How much paint do I need for a room?

It depends on the wall area, number of coats, and paint coverage rate. As a rule of thumb, one U.S. gallon of paint covers about 350 square feet per coat. Our calculator does the full math for your exact room.

How is wall area calculated?

We calculate the room's perimeter (2 × (length + width)) and multiply by wall height to get the gross wall area, then subtract the area of doors and windows to get the net paintable area.

How much area does a standard door or window take up?

We use standard estimates of 21 square feet per door and 15 square feet per window (or 1.95 m² and 1.4 m² in metric), which you can adjust by changing the door/window counts to match your room.

Why does the calculator include a waste factor?

Paint is lost to rollers, trays, uneven surfaces, touch-ups, and spills. A 10% waste factor is standard for most projects; use a higher percentage (15–20%) for textured walls or a first-time DIY job.

How many coats of paint should I apply?

Two coats is standard for most walls, especially when changing colors or painting over a darker shade. A single coat may suffice for touch-ups or when using the same color, while three coats may be needed for very light colors over dark surfaces.

Does paint coverage vary by brand or paint type?

Yes. Coverage typically ranges from 250–400 square feet per gallon depending on the paint's formulation, sheen, and the porosity of your wall surface. Always check the coverage rate listed on your specific paint can and adjust the calculator's coverage field accordingly.

Should I paint the ceiling too?

This calculator estimates wall paint only. For ceilings, multiply the room's length by width to get ceiling area, then apply the same coverage-rate math separately since ceiling paint often has different coverage and formulation.

How do I calculate paint for an accent wall?

Enter the accent wall's length as the room length and set the width to a very small number (or manually calculate length × height) to isolate just that one wall's area, then subtract any doors or windows on it.

What if my room has more than four walls?

The perimeter-based formula works for rectangular rooms with four walls. For irregular or multi-sided rooms, measure and sum each wall's area individually, then subtract door/window areas manually.

How accurate is the cost estimate?

The cost estimate multiplies the rounded number of gallons or liters needed by the price you enter per unit. It's only as accurate as your local paint pricing, so check current prices from your preferred retailer or brand.

AI Insights

Paint Calculator

Find out exactly how many gallons or liters of paint your room needs, with cost estimates.

Last updated August 1, 2026

Room length (ft)

Room width (ft)

Wall height (ft)

Doors

Windows

Number of coats

Coverage (sq ft per gal)

Waste factor (%)

Price per gallon ($)

You'll need

3 gallons

(2.73 gallons exact)

Net wall area

435.0 sq ft

Total area to paint (with coats)

870.0 sq ft

Estimated cost

$90.00

With walls this expansive, slight variations in lighting can make a huge difference in how your paint color looks from corner to corner. Tint your primer to match your topcoat so you can easily spot missed patches and cut down on the total number of coats needed to get a rich, solid finish. Tackling the job section by section while keeping a wet edge will prevent unsightly roller marks from drying unevenly across the large surface area.

AI-generated insight

How to use the paint calculator

1
Enter your room's length, width, and wall height in feet or meters.
2
Enter the number of doors and windows so they're subtracted from the paintable wall area.
3
Set the number of coats, your paint's coverage rate, and a waste factor.
4
Your gallons/liters needed and estimated cost appear instantly and update as you adjust any field.

How much paint do I need? The coverage formula

Wall area is height × length, minus door and window openings.

Estimating paint accurately means working through the room geometry step by step instead of guessing. Here's how the calculator gets to your number:

Wall perimeter = 2 × (length + width)
Gross wall area = perimeter × wall height
Openings area = (doors × 21 sq ft) + (windows × 15 sq ft)
Net wall area = gross wall area − openings area
Total area to paint = net wall area × number of coats
Area with waste = total area × (1 + waste factor)
Paint needed = area with waste ÷ coverage per gallon

Say you've got a 15 ft × 12 ft room with 9 ft ceilings. Perimeter is 2 × (15 + 12) = 54 ft, giving a gross wall area of 54 × 9 = 486 sq ft. Subtract one door (21 sq ft) and two windows (30 sq ft) and you're left with 435 sq ft of net wall area. With two coats, that's 870 sq ft to cover. Add a 10% waste factor and it's 957 sq ft. At 350 sq ft per gallon, that's 957 ÷ 350 = 2.73 gallons, rounded up to 3, since paint doesn't come in partial cans.

Coverage and waste factor quick reference

Surface condition	Suggested waste factor
Smooth, previously painted wall	5–10%
Standard drywall, first coat	10%
Textured or stucco wall	15–20%
Unprimed or porous surface	20%+

Tips for estimating paint accurately

Paint coverage rates printed on the can are based on ideal application onto a smooth, primed, single-color surface. Textured walls, unprimed drywall, or a significant color change (e.g., painting white over dark red) can reduce real-world coverage by 20% or more. If you're unsure, use a lower coverage number and a higher waste factor to avoid a mid-project trip to the store.

It's almost always worth rounding up to the next full gallon or can. Paint is sold in fixed sizes anyway, and having a little extra on hand for touch-ups later matters: custom tinted colors especially can be tough to match exactly if you run out and need more.

For a bigger or multi-room job, run each room through this tool separately, then add up the gallons before you head to the store. Most retailers price 5-gallon buckets better per gallon than individual cans.

Hanging new drywall before you paint? Try the Drywall Calculator →

Not sure of your exact wall square footage yet? Try the Square Footage / Area Calculator →

Frequently asked questions

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