Paint Calculator: How Many Gallons Do You Need?
Calculate paint coverage for any room or exterior surface. Account for doors, windows, multiple coats, and different paint types to get the right amount.
Paint is cheap compared to your time. Doing a second trip to the store because you ran out mid-wall — especially if you mixed a custom color — is one of those annoying situations that's easy to avoid with a quick calculation upfront. The CalcHub Paint Calculator figures out how many gallons you need based on your room dimensions, number of coats, and the type of paint.
Coverage Rates by Paint Type
Different paints have different coverage rates, and the manufacturer's number (typically 400 sq ft per gallon) is optimistic — that's for a smooth surface with a single coat. Real-world coverage is lower:
| Paint Type | Label Coverage | Realistic Coverage |
|---|---|---|
| Flat/Matte | 400 sq ft/gal | 300–350 sq ft/gal |
| Eggshell / Satin | 400 sq ft/gal | 300–350 sq ft/gal |
| Semi-Gloss | 400 sq ft/gal | 300–380 sq ft/gal |
| Primer | 300 sq ft/gal | 200–280 sq ft/gal |
| Textured paint | 150–200 sq ft/gal | 100–150 sq ft/gal |
| Exterior paint | 350 sq ft/gal | 250–300 sq ft/gal |
Calculating a Room
Step 1: Total wall area Add up the perimeter of the room and multiply by ceiling height.Example room: 12 ft × 14 ft, 9 ft ceiling
- Perimeter = (12 + 14) × 2 = 52 ft
- Wall area = 52 × 9 = 468 sq ft
Step 2: Subtract doors and windows
A standard interior door is about 20 sq ft (2.5 × 8 ft). A standard window is 12–15 sq ft. If you have two doors and two windows:
- Subtract: 40 + 26 = 66 sq ft
- Paintable wall area = 468 − 66 = 402 sq ft
Step 3: Multiply by coats
For two coats: 402 × 2 = 804 sq ft to cover Step 4: Divide by coverage rate At 350 sq ft/gallon: 804 ÷ 350 = 2.3 gallons
Round up to the nearest quart or half-gallon — so 2.5 gallons is your order.
The calculator handles all of this with sliders and room inputs. Add multiple walls, toggle whether you're painting the ceiling, and it updates the total live.
Ceiling and Trim: Don't Forget These
People often calculate walls but forget that the ceiling is its own job. A 12 × 14 room has 168 sq ft of ceiling — just over a quart for one coat.
Trim (baseboards, door casings, window trim) is trickier to measure precisely. A rough rule: for a typical bedroom, budget about half a quart for trim. For a large living room with lots of crown molding, a full quart.
Ceiling paint and trim paint are usually different products than wall paint — factor in separate gallons for each.
Going Dark: Account for Extra Coats
Painting a dark accent wall? Going from white to deep navy or forest green? Plan for three coats, possibly four, especially if the current wall is white. Some deep colors have poor hide and the primer layer is almost mandatory.
On the flip side, painting a light color over another similar light color in good condition might need only one coat. If you're touching up rather than repainting a full room, a quart is often plenty.
Leftover Paint Is Not Waste
Buying a little extra is smart. You want leftover paint for touchups — nail holes from future picture hanging, scuffs, the inevitable doorknob dent. Store it properly (lid sealed tight, stored at room temperature, not in a garage that freezes) and it'll last a few years.
A half-gallon extra on a $35/gallon project is $17.50 of insurance.
How much paint for a bedroom?
A typical 10×12 bedroom with 8 ft ceilings needs about 1.5–2 gallons for two coats on the walls. Add a quart for the ceiling and another quart for trim.
Do I need primer?
Primer is worth it when: you're painting over a dark color, painting new drywall (mandatory), covering stains or smoke damage, or switching from oil-based to latex paint. For repainting the same color or going lighter, a quality paint+primer product often skips the separate primer step.
Can I return unused paint to the store?
Most stores accept returns of unopened gallons. Tinted/custom-mixed paint typically cannot be returned. When in doubt, buy one less gallon and run back if needed — or buy the extra and store it for touchups.
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