Interior Painting Cost & Paint Calculator
Find out how many gallons of paint a room needs and what the job costs from the wall area, your coats and your own price per gallon.
Calculator
A 12×15 ft room has ≈ 432 sq ft of wall; at 2 coats and 375 sq ft/gal that is 3 gallons ≈ $120.00 with labor.
Paint is sold by the gallon, but rooms are measured in square feet, so the question “how much paint do I need?” is really two calculations: first the wall area you are covering, then the gallons that area needs at your chosen number of coats. This tool does both, then multiplies by your price per gallon so the cost reflects the paint you actually buy — not a national average that goes stale.
Wall area is the room perimeter times the ceiling height, minus the doors and windows you are not painting. Coverage — how far one gallon goes per coat — is a product number, so it is a field you can override; a smooth, previously painted wall stretches a gallon further than a porous or freshly patched one. We default to 375 sq ft per gallon per coat (a mid-band typical), two coats, and always round up to whole gallons, because you cannot buy 2.1 cans.
Formula
Geometry then take-off:
perimeter = 2 × (length + width)\nwall_area = perimeter × ceiling_height − openings\ngallons = ceil( wall_area × coats ÷ coverage )\ntotal = gallons × price_per_gallon + wall_area × labor_per_sqftCoverage and coats are the two levers that move the gallon count; the price is entirely yours. Ceiling and trim paint, if you buy them separately, are additional line items.
Worked example
Take a 12 × 15 ft room with 8 ft walls, one door and one window totaling 36 sq ft, at two coats:
- Perimeter = 2 × (12 + 15) = 54 ft
- Wall area = 54 × 8 − 36 = 396 sq ft
- Gallons = ceil(396 × 2 ÷ 375) = ceil(2.112) = 3 gallons
So a standard bedroom needs 3 gallons for two coats. At $40/gal that is $120 of paint before any labor — swap in your real price to finish the estimate.
Coats, coverage & buying round
Round up, then buy one extra quart for touch-ups if the wall color is changing dramatically or the surface is patchy. Primer counts as its own coat: a deep color over white (or white over a deep color) often behaves like three coats, which is why the coats selector matters more than people expect.
The coverage figure is where estimates drift. Manufacturers quote 350-400 sq ft per gallon on smooth drywall in good condition; textured, porous, or hot (freshly mudded) surfaces drink more paint, so a lower coverage number is the honest input there. Trim, doors and ceilings use different products and are usually estimated separately — this calculator sizes the wall paint.
Basis & sources. Wall-area geometry and the ceil() gallon rule are standard take-off math; the 350-400 sq ft/gal band is a labeled planning typical, not a spec. The prices are the ones you enter. See sources and methodology for how each formula is derived and self-checked.
Reference table
Paint coverage is a labeled planning band — always check your product label. Typical whole-room gallon counts at two coats, 375 sq ft/gal:
| Wall area | 1 coat | 2 coats | 3 coats |
|---|---|---|---|
| 200 sq ft | 1 gal | 2 gal | 2 gal |
| 300 sq ft | 1 gal | 2 gal | 3 gal |
| 400 sq ft | 2 gal | 3 gal | 4 gal |
| 500 sq ft | 2 gal | 3 gal | 4 gal |
| 600 sq ft | 2 gal | 4 gal | 5 gal |
Band: 350–400 sq ft/gal/coat. Deep color changes behave like an extra coat.
Frequently asked questions
How much paint do I need for a 12x15 room?
A 12 × 15 ft room with 8 ft walls has about 396 sq ft of wall after subtracting a door and a window. At two coats and 375 sq ft/gal that is 3 gallons. One gallon covers a single coat of a small room; most bedrooms land at 2-3 gallons for two coats.
Does the calculator include the ceiling?
No — it sizes the wall paint. Ceilings usually use a dedicated flat ceiling paint and are estimated as a separate area (length × width). Add that gallon count and price on its own line if you are painting the ceiling too.
What coverage number should I use?
Start with the label. Smooth, previously painted drywall runs 350-400 sq ft per gallon per coat; textured, porous or freshly patched walls cover less, so drop the number toward 300. Because coverage is a field you control, the estimate stays accurate for whatever product you buy.
Why does it always round up to whole gallons?
Because paint is sold in whole cans and colors are hard to re-match if you run short mid-wall. The tool uses ceil(), so 2.1 gallons becomes 3. Buying a spare quart for touch-ups on top of that is sensible on a color change.
Is the total a quote for a painter?
No. It is a planning estimate on the numbers you enter. If you add a labor rate per square foot it approximates a painted price, but a real painter prices prep, trim, ceilings and access too — get an itemized written quote before you commit.