Paint Calculator

Step 1: Measure each room. Enter the length and width along the floor and the height of the wall (floor to ceiling). All measurements in feet.

Step 2: Enter the number of standard doors (assumed 80 × 36 inches = 20 sqft each) and standard windows (assumed 36 × 48 inches = 12 sqft each). Doors and windows are subtracted from the wall area you need to paint.

Step 3: Check "Include ceiling" if you are painting the ceiling too. Ceilings count as length × width additional sqft.

Step 4: Click "+ Add Room" if you are painting multiple rooms — the calculator totals across all rooms in a single estimate. You can rename rooms (Living Room, Bedroom, etc.) for the breakdown table.

Step 5: Set the number of coats. Two coats is the standard for proper coverage. One coat works only when going light-over-light with high-quality paint; three coats is needed for deep colors over white or when going light over a much darker color.

Step 6: Adjust the coverage. Most interior latex paints cover 350–400 sqft per gallon on a smooth, primed wall. Use the porosity slider to bump coverage down for textured, porous, or unprimed surfaces — fresh drywall absorbs 15–20% more paint than primed walls.

Step 7: Optionally check "Include primer coat" if painting new drywall, going over patches, or making a major color change. Optionally enter price per gallon to see total cost.

Step 8: Click Calculate. The result shows total gallons needed (always rounded up — you cannot buy partial gallons), with a per-room breakdown and step-by-step math.

Most people underestimate paint needs and end up making a second store trip mid-job. The calculation is straightforward — wall area minus doors/windows, multiplied by coats, divided by coverage — but the real-world variables move the number more than the formula does.

Coverage varies by surface. A gallon of interior latex paint covers about 350–400 sqft on a smooth, previously-painted wall. On unprimed drywall, that drops to 300 sqft. On heavily textured or porous surfaces (popcorn ceilings, brick, stucco), you can lose another 20–30%. The porosity slider in this calculator lets you adjust for that.

Color and coats interact. Light colors over light primer usually achieve coverage in two coats. Dark or saturated colors (deep reds, navy, forest green) typically need three coats — sometimes four — to reach full opacity. Going light over dark almost always requires a tinted primer plus two coats of finish, or three coats of finish paint without primer.

Primer changes the math. Fresh drywall, drywall patches, and walls being repainted in a much different color all benefit from primer. A primer coat covers about as much area as a paint coat — so adding primer adds roughly one paint coat's worth of gallons. This calculator's primer toggle adds that automatically.

Trim, doors, and windows are separate jobs. This calculator focuses on walls and ceilings (the big surface). Trim and door painting use a different paint (semi-gloss enamel typically) and cover much less area but at a higher per-sqft cost. For trim, plan on roughly 1 gallon per 300 linear feet of standard baseboard plus door frames.

Always buy 10% extra. Even with a careful calculation, you will spill, drop a brush, miss patches, and need touch-ups months later. Buying an extra quart or rounding up a gallon ensures you have the same batch (color matching across paint mixes is imperfect) for future touch-ups.

For each room:

Wall area = 2 × (Length + Width) × Height

Subtract openings: Doors at 20 sqft each (standard 80 × 36 inch interior door) + Windows at 12 sqft each (standard 36 × 48 inch window)

Add ceiling (if including): Length × Width

Paintable area per room = Wall area − Door area − Window area + Ceiling area (optional)

Across all rooms:

Effective coverage = Listed coverage ÷ (1 + porosity adjustment %)

Gallons for walls and ceiling = ⌈(Total paintable area × Coats) ÷ Effective coverage⌉

Gallons for primer (if including) = ⌈Total paintable area ÷ Effective coverage⌉

Total gallons = Walls + Primer (always rounded up; partial gallons cannot be purchased)

Example: 12 × 12 × 8 ft bedroom with 1 door + 2 windows, no ceiling, 2 coats:
- Wall area = 2 × (12 + 12) × 8 = 384 sqft
- Less openings = 384 − 20 − 24 = 340 sqft
- × 2 coats = 680 sqft to cover
- ÷ 350 sqft/gal = 1.94 → 2 gallons

Example 1: One bedroom. 12 × 12 × 8 ft, 1 door, 2 windows, 2 coats, no ceiling. Wall area: 384 sqft. After openings: 340 sqft. Two coats: 680 sqft to cover. At 350 sqft/gal: 1.94 → 2 gallons.

Example 2: Bedroom + ceiling. Same room as above but painting the ceiling too. Ceiling adds 12 × 12 = 144 sqft. New paintable area: 484 sqft. Two coats: 968 sqft. → 3 gallons.

Example 3: Whole house (3 rooms). Bedroom (12×12×8), living room (16×14×9), kitchen (10×12×8). Total wall area: 384 + 540 + 352 = 1,276 sqft. Subtract 3 doors + 6 windows: 1,196 sqft. Two coats at 350 sqft/gal: 2,392 ÷ 350 = 6.83 → 7 gallons. Add primer for the kitchen (10 × 12 × 8 = 352 sqft minus openings ≈ 320 sqft, ÷ 350 = 0.91 → 1 gallon primer). Total: 8 gallons + buy 1 quart extra for touch-ups.

Buy 10% extra. Spills, drops, missed patches, and future touch-ups all need paint from the same mix. Color-matching across different batches is imperfect, so getting more later may not match exactly.

One large can beats four quarts. A gallon mixed from the same paint base is uniform; four quarts mixed separately have slight color variation that shows under direct light. If you need 3.5 gallons, buy 4 gallons rather than 3 gallons + 2 quarts.

Prime when: the wall is new drywall, has been patched in multiple spots, was previously painted dark and you are going light, or has stains (water, smoke, marker). Primer is also recommended over high-gloss finishes that fresh paint would otherwise slide off.

Skip primer when: you are repainting in the same color family on an already-painted wall, and the existing finish is matte or eggshell (not high-gloss). Modern self-priming paints can handle this in 2 coats.

Wall texture matters more than the can label says. Manufacturer coverage numbers assume smooth primed walls. Knockdown or orange-peel texture loses 10–15%, popcorn ceilings or stucco loses 25–30%. The porosity slider compensates for this.

Color depth changes coat count. Standard whites and light pastels: 2 coats. Mid-tone colors (sage greens, beige, blue-grays): 2–3 coats. Deep saturated colors (red, navy, charcoal): 3 coats minimum, plus tinted primer is strongly recommended.

Store leftover paint properly. Press a layer of plastic wrap onto the paint surface before resealing — air exposure causes skinning. A tightly sealed gallon stored away from temperature extremes lasts 2–10 years for touch-ups.