How much paint do I need for one room?

Measure the room (or wall area), subtract doors and windows, multiply by coats and your waste percent, then divide by the coverage on the can. This page does that math and rounds up to whole gallons. A typical 14×12×8 room with two doors, two windows, two coats, 350 sq ft/gal coverage, and 5% waste needs about 2.08 gallons exact—3 gallons to buy.

How many square feet does a gallon of paint cover?

It depends on the product and the surface. Many US interior latex labels cite about 350–400 sq ft per gallon per coat on sealed smooth drywall. This calculator defaults to 350 as a slightly conservative planning number; change the coverage field to match your can.

Should I subtract doors and windows?

Yes, when those openings will not receive the wall color. The defaults assume 20 sq ft per door and 15 sq ft per window. Measure unusual sizes and edit those fields so the gallon count is not inflated.

How many coats of paint should I plan for?

Two coats is a common interior plan on ordinary drywall when the color change is modest. One coat is sometimes enough for a similar color over sound paint; new drywall, stains, or a big color shift often need primer plus two finish coats. Set the coats field to what you will actually apply.

How do I calculate paint for multiple rooms?

Run the calculator once per color (or once per room if sizes differ), then add the gallons to buy. Do not average different coverages or coat counts into one run—different products need separate inputs.

Do I subtract trim I will paint a different color?

If walls and trim are different SKUs, subtract trim area from the wall color and calculate trim separately (often a quart).

Paint Calculator: Gallons for Walls and Ceilings

Figure out how much paint you need before you buy. Enter a room’s length, width, and height—or a wall area you already measured—subtract doors and windows, set coats and the coverage on your can, and get gallons rounded up so you are not one quart short.

How to use this calculator

Measure once, then match the fields to what you measured. The tool keeps the math in US feet and square feet even if you toggle display units elsewhere on the page.

Choose room dimensions (length, width, ceiling height) or switch to “I already know wall area” if you measured walls with a laser or took off from a plan.
Count doors and windows that will not get this wall color. Defaults are 20 sq ft per door and 15 sq ft per window—edit those if your openings are larger or smaller.
Leave “Include ceiling” unchecked when the ceiling is a different product. Check it only when the same gallons cover walls and ceiling.
Set coats (default 2) and coverage per gallon per coat. Prefer the number printed on your can; the planning default here is 350 sq ft per gallon per coat.
Adjust waste if you expect texture, cutting-in, or color change. The result rounds up to whole gallons to buy.

How this calculation works

For a box room, wall area is 2 × (length + width) × height.
Doors and windows subtract. Defaults (20 and 15 sq ft) are typical; measure odd sizes.
Optional ceiling area is length × width when that checkbox is on.
Paintable area is increased by your waste percent, multiplied by coats, then divided by coverage. Exact gallons are shown; gallons to buy round up to a whole gallon.

walls = 2(L+W)H − doors − windows; total = walls + optional ceiling; gallons = ceil(total × coats × (1+waste%) / coverage)

Worked example

Example inputs: a 14 ft × 12 ft room, 8 ft ceilings, 2 doors (20 sq ft each), 2 windows (15 sq ft each), 2 coats, 350 sq ft per gallon coverage, 5% waste, ceiling not included.

Wall area = 2 × (14 + 12) × 8 = 416 sq ft.
Openings = 2 × 20 + 2 × 15 = 70 sq ft. Paintable walls = 416 − 70 = 346 sq ft.
With 5% waste: 346 × 1.05 = 363.3 sq ft of area to cover per coat.
Exact gallons = 363.3 × 2 ÷ 350 ≈ 2.08. Rounded up to buy: 3 gallons.

Result: 346 sq ft of wall area, 2.08 exact gallons, 3 gallons to buy—the same numbers this calculator returns for those inputs.

Common mistakes

Including the ceiling in the wall takeoff when you will use a different ceiling paint. Use the ceiling checkbox only when one gallon count covers both.
Forgetting to subtract doors and windows, which inflates gallons on rooms with large openings.
Planning one coat when you will apply two. Coats multiply the paint needed; change the coats field instead of guessing.
Using a generic coverage number when the can says something else. Labels often cite about 350–400 sq ft per gallon per coat on smooth sealed drywall—texture and drastic color changes use more.
Mixing trim or accent colors into the wall gallon count. Calculate those SKUs separately (often a quart).

What to do next

Use the coverage number on your can, not the default, if they disagree.
Buy all wall color from the same batch. Keep a labeled quart for scuffs.
If you are painting over dark colors or new drywall, add primer as extra coats or a second run of the math.

After paint, many rooms need a flooring takeoff—use the flooring calculator for area, waste, and boxes. For a full gut, the kitchen remodel cost calculator or bathroom remodel cost calculator can sketch an editable budget range alongside this gallon count.

FAQ

How much paint do I need for one room?: Measure the room (or wall area), subtract doors and windows, multiply by coats and your waste percent, then divide by the coverage on the can. This page does that math and rounds up to whole gallons. A typical 14×12×8 room with two doors, two windows, two coats, 350 sq ft/gal coverage, and 5% waste needs about 2.08 gallons exact—3 gallons to buy.
How many square feet does a gallon of paint cover?: It depends on the product and the surface. Many US interior latex labels cite about 350–400 sq ft per gallon per coat on sealed smooth drywall. This calculator defaults to 350 as a slightly conservative planning number; change the coverage field to match your can.
Should I subtract doors and windows?: Yes, when those openings will not receive the wall color. The defaults assume 20 sq ft per door and 15 sq ft per window. Measure unusual sizes and edit those fields so the gallon count is not inflated.
How many coats of paint should I plan for?: Two coats is a common interior plan on ordinary drywall when the color change is modest. One coat is sometimes enough for a similar color over sound paint; new drywall, stains, or a big color shift often need primer plus two finish coats. Set the coats field to what you will actually apply.
How do I calculate paint for multiple rooms?: Run the calculator once per color (or once per room if sizes differ), then add the gallons to buy. Do not average different coverages or coat counts into one run—different products need separate inputs.
Do I subtract trim I will paint a different color?: If walls and trim are different SKUs, subtract trim area from the wall color and calculate trim separately (often a quart).

Sources

Wall area ≈ perimeter × height, or 2×(L+W)×H for a rectangular room
Many US interior latex labels cite about 350–400 sq ft per gallon per coat on sealed smooth drywall

Cost defaults are typical US planning bands, last reviewed 2026-08-25. Edit the price fields on this page rather than treating them as a local market index.