Paint Calculator
Find how much paint you need — enter your room size, ceiling height, number of coats and doors and windows to get the gallons of wall paint (and ceiling), with cost.
United States — national average · 2026 prices
Material only unless stated as installed. Excludes tax, permits and local labour. Verify with local suppliers — see how we calculate.
How this calculator works
Every figure above comes from the formula below, applied to the assumptions listed. Nothing is looked up from a hidden table — you can reproduce the result by hand.
Formula
wall area = 2 × (length + width) × ceiling height
openings = doors × 21 sq ft + windows × 15 sq ft
paintable = (wall area − openings) + ceiling area if painted
gallons = paintable × coats ÷ 350 sq ft per gallon, rounded up
Assumptions & limits
- Coverage is 350 sq ft per gallon, the middle of the 300–400 range manufacturers state for a smooth primed surface.
- A door is taken as 21 sq ft and a window as 15 sq ft. Large picture windows and sliding doors are considerably bigger — subtract those yourself.
- Two coats is the default and the realistic assumption for a colour change. One coat only works when refreshing the same colour.
- Textured, porous or previously unpainted surfaces absorb noticeably more, sometimes 20–30% more. Deep colours over light often need three coats.
- Gallons are rounded up to whole cans, and paint is priced at $42 per gallon plus about $35 of supplies.
- Trim, doors, ceilings and walls in the same room are usually different products. This sizes the wall paint, with a separate primer line.
Worked example
A 12 × 12 ft room, 8 ft ceilings, one door, two windows, two coats, walls only
- Wall area = 2 × (12 + 12) × 8 = 384 sq ft
- Openings = 1 × 21 + 2 × 15 = 51 sq ft
- Paintable = 384 − 51 = 333 sq ft
- Gallons = 333 × 2 ÷ 350 = 1.90 → 2 gallons
Result:2 gallons for two coats on 333 sq ft — about $101 to $155 with supplies.
Built and maintained by Christoph Ballmer, Honeygoat GmbH
Method and constants last checked against the calculator code on August 17, 2026. Independent trade review is not yet in place for this calculator — see how we handle review.
Read the site-wide calculation methodology · estimate vs. quote and safety limits
How much paint do I need?
Paint is figured from wall area, not floor area. Add up the length of every wall to be painted to get the room perimeter, multiply by the ceiling height for the gross wall area, subtract the doors and windows you are leaving unpainted, multiply by the number of coats, and divide by the paint's coverage rate — about 350 to 400 square feet per gallon for interior latex. Round up to whole gallons (or quarts) and add a touch-up allowance. The calculator above does all of this and adds the ceiling if you want it.
gallons = (perimeter × height − openings) × coats ÷ 350
Manufacturer calculators from the major paint brands use the same arithmetic: Sherwin-Williams and Benjamin Moore both rate a gallon of wall paint at roughly 350–400 square feet per coat and tell you to subtract about 20 sq ft per door and 10–15 sq ft per window. A quart covers about 100 square feet, which is handy for accents and small rooms.
Worked example: a 12 × 14 ft bedroom, two coats
Take a 12 ft × 14 ft bedroom with 9-foot ceilings, one door and two windows:
- Perimeter: 12 + 14 + 12 + 14 = 52 feet
- Gross wall area: 52 ft × 9 ft = 468 sq ft
- Subtract openings: 1 door (20 sq ft) + 2 windows (2 × 15 = 30 sq ft) = 50 sq ft, so 468 − 50 = 418 sq ft of paintable wall
- Two coats: 418 sq ft × 2 = 836 sq ft of coverage required
- Divide by coverage: 836 ÷ 350 = 2.39 gallons → round up to 3 gallons (or buy 2 gallons plus 2 quarts to trim waste)
If you also paint the ceiling (12 × 14 = 168 sq ft), that is 168 ÷ 350 ≈ 0.48 gallon for one coat — about 1 extra gallon of ceiling-flat for two coats. Painting the trim, baseboard and door casing typically takes 1–2 quarts on top of the wall figure.
