Paint Calculator by Square Feet — Gallons of Paint Needed

Paint Calculator by Square Feet — How Much Paint Do I Need?

One gallon of interior paint covers 400 sq ft per coat; exterior paint covers 350 sq ft per coat. Enter your room dimensions in the paint calculator below for an instant gallons estimate, or use the charts and formula further down. This is one of the guides on OnSiteCalculator.

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How Many Square Feet Does a Gallon of Paint Cover?

Contractor-tested coverage rates used in this guide:

Paint Type	Coverage per Gallon	Coverage per 5-Gal Bucket
Interior wall paint (latex)	400 sq ft	2,000 sq ft
Exterior paint	350 sq ft	1,750 sq ft
Ceiling paint	400 sq ft	2,000 sq ft
Standard primer	400 sq ft	2,000 sq ft
PVA primer (new drywall)	300 sq ft	1,500 sq ft
Chalk paint	150 sq ft	750 sq ft
Milk paint	200 sq ft	1,000 sq ft
Aerosol spray can (~12 oz)	~20 sq ft	—

Coverage drops to 300–350 sq ft/gal on porous, textured, or bare surfaces.

For a full breakdown by finish (flat, eggshell, semi-gloss, gloss), surface type, and exterior siding material, see the paint coverage per gallon guide.

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Paint Coverage Formula

Step 1 — Calculate wall area:

Wall Area (sq ft) = 2 × (Length ft + Width ft) × Ceiling Height ft

Step 2 — Subtract openings:

Paintable Area = Wall Area − (21 sq ft × doors) − (15 sq ft × windows)

Step 3 — Calculate gallons:

Gallons per coat = Paintable Area ÷ Coverage Rate

Total Gallons = Gallons per coat × Coats × 1.10 (waste factor)

Quick Shortcut Formulas

Situation	Formula
Interior, 1 coat	sq ft ÷ 400
Interior, 2 coats + 10% waste	sq ft ÷ 400 × 2 × 1.10
Exterior, 2 coats + 10% waste	sq ft ÷ 350 × 2 × 1.10
Ceiling only	room sq ft ÷ 400 per coat

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How Much Paint Do I Need? — Room Size Chart

Gallons of interior paint needed by room size (400 sq ft/gal, 8 ft ceilings, minus standard doors and windows):

Room Size	Wall Area	1 Coat	2 Coats	2 Coats + 10% Waste
8×8 bedroom	~224 sq ft	0.56 gal	1.12 gal	1.2 gal
10×10 bedroom	~260 sq ft	0.65 gal	1.30 gal	1.4 gal
10×12 bedroom	~292 sq ft	0.73 gal	1.46 gal	1.6 gal
12×12 bedroom	~324 sq ft	0.81 gal	1.62 gal	1.8 gal
12×15 living room	~378 sq ft	0.95 gal	1.89 gal	2.1 gal
14×16 living room	~480 sq ft	1.20 gal	2.40 gal	2.6 gal
20×20 great room	~660 sq ft	1.65 gal	3.30 gal	3.6 gal

Wall area = 2 × (L+W) × 8 ft, minus one door (21 sq ft) and two windows (30 sq ft).

Contractor tip: When in doubt, round up to the next gallon. Leftover paint stores for touch-ups; running short mid-project means a second trip and a possible color mismatch between batches.

For room-by-room estimates including kitchens, bathrooms, garages, and accent walls, see the how much paint for a room guide.

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How to Calculate How Much Paint You Need — Step by Step

1. Measure the room — length, width, and ceiling height in feet.
2. Calculate wall area: 2 × (length + width) × ceiling height Example: 12×15 room, 9 ft ceilings → 2 × (12+15) × 9 = 486 sq ft
3. Subtract openings:
   • Standard door: subtract 21 sq ft each
   • Standard window: subtract 15 sq ft each Example: 2 doors + 2 windows = 42 + 30 = 72 sq ft → 486 − 72 = 414 sq ft
4. Divide by coverage rate: 414 ÷ 400 = 1.04 gallons per coat
5. Multiply by coats: 1.04 × 2 coats = 2.07 gallons
6. Add 10% waste: 2.07 × 1.10 = 2.28 gallons → buy 2.5 gallons (2 gal + 2 qt)
7. Add primer if needed: Same formula ÷ 300 (PVA on new drywall) or ÷ 400 (standard primer)

The paint calculator applies this formula automatically. Enter your room dimensions, number of doors and windows, paint type, and coats — it outputs gallons of paint and primer with waste included.

