape while the final coat is still slightly wet, at a 45-degree angle, and the line stays crisp.
Where the 350 sq ft rule bends
The calculator's coverage rate — 350 sq ft per gallon — is the number Sherwin-Williams, Benjamin Moore, and Behr all publish for interior latex on smooth, primed drywall. Three situations pull real coverage below it. Texture is the first: the same orange peel that wants a longer nap also has more surface area, so plan on 280 to 300 sq ft per gallon. New or unprimed drywall is the second — the paper face and joint compound drink the first coat, and since primer costs less than paint, priming first and then applying two topcoats is cheaper than three coats of the good stuff. A drastic color change is the third: covering a deep color with a light one, or painting a saturated red, orange, or bright yellow, routinely takes a third coat, because deep and clear bases carry less of the titanium dioxide that does the hiding. A gray-tinted primer under a deep color is the trade secret there — it kills the old color in one coat so the finish coats only have to build depth.
“Paint and primer in one” deserves its own sentence: it works when you're repainting a sound, previously painted wall in a similar color, and it is not a substitute for real primer on bare drywall, patches, or stains. If any of these caveats apply to your room, the coats selector above is the honest fix — set it to 3 and let the math absorb the extra coat.
Buy every gallon at once, box them, and keep the leftover
Tint is mixed per can, and two gallons of the same color mixed on different machines — or the same machine on different days — can land a visible shade apart. The fix is old and free: buy all your gallons in one batch, then “box” them by pouring every can into one larger bucket and stirring before you start. One uniform color, no mid-wall shift where can two took over from can one. It's also the reason to trust the calculator's round-up rather than buying gallon by gallon as you run short.
Work order matters more than technique: spackle and sand first, wipe the dust, tape, then cut in one wall with the sash brush and roll that same wall while the cut-in edge is still wet. Keeping that wet edge is what prevents the picture-frame effect, where brushed edges dry darker than the rolled field. Roll in a loose W and fill it in without lifting the roller. When the room is done, seal the leftover paint in its can, label it with the room and date, and keep it — touch-ups from the original batch disappear; touch-ups from a new can often don't. Before any of this, the finish decision is worth five minutes: our paint sheen guide covers which sheen fits which room, and how to test paint samples covers proving the color itself before you commit to gallons.
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Next Steps After Calculating
Once you know how much paint to buy, use these free Paint Color HQ tools to finalize your project:
- Preview your color — open the Room Visualizer to see your chosen shade on walls before purchasing.
- Compare shades side by side — the Color Comparison tool shows colors from Sherwin-Williams, Benjamin Moore, Behr, Valspar, PPG, Dunn-Edwards, and Farrow & Ball next to each other with Delta E difference scores.
- Choose the right finish — read our paint sheen guide covering flat, eggshell, satin, semi-gloss, and high-gloss — with recommendations by room type.
- Build a full palette — use the Palette Generator to create coordinating color schemes for your entire home.