Paint by room size (two coats, walls only)
| Room | Wall area (8-ft ceiling) | Paint (2 coats) |
|---|---|---|
| 8 × 8 ft (small bath) | ≈ 256 sq ft | ≈ 1.5 gallons |
| 10 × 10 ft (bedroom) | ≈ 320 sq ft | ≈ 2 gallons |
| 12 × 12 ft (bedroom) | ≈ 384 sq ft | ≈ 2 gallons |
| 15 × 15 ft (living room) | ≈ 480 sq ft | ≈ 3 gallons |
| 20 × 20 ft (great room) | ≈ 640 sq ft | ≈ 4 gallons |
Before subtracting doors and windows. Two coats is standard for an even finish. Taller ceilings (9–10 ft) and dark-over-light changes push these figures up.
Coverage by finish (sheen)
Glossier paints are thinner and spread further per gallon, but they highlight every surface flaw, so walls usually get eggshell or satin and trim gets semi-gloss. Use the right coverage figure for the sheen you are buying:
| Finish | Coverage per gallon | Typical use |
|---|---|---|
| Flat / matte | ≈ 400 sq ft | Ceilings, low-traffic walls |
| Eggshell | ≈ 350–400 sq ft | Living rooms, bedrooms |
| Satin | ≈ 350 sq ft | Kitchens, baths, hallways |
| Semi-gloss | ≈ 300–350 sq ft | Trim, doors, cabinets |
| Primer | ≈ 200–300 sq ft | New drywall, stains, big color changes |
Coverage figures are manufacturer averages on a smooth, primed surface; rough, porous or unpainted drywall covers less.
Do I need primer?
Primer is a separate coat, so price and figure it on its own line. Plan on a primer coat — coverage of about 200–300 sq ft per gallon (divide your wall area by 300) — when you are:
- Painting new, bare drywall or fresh joint compound (it soaks up finish paint unevenly).
- Making a dramatic color change, especially dark-to-light or light-to-dark.
- Covering stains, smoke or water marks with a stain-blocking primer.
- Painting glossy, slick or previously oil-based surfaces that need a bonding primer.
Repainting a similar color over a sound, previously painted wall usually needs no separate primer — a quality two-coat paint is enough. A tinted primer can also save you a finish coat on big color swings.
How much does interior paint cost?
Paint price is driven mostly by quality tier, which also affects how well it covers — premium paints hide better and often save a coat. Use these per-gallon ranges to budget the example above:
| Tier | Cost per gallon | Examples |
|---|---|---|
| Budget / contractor | ≈ $22–$40 | Glidden, Behr Premium Plus |
| Mid-grade | ≈ $45–$72 | Behr Marquee, SW Cashmere, BM Regal Select |
| Premium | ≈ $78–$95 | SW Emerald, BM Aura |
| Primer | ≈ $14–$35 | PVA drywall, stain-blocking, bonding |
Cost figures are 2026 estimates and vary by region, brand promotions and sheen — coastal urban markets can run 5–15% higher than rural ones. Check current local pricing before you buy.
For the 12 × 14 bedroom (3 gallons of mid-grade wall paint at about $55) you are looking at roughly $165 in paint for the walls, plus a gallon of ceiling-flat and a quart or two of trim enamel. Painting and want the floor or wall behind it too? See the flooring calculator, tile calculator, and drywall calculator.
How much extra paint for waste?
Unlike concrete or mulch you do not need a big waste percentage, but you should always finish a wall with paint to spare rather than one roller-width short of the can. Build in a small cushion:
- Buy from one batch: order all your gallons at once so the color matches; cans mixed weeks apart can vary slightly in shade.
- Keep 1–2 quarts for touch-ups: nicks and scuffs are inevitable, and an exact-match touch-up later is hard to find.
- Round up, never down: a job that calculates to 2.4 gallons should be bought as 3 gallons (or 2 gallons + 2 quarts).
- Add 10% for textured or rough surfaces: knockdown, popcorn and masonry drink more paint than smooth drywall.
Common mistakes
- Using floor area instead of wall area. Paint covers the walls — a 12 × 12 floor is 144 sq ft, but its walls are nearly 384 sq ft.
- Forgetting the second coat. Most jobs need two coats; sizing for one is the single most common reason people run out.
- Ignoring the ceiling and trim. They use different products (flat ceiling paint, semi-gloss trim enamel) and must be figured separately.
- Over-subtracting openings. Skip the door/window deductions on small rooms — the extra paint is cheap insurance and undershooting is worse.
