Paint Calculator: How to Calculate Paint for Any Project

Getting the paint quantity right is one of those things that seems simple until you’re three gallons short on a Saturday afternoon with a client expecting the job done Monday. Or worse, you overbuy by 10 gallons and eat the cost because the paint store won’t take back tinted returns. A paint calculator takes the guesswork out of both scenarios.

Use our free Paint Calculator to get accurate gallon estimates in seconds, or keep reading to understand the math behind the numbers so you can verify any calculator’s output.

Quick Answer: How Does a Paint Calculator Work?

A paint calculator converts your room dimensions into paintable surface area, then divides by the paint’s coverage rate to determine how many gallons you need. The basic formula is: (wall area - doors/windows) x number of coats / coverage rate per gallon = gallons needed. Most interior paints cover 300-400 sq ft per gallon per coat, with 350 sq ft being the standard assumption. Add a 10-15% waste factor for cutting in, touch-ups, and roller absorption, and you have your order quantity.

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The Inputs Every Paint Calculator Needs

Before you punch numbers into any calculator, you need accurate measurements. Here is what matters:

Room length and width. Measure at floor level in feet. For L-shaped or irregular rooms, break them into rectangles and calculate each section separately.
Ceiling height. Standard is 8 feet. Taller ceilings mean more wall area per room, which changes your gallon count significantly.
Number of doors. A standard interior door is about 20 sq ft (3 x 6.8 ft). Exterior and sliding doors are larger.
Number of windows. Average window opening is about 15 sq ft (3 x 5 ft). Large picture windows can be 25-40 sq ft.
Number of coats. Two coats is standard for most repaints. New drywall, dramatic color changes, or going from dark to light may need primer plus two coats.
Paint coverage rate. This varies by brand, finish, and surface texture. The calculator needs this to convert area into gallons.

Measurements That Beginners Miss

Two things catch people off guard when measuring for paint:

Closet interiors. If the scope includes painting inside closets, those walls add up fast. A standard 6 x 3 ft closet adds about 60 sq ft of wall area.
Alcoves, soffits, and bulkheads. Kitchen soffits, hallway alcoves, and architectural features all have paintable surfaces that standard room calculators skip.

Paint Coverage Rates by Type and Finish

Not all paint covers the same. Here is what to expect from different products:

Paint Type	Coverage (sq ft/gallon)	Notes
Flat/matte interior	350-400	Best coverage, hides imperfections
Eggshell interior	350-400	Standard for living areas
Satin interior	300-350	Slightly lower coverage than flat
Semi-gloss interior	300-350	Trim, doors, bathrooms, kitchens
High-gloss interior	250-350	Lowest coverage, most prep needed
Ceiling paint	350-400	Thick formula, good one-coat hide
Primer	300-400	Varies by type (PVA, shellac, oil)
Exterior latex	250-350	Depends on surface texture
Exterior stain	150-300	Wood grain absorbs more

Key takeaway: Flat and eggshell finishes stretch the furthest. Glossier finishes cover less per gallon because they are thinner and show application marks if applied too thick.

Surface Texture Changes Everything

A smooth drywall wall at 350 sq ft per gallon is straightforward. But textured surfaces absorb more paint:

Knockdown texture: Reduce coverage by 15-20%. Budget 280-300 sq ft per gallon.
Orange peel texture: Reduce by 10-15%. Budget 300-320 sq ft per gallon.
Heavy stucco/sand texture: Reduce by 25-30%. Budget 250-280 sq ft per gallon.
Brick or block (interior): Reduce by 30-40%. Budget 200-280 sq ft per gallon.

If you are using a paint calculator that only assumes 350 sq ft per gallon, adjust the output manually for textured surfaces.

The Paint Calculation Formula (Step by Step)

Here is the exact method professional painters use:

Step 1: Calculate gross wall area Wall area = perimeter x ceiling height Perimeter = 2 x (length + width)

Step 2: Subtract doors and windows Net paintable area = gross wall area - (doors x 20 sq ft) - (windows x 15 sq ft)

Step 3: Calculate gallons per coat Gallons per coat = net paintable area / coverage rate

Step 4: Multiply by number of coats Total gallons = gallons per coat x number of coats

Step 5: Add waste factor Order quantity = total gallons x 1.10 (for 10% waste)

Step 6: Round up to whole gallons Paint stores sell in gallons and quarts. Round up to the next gallon for walls. For small areas like accent walls, a quart or two may be enough.

