Paint Calculator
By BRIX Editorial Team · Updated
To figure paint, multiply the room's perimeter by the wall height, subtract doors and windows, multiply by the number of coats, and divide by the coverage on the can, often 350–400 sq ft per gallon. A 14 × 12 ft room with 8 ft walls takes about 2.1 gallons for two coats on the walls.
- Wall area (less doors and windows)
- 365 sq ft
- Wall paint
- 2.09 gal → buy 2 gal + 1 qt
- Ceiling area
- 168 sq ft
- Ceiling paint
- 0.96 gal → buy 1 gal
2 coats on 365 sq ft of wall at 350 sq ft per gallon takes 2.09 gallons. Bare drywall, deep colors and rough texture eat more paint than the can says, so keep a quart extra for touch-ups.
Key takeaways
- Wall area = 2 × (length + width) × height, minus doors and windows.
- Gallons = area × coats ÷ coverage per gallon. Use the low end of the coverage range on the data sheet.
- Sherwin-Williams lists 350–400 sq ft per gallon for SuperPaint interior flat; Benjamin Moore lists 400–450 for Regal Select matte.
- Plan two coats on new drywall, color changes and anything with a primer you skipped.
- Buy walls and ceilings separately: they're usually different products and sheens.
How do you calculate how much paint you need?
Find the paintable area in square feet, multiply by the number of coats, and divide by the coverage per gallon from the product data sheet. Round up to whole gallons, or gallons plus a quart or two when the overage is small.
Gallons = (2 × (L + W) × H − openings) × coats ÷ coverage
- L, W
- room length and width in feet
- H
- wall height in feet
- openings
- doors (21 sq ft each for a 3 × 7 ft door) and windows (15 sq ft for a 3 × 5 ft window)
- coats
- usually 2
- coverage
- sq ft per gallon from the can or data sheet
- 1
Measure the walls.
Add up the length of every wall (the perimeter) and multiply by the wall height. For a rectangular room, that's 2 × (length + width) × height.
- 2
Subtract openings.
Take off doors and windows. Measure odd sizes; a 6 ft slider or a big picture window is much more than the standard deduction.
- 3
Add the ceiling if you're painting it.
Ceiling area is length × width. Keep it separate, since ceiling paint is a different product.
- 4
Multiply by coats and divide by coverage.
Use the low end of the coverage range on the data sheet. Rough, porous or bare surfaces take more.
- 5
Round to cans.
Round up to whole gallons. If you're only a little over, buy a quart or two instead of another gallon.
How many square feet does a gallon of paint cover?
Most interior wall paints cover 250–450 sq ft per gallon per coat, depending on the product, the surface and how it's applied. The number on the data sheet assumes a smooth, sealed surface at the recommended film thickness. Use the low end of the range for estimating.
| Product | Coverage per gallon (per coat) | Notes |
|---|---|---|
| Sherwin-Williams SuperPaint Interior Latex Flat | 350–400 sq ft | At 4 mils wet; data sheet systems call for 2 coats |
| Benjamin Moore Regal Select Matte | 400–450 sq ft | 1 or 2 coats recommended |
| Behr Premium Plus Interior Flat | 250–400 sq ft | Depends on application method and surface porosity |
| KILZ 2 All-Purpose primer | 300–400 sq ft | Primer coat on bare or patched surfaces |
How much paint for common rooms?
| Room | Size | Wall sq ft (net) | Wall paint to buy | Ceiling paint to buy |
|---|---|---|---|---|
| Half bath | 5 × 8 ft, 8 ft walls | 187 | 1 gal + 1 qt | 1 qt |
| Small bedroom | 10 × 10 ft, 8 ft walls | 284 | 2 gal | 1 gal |
| Primary bedroom | 14 × 16 ft, 8 ft walls | 408 | 2 gal + 2 qt | 1 gal + 2 qt |
| Living room | 16 × 20 ft, 9 ft walls | 561 | 3 gal + 1 qt | 2 gal |
| Great room, 10 ft walls | 20 × 24 ft, 10 ft walls | 778 | 4 gal + 2 qt | 3 gal |
When do you need two coats or primer?
- New drywall and patches. Bare paper and joint compound soak up paint unevenly. Prime first or use a self-priming paint with two coats.
- Big color changes. Going light over dark, or covering reds and deep blues, usually takes two coats and sometimes a tinted primer.
- Same color refresh. One coat may cover if the old paint is sound and the color matches. Most pros still quote two coats.
- Stains, smoke and water marks. Spot-prime with a stain-blocking primer or they'll bleed through.
Common paint estimating mistakes
- Using the high end of the coverage range. That figure assumes ideal conditions. Rough plaster, textured walls and bare drywall take noticeably more.
- Forgetting the second coat. Most data sheets spec two coats. Quote it unless you know one will cover.
- Subtracting every opening. Small closet doors and windows barely matter; skipping deductions builds in a little touch-up paint.
- Ignoring trim. Doors, casing and baseboards are a separate product and sheen. Figure them separately.
- Not buying a single lot. Mix all cans of the same color together (boxing) so slight batch differences don't show on the wall.
Frequently asked questions
How much paint do I need for a 12 × 12 room?
With 8 ft walls, one door and two windows, a 12 × 12 room has about 333 sq ft of wall. Two coats at 350 sq ft per gallon is 1.9 gallons, so buy 2 gal.
How many square feet does a gallon of paint cover?
Usually 250–450 sq ft per coat, depending on the product and surface. Sherwin-Williams lists 350–400 sq ft per gallon for SuperPaint interior flat and Benjamin Moore lists 400–450 for Regal Select matte. Use the low end for estimating.
Do I need two coats of paint?
Plan on two coats for new drywall, patched walls and color changes; many manufacturers' recommended systems specify two. A same-color refresh over sound paint can sometimes be done in one.
Should I subtract doors and windows when estimating paint?
For a close number, yes: about 21 sq ft for a standard 3 × 7 ft door and 15 sq ft for a 3 × 5 ft window. Measure big openings like sliders. Skipping small deductions leaves a bit of paint for touch-ups.
How much paint do I need for a ceiling?
Ceiling area is length × width. A 14 × 12 ft ceiling is 168 sq ft, so two coats at 350 sq ft per gallon takes 0.96 gallons. Buy ceiling paint separately from wall paint.
Is it better to buy gallons or 5-gallon buckets?
Once a job needs four or more gallons of one color, a 5-gallon bucket is often cheaper per gallon and keeps the color consistent. Compare the price per gallon at your supplier.
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Sources
- Sherwin-Williams — SuperPaint Interior Latex Flat product data sheet (checked October 12, 2026)
- Benjamin Moore — Regal Select Waterborne Interior Paint, Matte (checked October 12, 2026)
- Behr — Premium Plus Interior Flat (Pro) (checked October 12, 2026)
- KILZ — KILZ 2 All-Purpose primer (checked October 12, 2026)