- Not priming bare drywall or stains. Skipping primer there means extra finish coats and a blotchy result, which costs more paint, not less.
When to hire a pro
A single bedroom is a classic DIY weekend. Consider hiring a professional painter when the job involves high or vaulted ceilings and stairwells (ladder and scaffold work), extensive drywall or plaster repair, oil-to-latex conversions that need careful prep, lead paint in homes built before 1978 (which has federal handling rules), or simply a whole-house repaint on a deadline. Interior painting commonly runs about $2–$6 per square foot of floor area installed, so a pro quote is a useful sanity check against your material estimate.
These are planning estimates only — coverage, paint and labor costs vary by product, surface, region and year. Verify quantities and current prices with a local paint supplier or a licensed painting contractor before you buy.
Frequently asked questions
How much paint do I need for a 12x14 room?
A 12 × 14 ft room with 8-foot ceilings has a 52-foot perimeter and about 416 square feet of wall. Subtracting one door and two windows leaves roughly 366 sq ft, so two coats need about 366 × 2 ÷ 350 ≈ 2.1 gallons — round up to 3 gallons, or buy 2 gallons plus 2 quarts. With 9-foot ceilings it climbs toward 3 gallons. Add about 1 gallon of flat ceiling paint and 1–2 quarts for the trim. Enter your exact size above for the precise figure.
How much paint do I need for the ceiling and trim?
Figure the ceiling and trim separately from the walls because they use different products. For the ceiling, multiply length × width and divide by 350–400 (flat ceiling paint covers a bit further), so a 12 × 14 ceiling of about 168 sq ft needs roughly half a gallon per coat. Trim, baseboards and door casings in an average room usually take 1–2 quarts of semi-gloss enamel. The calculator above can include the ceiling; budget the trim as an extra quart or two.
Do I need primer before painting?
Use a separate primer coat on bare or new drywall, over stains, smoke or water marks, for big color changes such as dark-to-light, and on glossy or previously oil-based surfaces. Primer covers about 200–300 square feet per gallon, so divide your wall area by 300 to size it. Repainting a similar color over a sound, already-painted wall usually needs no separate primer — a quality two-coat paint is enough, and a tinted primer can save a finish coat on dramatic color swings.
How much does it cost to paint a room yourself?
For a standard bedroom, DIY paint costs are modest: 2–3 gallons of mid-grade wall paint at roughly $45–$72 each is about $100–$200, plus a gallon of ceiling paint, a quart or two of trim enamel, and rollers, brushes, tape and drop cloths. Budget paint starts around $22–$40 a gallon and premium lines run $78–$95. Hiring a pro instead typically runs about $2–$6 per square foot of floor area. Cost figures are 2026 estimates and vary by region and brand — check local pricing before you buy.
How much paint do I need?
Work out the wall area — the room perimeter times the ceiling height — subtract the doors and windows, then multiply by the number of coats and divide by the coverage (about 350 square feet per gallon). The calculator above does this and rounds up to whole gallons and quarts.
How many gallons does it take to paint a room?
A typical 12×12 room with 8-foot ceilings has about 384 square feet of wall. At one coat that is roughly 1.1 gallons; for two coats plan on about 2 gallons. Add more for the ceiling and any extra coats — enter your room above for an exact figure.
How much does a gallon of paint cover?
One gallon of wall paint covers about 350–400 square feet in a single coat on a smooth, primed surface. Rough, porous or dark-to-light surfaces cover less and often need an extra coat. The calculator uses 350 sq ft per gallon as a practical figure.
How many coats of paint do I need?
Two coats is the standard for an even, durable finish, especially over a new color. One coat can work when repainting a similar color over a primed wall, while big color changes or patchy surfaces may need a primer plus two coats. Set the coats above to match your job.
What is the formula for calculating paint?
Gallons = (wall area - openings) x coats / 350. Wall area is 2 x (length + width) x ceiling height; subtract about 21 sq ft per door and 15 per window; 350 sq ft is the typical one-coat coverage of a gallon on a smooth, primed surface. Example: a 12x12 room with 8 ft ceilings is 384 sq ft of wall, minus 51 for one door and two windows = 333 sq ft - times two coats, divided by 350, is 1.9, so you buy 2 gallons.