Worked Example 1: Standard 3-Bedroom Interior

Project: Paint all walls in three bedrooms, a living room, a kitchen, a hallway, and two bathrooms. Standard 8 ft ceilings, smooth drywall, two coats of eggshell, single color throughout.

Room	L x W	Perimeter	Wall Area (8 ft)	Doors	Windows	Net Wall Area
Bedroom 1	12 x 12	48 ft	384 sq ft	2	1	329 sq ft
Bedroom 2	11 x 12	46 ft	368 sq ft	2	1	313 sq ft
Bedroom 3	10 x 11	42 ft	336 sq ft	2	1	281 sq ft
Living Room	16 x 14	60 ft	480 sq ft	2	3	395 sq ft
Kitchen	12 x 10	44 ft	352 sq ft	2	1	297 sq ft
Hallway	30 x 4	68 ft	544 sq ft	5	0	444 sq ft
Bath 1	8 x 6	28 ft	224 sq ft	1	1	189 sq ft
Bath 2	7 x 5	24 ft	192 sq ft	1	1	157 sq ft
Total						2,405 sq ft

Calculation:

Gallons per coat: 2,405 / 350 = 6.9 gallons
Two coats: 6.9 x 2 = 13.8 gallons
Add 10% waste: 13.8 x 1.10 = 15.2 gallons
Order: 16 gallons (4 five-gallon buckets is more cost-effective if same color)

Material cost: 16 gallons of standard eggshell at $30-$55 per gallon = $480-$880

This is for walls only. Ceilings, trim, and doors would be quoted separately.

Worked Example 2: Single Accent Wall

Project: One accent wall in a living room, 16 ft wide, 9 ft ceiling, deep navy blue over existing beige. Needs primer plus two coats of satin.

Wall area: 16 x 9 = 144 sq ft (no doors or windows on this wall)

Primer:

144 / 350 = 0.41 gallons per coat
One coat of primer: 0.41 x 1.10 (waste) = 0.45 gallons
Order: 1 quart of tinted primer (or 1 gallon if you want leftover for touch-ups)

Paint (satin finish, 325 sq ft/gallon assumed):

144 / 325 = 0.44 gallons per coat
Two coats: 0.44 x 2 = 0.89 gallons
Add waste: 0.89 x 1.10 = 0.98 gallons
Order: 1 gallon of satin finish

Material cost: 1 gallon premium paint at $45-$85 + 1 quart primer at $10-$15 = $55-$100

The takeaway here: even a dramatic color change on a single wall only needs about a gallon of paint. The primer step is what makes it work.

When Do You Need Primer?

Not every paint job needs a separate primer. Here is a quick reference:

Situation	Primer Needed?	Why
Same color or similar shade repaint	No	Paint covers itself fine
Light over light color change	No	Two coats usually covers
Dark to light color change	Yes	Dark pigment bleeds through without primer
New (unpainted) drywall	Yes	PVA primer seals the paper and mud
Smoke or water stains	Yes	Shellac-based primer to block stains
Glossy surface being repainted	Yes	Bonding primer for adhesion
Bare wood	Yes	Wood primer prevents tannin bleed

When primer is needed, add it as a separate line item in your estimate. Primer is cheaper per gallon ($18-$35) than finish paint, but it still needs to be calculated based on surface area and coverage rate.

Paint Calculator vs. Real-World Adjustments

A paint calculator gives you a solid starting number. But experienced painters know the calculator output is a baseline, not a final answer. Here is what to adjust for:

Add More Paint For:

Textured walls. Add 15-30% depending on texture depth.
Dark or vivid colors. Deep reds, greens, and blues have lower hide. Budget an extra 10-15%.
First-time painters or less experienced crew. More paint gets absorbed by rollers, spilled, or applied too thick. Add 15%.
Spraying vs. rolling. Spraying uses 20-30% more paint due to overspray, even with good masking.

