Buy Exactly What You Need — No Second Trip
The most expensive paint mistake isn’t choosing the wrong color — it’s guessing the quantity. Buy too little and you stall mid-project; buy too much and gallons gather dust in the garage. This free paint calculator removes the guesswork: enter your room dimensions, doors, windows, coats and coverage, and get the exact gallons to buy plus an estimated cost.
It uses the industry rule of thumb (about 350 ft² per gallon per coat), subtracts standard door and window areas, and always rounds up — because nobody sells 1.7 gallons. No signup, everything runs privately in your browser.
Walls · Doors & windows · Coats
Please enter room dimensions greater than zero.
Paint needed (rounded up)
Estimates only. Porous, textured or dark-to-light repaints drink more paint — buy one extra gallon for touch-ups on big jobs. Primer is calculated separately: most primers cover about 300 ft² per gallon.
How to Use the Paint Calculator
- Enter the room. Length, width and ceiling height — in feet or meters. For a single accent wall instead of a room, just enter that wall’s dimensions as length × height with width 1.
- Count openings. Doors subtract ~20 ft² each, windows ~15 ft² — you don’t paint glass or door slabs.
- Set coats and coverage. Two coats is the standard for good coverage; check your paint can for its rated ft²/gallon (350 is typical).
- Add the price per gallon for an instant budget estimate, and tick the ceiling box if it gets the same paint.
The Formula
Paintable = wall area − doors × 20 − windows × 15
Gallons = paintable × coats ÷ coverage → round up
Worked example: a 12 × 10 ft room with 8 ft ceilings, 1 door, 2 windows, 2 coats. Walls = 2 × 8 × 22 = 352 ft². Minus openings (20 + 30) = 302 ft². Paint = 302 × 2 ÷ 350 = 1.73 → 2 gallons. At $45/gallon, that’s $90.
Quick Reference: Common Rooms (2 coats)
| Room | Approx. wall area | Gallons (2 coats) |
|---|---|---|
| Small bathroom | 150–200 ft² | 1 |
| Bedroom (12×12) | ~380 ft² | 2–3 |
| Living room (16×20) | ~580 ft² | 3–4 |
| Open floor plan | 1,200+ ft² | 7+ |
Who This Helps
- DIY homeowners painting a room on a Saturday without a mid-project store run.
- Landlords budgeting turnovers across multiple units.
- Contractors turning measurements into material lists fast.
Pro tips
- Cheap paint is usually more expensive: budget paint often needs 3 coats where mid-range ($40–50/gal) covers in 2 — do the math before you “save.”
- Dark-to-light color changes almost always need an extra coat or a tinted primer.
- Keep one leftover gallon (or at least a labeled jar) for touch-ups — future you will be grateful.
Painting a House Exterior? Same Formula, Bigger Walls
Everything above works outdoors too — only the inputs change. Measure the house perimeter and multiply by the wall height to get gross exterior area, subtract windows and doors, then divide by coverage as usual. Worked example: a two-story house with a 150 ft perimeter and 18 ft walls has 150 × 18 = 2,700 ft² of wall. Subtract roughly 300 ft² for windows and doors and 2,400 ft² remains. Two coats at 350 ft²/gallon needs 2,400 × 2 ÷ 350 = 13.7, so buy 14 gallons of body paint.
Two exterior caveats. First, texture eats paint: rough stucco, bare wood and unprimed masonry can drop real coverage to about 250 ft²/gallon, which pushes the same house to roughly 19–20 gallons. Second, trim, fascia and doors are usually a different color and sheen — estimate them separately, and expect about 1 gallon of trim paint per 8–10 gallons of body paint.
Frequently Asked Questions
How much paint do I need for a 12×12 room?
About 2–3 gallons for two coats on the walls (roughly 380 ft² of wall area at 8 ft ceilings, minus a door and window, divided by 350 ft²/gallon, rounded up).
How many square feet does a gallon of paint cover?
About 350 ft² per coat for standard interior latex on smooth walls. Porous drywall, textured surfaces and drastic color changes reduce real-world coverage.
How do you calculate paint for a room?
Multiply wall length by height for each wall (or use 2 × height × (length + width) for a rectangular room), subtract ~20 ft² per door and ~15 ft² per window, multiply by coats, and divide by coverage per gallon.
Do I need primer?
On bare drywall, patched spots, or dark-to-light changes — yes. Primer is calculated separately at roughly 300 ft² per gallon. Paint-and-primer-in-one works for same-color refreshes.
How much does it cost to paint a room?
DIY material cost is typically $80–$200 for an average bedroom (2–3 gallons at $40–60 each, plus supplies). Enter your price per gallon above for your exact estimate.
Should I include the ceiling?
Only if you’re actually painting it. Ceiling paint is usually flat white and bought separately — but if it’s the same color and sheen, tick the ceiling box and let the calculator add it.
How much paint do I need for one wall?
A standard 12 × 8 ft wall is 96 ft². Two coats cover 192 ft², and at 350 ft² per gallon that is about 0.55 gallons — roughly 2 quarts. Buy a full gallon if you want leftover paint for touch-ups, which you almost always will.
Does spraying use more paint than rolling?
Yes. A sprayer loses paint to overspray and bounce-back, so plan on roughly 30% more paint than the calculator’s roller-based estimate for the same area. Rolling and brushing transfer nearly all the paint to the wall. Indoors, that waste difference is why most DIY painters roll.
How much paint do I need to paint a house exterior?
Most two-story homes need about 10–15 gallons of body paint for two coats, depending on size and siding texture. The worked example above — 2,400 ft² of paintable wall — comes to 14 gallons at standard coverage, or around 19–20 gallons on rough stucco. Trim and primer are extra.
Final Thoughts
Two minutes with a tape measure and this calculator beats every alternative: the eyeball method, the “buy extra just in case” method, and especially the mid-Sunday store run in paint-spattered clothes. Measure once, buy once, paint once.
Try our tile calculator for flooring projects.