Painting the wrong amount hurts either way: run short mid-wall and you get banding; over-buy and you're stuck with leftover latex you won't use in four years. Here's how much paint you need for any room, exterior, or trim job — with the formulas the pros use.
The paint coverage formula
Every gallon of paint covers about 350–400 square feet in one coat on smooth drywall. The basic formula is:
Gallons = (Wall area − Openings) ÷ 350 × Number of coats
Coverage assumes average texture and mid-body viscosity. Rough walls, porous drywall, or dark-to-light color changes reduce coverage by 20–40%.
Step 1: Calculate wall area
For each room, measure the perimeter (add up the length of all walls) and multiply by ceiling height:
- 12 × 14 ft room with 8 ft ceilings: perimeter = 52 ft × 8 ft = 416 sq ft of wall
- Add ceiling area if painting: 12 × 14 = 168 sq ft
Step 2: Subtract openings
Standard deductions:
- Standard door: 21 sq ft
- Standard window: 15 sq ft
- Large picture window: 30 sq ft
For the example room with 2 doors and 2 windows: 416 − 42 − 30 = 344 sq ft paintable.
Step 3: Account for coats
Most walls need two coats. Exceptions:
- 1 coat: Same color refresh, premium paint-and-primer in a similar shade
- 2 coats: Standard for all new color applications
- 3 coats: Dark-to-light changes, red/yellow tones (poor hide), new drywall without primer
Worked example — full room
Bedroom, 12×14 ft, 8 ft ceilings, 2 doors, 2 windows, painting walls + ceiling, two coats:
- Walls paintable: 344 sq ft × 2 coats = 688 sq ft
- Ceiling: 168 sq ft × 2 coats = 336 sq ft
- Total: 1,024 sq ft ÷ 350 = 2.9 gallons
- Buy 3 gallons (or 1 gallon + 1 two-gallon box)
Whole-house estimates
| House size | Interior paint | Exterior paint |
|---|---|---|
| 1,000 sq ft | 8–10 gal | 6–8 gal |
| 1,500 sq ft | 12–15 gal | 10–12 gal |
| 2,000 sq ft | 15–18 gal | 12–15 gal |
| 2,500 sq ft | 18–22 gal | 15–18 gal |
| 3,000 sq ft | 22–26 gal | 18–22 gal |
Interior estimates assume walls and ceilings in two coats; exterior assumes siding only (no trim/soffit).
Trim, doors, and windows
Trim paint is bought separately because it's usually semi-gloss or satin vs matte/eggshell on walls. Rule of thumb:
- One gallon of trim paint = 4–6 average rooms of baseboard and door/window casing
- One door (both sides): ~1/4 gallon
- Window sash and frame: ~1/8 gallon
Exterior paint
Exterior coverage drops to 250–350 sq ft per gallon because siding, stucco, and brick are more porous. Calculate the wall area the same way:
- Measure house perimeter × average height
- Add gable triangles: base × height ÷ 2
- Subtract windows, doors, garage doors
- Divide by 300 × coats
Primer — when you need it
- Always: New drywall, bare wood, stains, patches, glossy surfaces
- Probably: Dark-to-light color change, high-traffic kitchen/bath refresh
- Skip: Repainting similar color with paint-and-primer-in-one
Primer covers at the same rate as paint (~350 sq ft/gal), one coat.
Common mistakes
- Forgetting the second coat — always double your estimate for new colors.
- Mixing batches — slight tint variation between cans shows on large walls. "Box" all gallons into one bucket before rolling.
- Ignoring texture — knockdown and popcorn eat paint; add 15–20%.
- Not saving leftovers — keep a quart for touch-ups; label the room and date.
Buying strategy: gallons vs 5-gallon buckets
If your estimate is above 4 gallons, consider the 5-gallon bucket. Most brands price the 5-gallon at 15–25% less per gallon than individual cans. Boxed paint also guarantees batch consistency, eliminating the tint-variation problem entirely. For interior jobs needing 3 gallons or less, individual cans make sense because you can return unopened ones at most big-box stores.
Also plan your tinting: custom colors must be tinted at the store, and mixing small batches at different times can produce visible differences. Buy all gallons at once from the same store, same day, and note the formula printed on the lid for future touch-up.
Storing leftover paint properly
Leftovers are valuable for touch-ups — nail holes, scuffs, kid-art cleanup. Storage best practices:
- Wipe the rim clean before sealing the lid (dried paint breaks the seal)
- Store cans upside-down — creates an air-tight seal with the paint film itself
- Keep cans in a climate-controlled space (not a shed or garage that freezes)
- Label with room, date, brand, and color code (including the tint formula from the sticker)
- Latex paint stored properly lasts 3–5 years; oil-based 5–7 years
When to call a pro for estimating
For most single rooms, DIY estimation plus 10% safety works fine. But for whole-house jobs, complex trim profiles, or commercial work, a professional's experience saves real money. Painters carry around mental coverage rates for every paint they use — and know when a "2-coat" quote is actually going to need 3 because of the existing color. If you're unsure, have 2–3 painters measure and estimate and compare their gallon counts.
Get your exact number
Every room is different — use our paint calculator to enter room dimensions, openings, and coat count for a precise gallon estimate in seconds.