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.

Paint Calculator

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

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

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.

The paint coverage formula

Estimating paint accurately means working through the room geometry step by step rather than guessing. Here's exactly how the calculator arrives at 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

Worked example: A 15 ft × 12 ft room with 9 ft ceilings has a perimeter of 2 × (15 + 12) = 54 ft, giving a gross wall area of 54 × 9 = 486 sq ft. Subtracting one door (21 sq ft) and two windows (30 sq ft) leaves 435 sq ft of net wall area. With two coats, that's 870 sq ft to cover. Adding a 10% waste factor brings it to 957 sq ft. At 350 sq ft per gallon, you'd need 957 ÷ 350 = 2.73 gallons — rounded up to 3 gallons, since paint is sold in whole cans.

Tips for a more accurate estimate

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, both because paint is sold in fixed sizes and because having slightly extra paint on hand for future touch-ups is valuable — paint colors can be difficult to match exactly if you need to buy more later, especially with custom tinted colors.

For large or multi-room projects, calculate each room separately using this tool, then sum the gallon requirements before your store visit — most retailers offer better per-gallon pricing on larger quantities like 5-gallon buckets.

Frequently asked questions

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