How to measure for your paint project
- Measure your wall area. Add up the perimeter of the room (length + width + length + width) and multiply by the ceiling height to get total wall square footage. A 12 × 12 room with 8 ft ceilings gives 2 × (12 + 12) × 8 = 384 sq ft. Subtract roughly 21 sq ft per standard door and 15 sq ft per average window if you want a tighter estimate, but many painters skip the deductions and use the extra paint for touch-ups.
- Decide on coats and coverage. Enter the number of coats in the "Coats" field (typically 2). The default coverage of 350 sq ft per gallon is a safe figure for most latex paints on previously painted walls; bump it to 400 for smooth, primed drywall or if your can label says 400. For ceilings or trim, run a separate calculation — they are different products and not included in the wall area estimate.
- Read the result and buy whole gallons. The calculator outputs whole gallons (it always rounds up). Grab one extra quart for touch-ups and small mistakes — it is far cheaper to buy it now than to rematch the color months later when the batch dye lot may have shifted.
How the paint calculator works
Paint coverage is simple division: multiply wall area by the number of coats to get total painted square footage, then divide by how many square feet one gallon covers. Because you can only buy whole gallons, the result is rounded up. Worked example: a 12 × 16 room with 8 ft ceilings has 2 × (12 + 16) × 8 = 448 sq ft of wall. Two coats = 448 × 2 = 896 sq ft to cover. At the default 350 sq ft per gallon: 896 ÷ 350 = 2.56, rounded up to 3 gallons — which matches the reference table for that room size. If your paint can says 400 sq ft per gallon instead, the same room takes ceil(896 ÷ 400) = ceil(2.24) = 3 gallons, so in this range the difference is small. On a larger wall — say a great room with 640 sq ft — two coats at 350 needs ceil(1,280 ÷ 350) = 4 gallons versus ceil(1,280 ÷ 400) = 4 gallons — still the same. The coverage number matters most on borderline jobs right at a gallon boundary.
Which type are you estimating?
Standard repaint (light color over similar shade)
The everyday scenario: same or similar color going back on clean, previously painted walls. Two coats deliver full hiding without a primer coat, and 350–400 sq ft per gallon is achievable.
Enter: Coats: 2, Coverage: 350–400 sq ft/gal
Dark color over light (or light over dark)
High-contrast color changes — think navy over white or white over red — almost always require three coats or a tinted primer plus two topcoats. A tinted primer cuts the job back to two topcoats while preventing bleed-through. Plan coverage at 350 sq ft/gal because the loaded coverage is not as high when you are working opaque layers.
Enter: Coats: 3 (or primer + 2 topcoats), Coverage: 350 sq ft/gal
New drywall or bare surface
Fresh, unprimed drywall is extremely porous and will soak up an entire coat of paint as a pseudo-primer. Always apply a dedicated drywall primer first, then two coats of finish paint. Run the calculator twice — once for the primer coat (set Coverage to 300–350 for porous surfaces) and once for the two finish coats.
Enter: Primer run: Coats: 1, Coverage: 300–350. Finish run: Coats: 2, Coverage: 350–400
Ceiling only
Ceilings use flat ceiling paint (often sold as "ceiling white") and are measured separately from walls. Ceiling area is simply length × width of the room. A 12 × 12 ceiling is 144 sq ft; at 350–400 sq ft per gallon, one coat takes well under a gallon, but two coats will require 1 gallon. Run a fresh calculation with the ceiling square footage.
Enter: Coats: 2, Coverage: 350–400 sq ft/gal (ceiling paint only)
Exterior walls
Exterior paint typically covers 250–350 sq ft per gallon on rough or textured siding — less than smooth interior walls. Weathered, chalky, or bare wood soaks paint aggressively; use 250 sq ft/gal as your coverage figure and plan for a primer coat first. Two finish coats are standard on new or stripped surfaces.
Enter: Coats: 2, Coverage: 250–350 sq ft/gal depending on texture
Tips & ways to save
- Always buy paint in the same dye-lot batch — color can shift slightly between batches, and a visible seam where lots meet is nearly impossible to fix without repainting.
- Flat and matte finishes hide imperfections but scuff and stain easily — save them for ceilings and low-traffic bedroom walls. Eggshell or satin is more washable and suits living rooms, hallways, and kids' rooms. Semi-gloss or gloss holds up to scrubbing, making it the right call for trim, doors, kitchens, and bathrooms.
- Subtract door and window area to reduce waste: one standard door ≈ 21 sq ft, one average window ≈ 15 sq ft. Skipping deductions is also common — the overage becomes your touch-up supply.
- Stir, don't shake — shaking introduces air bubbles that dry as tiny craters in the finish. Give the can a slow stir for a full minute before rolling.
- A quality roller cover (3/8 in nap for smooth walls, 1/2 in for light texture) holds more paint per pass and leaves fewer roller marks than a cheap cover — the cover cost pays for itself in a single room.
Paint needed by room size (walls, 2 coats)
| Room (8 ft ceiling) | Wall area | Paint, 2 coats |
|---|---|---|
| 10 × 10 ft | 320 sq ft | 2 gallons |
| 10 × 12 ft | 352 sq ft | 3 gallons |
| 12 × 12 ft | 384 sq ft | 3 gallons |
| 12 × 14 ft | 416 sq ft | 3 gallons |
| 12 × 16 ft | 448 sq ft | 3 gallons |
Assumes 8 ft ceilings, ~350 sq ft per gallon, and no deduction for doors and windows (so it runs slightly generous). Smooth, primed walls cover closer to 400 sq ft per gallon — set coverage to 400 in the calculator for those.
Frequently asked questions
How many square feet does a gallon of paint cover?
How much paint do I need for a 12×12 room?
Do I need separate paint for the ceiling?
How many coats of paint do I need when going from a dark color to a light one?
Should I subtract doors and windows from my paint estimate?
What paint sheen should I use in a bathroom or kitchen?
Sources
Related calculators
Reviewed by the BackyardCalc editorial team. Figures are computed from the formula above and checked against manufacturer yields.