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Paint Coverage by Room — Worked Examples

How Much Paint for a 10×12 Room?

• Wall area: 2 × (10+12) × 8 = 352 sq ft
• Minus 1 door + 2 windows: 352 − 51 = 301 sq ft
• 1 coat: 301 ÷ 400 = 0.75 gal
• 2 coats + 10% waste: 0.75 × 2 × 1.10 = 1.65 gal → buy 2 gallons

How Much Paint for a 12×12 Room?

• Wall area: 2 × (12+12) × 8 = 384 sq ft
• Minus 1 door + 2 windows: 384 − 51 = 333 sq ft
• 1 coat: 0.83 gal
• 2 coats + 10% waste: 1.83 gal → buy 2 gallons

How Much Paint for a 10×10 Room?

• Wall area: 2 × (10+10) × 8 = 320 sq ft
• Minus 1 door + 1 window: 320 − 36 = 284 sq ft
• 1 coat: 0.71 gal
• 2 coats + 10% waste: 1.56 gal → buy 1 gallon + 2 quarts

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How Much Paint for a Whole House?

Home Size	Paintable Wall Area (est.)	2 Coats Interior	2 Coats Exterior
1-bedroom apt	~800 sq ft	4.4 gal	—
2-bedroom home	~1,200 sq ft	6.6 gal	7.5 gal
3-bedroom home	~1,600 sq ft	8.8 gal	10.0 gal
4-bedroom home	~2,400 sq ft	13.2 gal	15.1 gal
5-bedroom home	~3,200 sq ft	17.6 gal	20.1 gal

Interior: 400 sq ft/gal + 10% waste. Exterior: 350 sq ft/gal + 10% waste. Two coats.

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Square Feet Per Gallon — How Far Does Paint Go?

Amount	Interior Coverage (400 sq ft/gal)	Exterior Coverage (350 sq ft/gal)
1 quart (0.25 gal)	100 sq ft	87 sq ft
1 gallon	400 sq ft	350 sq ft
2 gallons	800 sq ft	700 sq ft
3 gallons	1,200 sq ft	1,050 sq ft
5 gallons	2,000 sq ft	1,750 sq ft
10 gallons	4,000 sq ft	3,500 sq ft

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Primer Coverage — When You Need It

Always prime in these situations:

Situation	Primer Type	Coverage Rate
New drywall	PVA primer	300 sq ft/gal
Dark to light color change	Tinted primer	350 sq ft/gal
Bare wood or MDF	Oil-based or shellac	300 sq ft/gal
Stain or water damage	Stain-blocking primer	300 sq ft/gal
Repaint, same color	Optional — skip if clean	400 sq ft/gal
Patched or skim-coated walls	Standard primer	400 sq ft/gal

You can skip primer when: repainting the same or similar color on clean, sealed walls in good condition.

Use the drywall calculator to estimate sheets and finishing materials for new drywall — PVA primer is always required before painting bare drywall.

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Common Paint Calculation Mistakes

Using floor area instead of wall area. A 1,000 sq ft home does not have 1,000 sq ft of paintable walls. Calculate wall area from dimensions: 2 × (L+W) × ceiling height.

Forgetting the ceiling. If you are painting the ceiling, add length × width to your total. A 12×15 room adds 180 sq ft overhead.

Skipping door and window deductions. Standard deductions: 21 sq ft per door, 15 sq ft per window. Forgetting these adds about half a gallon of unnecessary paint per room.

Using the wrong coverage rate for the surface. Interior latex on smooth drywall: 400 sq ft/gal. Exterior on rough stucco or brick: 250–300 sq ft/gal. Using interior rates for exterior projects means running short.

Buying separate gallons instead of a 5-gallon bucket for large jobs. For projects requiring 4+ gallons of the same color, a 5-gallon bucket is cheaper per gallon and ensures color batch consistency. Boxing (mixing all cans together) before painting eliminates subtle shade differences between cans.

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Related Tools

• Paint Calculator — Enter room dimensions, doors, windows, paint type, and coats for an instant gallons estimate with primer and waste included.
• Paint Coverage Per Gallon Guide — Full coverage rates chart for every paint type, finish, and surface — interior, exterior, and trim.
• How Much Paint for a Room Guide — Pre-calculated gallon amounts for every common room type: bedrooms, bathrooms, kitchens, garages, and accent walls.
• Drywall Calculator — Estimate sheets, joint compound, tape, and screws for new drywall before priming and painting.
• Flooring Calculator — Calculate square footage and boxes of flooring for rooms you are also painting.