You Can Use Less When:

Color-matched repaint. Same color going over the same color, good condition walls. Sometimes one coat is enough (though two is still recommended for bid purposes).
High-quality paint with built-in primer. Premium paints like Benjamin Moore Aura or Sherwin-Williams Emerald cover in fewer passes.

Pro Tips for Contractors Calculating Paint

Always round up. Running out of paint mid-job costs you a trip to the store, lost production time, and potential color-match issues if the batch runs differently. Leftover paint is cheap insurance. The client can keep it for touch-ups.
Buy in fives when you can. A five-gallon bucket costs less per gallon than five individual gallons. For whole-house jobs using one color, this saves real money. Standard paint at $30-$55/gallon often drops to $25-$45/gallon in five-gallon buckets.
Track your actual coverage rates. The label says 350 sq ft per gallon. Your actual rate depends on your crew, your roller covers, your technique, and the surfaces you work on. Track a few jobs and you will know YOUR number, not the manufacturer’s number.
Separate your paint order by finish. Walls get eggshell or satin. Trim gets semi-gloss. Ceilings get flat ceiling paint. Don’t lump these together in your calculator, run each surface type separately.
Keep a gallon for touch-ups. Tell the client you are leaving a gallon for future touch-ups. It builds trust and reduces callback costs when they inevitably bump the wall moving furniture.

Common Mistakes When Calculating Paint

Using floor area instead of wall area. A 200 sq ft room does not need 200 sq ft of paint coverage. It needs 500-700 sq ft of wall coverage depending on ceiling height. This is the most common calculator error.
Forgetting the second coat. One coat of coverage means exactly that: one pass. Most jobs need two coats. Your calculator should double the coverage requirement.
Ignoring doors, windows, and openings. Failing to subtract non-paintable surfaces wastes 10-20% in over-purchasing. On a big project, that is real money.
Not adjusting for texture. Using the smooth-wall coverage rate on a heavy knockdown texture means you run out of paint. Always adjust for surface condition.
Mixing up ceiling and wall calculations. Ceilings use floor dimensions (length x width), not wall dimensions. A 12 x 14 room has 168 sq ft of ceiling, not 416 sq ft.

FAQs

How many square feet does a gallon of paint cover?

One gallon of standard interior paint covers 300-400 sq ft per coat, with 350 sq ft being the most common manufacturer rating. Flat and eggshell finishes are at the high end. Semi-gloss and high-gloss finishes are at the low end. Textured surfaces reduce coverage by 15-30% depending on texture depth.

How many gallons of paint do I need for a 12x12 room?

A 12 x 12 room with 8 ft ceilings needs about 2 gallons for two coats of wall paint. The gross wall area is 384 sq ft. After subtracting one door and one window (about 35 sq ft), you have roughly 349 sq ft of paintable surface. At 350 sq ft per gallon, that is 1 gallon per coat, or 2 gallons for two coats plus waste.

Do I need to calculate paint differently for ceilings?

Yes. Ceiling paint quantity is based on floor area (length x width), not wall area. A 12 x 14 room has 168 sq ft of ceiling. At 350 sq ft per gallon, one coat takes about half a gallon. Most ceilings need one coat unless you are covering a dark color or stains. Ceiling paint is formulated thicker with good one-coat hide.

How much extra paint should I buy for waste?

Add 10% for standard roller application on smooth walls. Add 15-20% for textured surfaces, spraying, or inexperienced painters. It is always better to have one gallon left over than to run short. Leftover paint can be left with the homeowner for touch-ups, which is a nice gesture that reduces your callback costs.

Is a paint calculator accurate enough for bidding?

A paint calculator gives you a reliable material estimate that is within 5-10% of actual usage on smooth walls. For bidding purposes, it is a solid starting point. Experienced painters adjust the output based on surface condition, paint quality, and their own crew’s production rates. For a complete bid that includes labor, materials, and overhead, the Painting Estimate Calculator handles the full calculation.

All pricing in this guide reflects 2026 national averages. Prices vary by region based on local labor markets, cost of living, and material availability. Always verify with local contractors and suppliers to confirm rates for your specific market.

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