Paint Calculator
Subtracts 21 ft² each (3 × 7 ft)
Subtracts 15 ft² each (3 × 5 ft)
Check the can — it varies by sheen and surface.
Buy this much (2 coats, incl. 10% waste)
2.01 gal
- Wall area before openings
- 416 ft²
- Doors & windows deducted
- 51 ft²
- Area per coat
- 365 ft²
- Total painted area
- 730 ft²
Whole gallons
3 gallons
- Or buy it as gallons + quarts
- 2 gal + 1 qt (2.25 gal)
- Coverage used
- 400 ft² per gallon
Cost comparison
$135.00 all-gallon vs $108.00 with quarts
- All gallons (3 × $45.00)
- $135.00
- Gallons + quarts (2 gal, 1 qt)
- $108.00
- Verdict
- Topping off with quarts is cheaper here
Geometric estimate only — it assumes smooth walls and doesn't account for texture, unprimed drywall drinking up the first coat, or color changes that need extra passes. Trim, doors, and closets are not included. Replace the price placeholders with your store's shelf price before you buy.
A 12 × 14 ft room with 8 ft ceilings, one door and two windows, painted with two coats, needs 2.01 gallons — the cheapest way to buy it is 2 gallons plus 1 quart, which costs $108 against $135 for three whole gallons. Walls only: 365 ft² per coat, 730 ft² of brushwork, at a nominal 400 ft² per gallon. Add the ceiling and the same room needs 2.93 gallons.
How much paint do I need?
gross wall area (ft²) = 2 × (length + width) × height
openings (ft²) = doors × 21 + windows × 15
per coat (ft²) = gross wall area − openings (+ ceiling if painted)
paint area (ft²) = per coat × coats
gallons = paint area × (1 + waste%) ÷ coverage per gallon
Worked example — the page default, a 12 × 14 ft room, 8 ft ceilings, 1 door, 2 windows, 2 coats:
- Gross walls: 2 × (12 + 14) × 8 = 416 ft²
- Subtract openings: 1 door (21 ft²) + 2 windows (30 ft²) = 365 ft² per coat
- Two coats: 365 × 2 = 730 ft²
- Add 10% waste: 803 ÷ 400 = 2.01 gallons
Gallons or gallons-and-quarts?
Paint is sold in whole gallons and quarts, and the quart is where most people overspend — or underspend and run out. This calculator prices both ways:
| Room (8 ft ceilings, 2 coats) | Per coat | Gallons needed | All gallons | Gallons + quarts | Cheaper |
|---|---|---|---|---|---|
| 12 × 14 ft, 1 door, 2 windows | 365 ft² | 2.01 | 3 gal — $135 | 2 gal + 1 qt — $108 | 2 gal + 1 qt |
| Same room plus the ceiling | 533 ft² | 2.93 | 3 gal — $135 | 3 gal — $135 | tie |
| Same room, one coat (refresh) | 365 ft² | 1.00 | 2 gal — $90 | 1 gal + 1 qt — $63 | 1 gal + 1 qt |
| 10 × 10 ft, 1 window | 305 ft² | 1.68 | 2 gal — $90 | 1 gal + 3 qt — $99 | all gallons |
The pattern: quarts win when the leftover fraction is small (2.01 gal needs just one quart), and whole gallons win when it isn't (1.68 gal needs three quarts at $18 each). Prices here are placeholders — $45 per gallon, $18 per quart — and a quart typically costs more than a quarter of a gallon, which is exactly why the comparison swings.
The inputs that change the answer most
- Coats. One coat over a similar color is a refresh; a dark wall going light, or bare drywall, needs two — and primer is a separate product. Dropping the default room from 2 coats to 1 halves the paint: 2.01 → 1.00 gallon.
- Ceiling. Painting it adds the full floor area (12 × 14 = 168 ft²) to every coat. On this room that's 365 → 533 ft² per coat and 2.01 → 2.93 gallons — nearly a gallon more.
- Coverage. 400 ft²/gal is the nominal figure for a smooth, primed interior wall at one coat. Rough texture, bare drywall, deep color changes, and spraying all push consumption up; 300 ft²/gal is a safer number for the first coat on new drywall.
Doors and windows are subtracted at 21 ft² and 15 ft² — a standard 3 × 7 ft door and 3 × 5 ft window. If you're painting the doors and trim the same color, don't subtract them; if they're a different color, subtract them here and estimate the trim separately.
How to use this calculator
- Enter room length, width, and ceiling height in feet.
- Count doors and windows (the defaults are 1 and 2).
- Tick the ceiling only if you're painting it the same color.
- Set coats — 2 by default, 1 for a same-color refresh.
- Check coverage against the can and the price fields against the shelf; read the gallons, both buy options, and which is cheaper.
Frequently asked questions
How much paint do I need for a 12×12 room?
About 1.9 gallons for two coats on 8 ft walls with one door and one window: 384 ft² of gross wall minus 36 ft² of openings is 348 ft² per coat, 696 ft² for two coats, 1.91 gallons with waste — so buy 2 gallons, or 1 gallon plus 3 quarts if quarts are cheap where you shop. Painting the ceiling too adds 144 ft² per coat, roughly another three-quarters of a gallon.
How many coats of paint do I need?
Two for a color change or new drywall, one for a refresh in the same color over sound paint. Deep reds and yellows over white often need three. Primer is a separate product and is not counted in the coat number.
How much area does a gallon of paint cover?
Manufacturers typically rate interior paint at 350–400 ft² per gallon per coat on a smooth, primed surface. Bare drywall, textured walls, and dark-to-light color changes can drop that to 250–300 ft² for the first coat.
Should I buy quarts or an extra gallon?
Compare the two options: a quart usually costs more per ounce than a gallon, so quarts only win when you need a small top-up. On the default room, 2 gallons + 1 quart is $108 vs $135 for three gallons — a $27 difference.
Should I subtract doors and windows?
Yes if they're staying a different color — a standard door is about 21 ft² and a window about 15 ft². If you're painting doors and trim the same color as the walls, don't subtract them.
Do I need primer?
If the wall is bare drywall, patched, stained, or you're going from dark to light — yes, and price it separately, since primer isn't part of the coat count here.
Data sources: All figures are arithmetic from the dimensions and prices you enter. The 400 ft²/gal default is a typical manufacturer coverage rating for interior paint on a smooth primed wall — not a guaranteed figure; check the can you're buying. Door and window deductions use the common US estimating sizes of 21 ft² (3 × 7 ft door) and 15 ft² (3 × 5 ft window). $45 per gallon and $18 per quart are placeholders, not shelf prices. Every number on this page was produced by this calculator and can be reproduced with the same inputs. Related tools: tile calculator, flooring calculator, concrete calculator, all home project calculators, percentage calculator.