Wondering how much paint do I need? This free paint calculator works as a house paint calculator, room paint estimator, exterior paint calculator, and interior paint calculator all in one. Choose your project type, enter your measurements, and this paint coverage calculator gives you an instant gallon count with a waste buffer, optional cost estimate, and full step-by-step breakdown. The ultimate painting estimate calculator for any project.
Project Details
Project type
Paint Settings
Paint to order
3 gallons
2 coats - 345 sq ft - 10% buffer
345 sq ft
Net area
0.99 gal
Per coat
1.97 gal
All coats
Your 345 sq ft project needs approximately 0.99 gallons per coat x 2 coats = 1.97 gallons total. With 10% extra, order 3 gallons.
Getting started
How to use this paint coverage calculator
1
Choose project type
Room/Interior calculates wall area from room length, width, and ceiling height. Exterior uses house perimeter and wall height.
2
Enter dimensions
All measurements in feet. For exterior, measure the outside perimeter at ground level and use average wall height to eaves.
3
Enter doors and windows
Standard door ≈ 21 sq ft; standard window ≈ 15 sq ft. These are deducted from the total area.
4
Set coats and coverage
2 coats is the standard for most jobs. Coverage defaults to 350 sq ft/gal. Check your specific paint can label for the exact figure.
The calculation
Step-by-step: how your paint quantity was calculated
1
Calculate net paintable area
Net Area = Gross Area - Openings
= 396 - 51 sq ft
= 345 sq ft
2
Calculate gallons per coat
Gallons/coat = Net Area / Coverage (350 sq ft/gal)
= 345 / 350
= 0.99 gal per coat
3
Multiply by 2 coats
Total gallons = gallons/coat x 2
= 0.99 x 2
= 1.97 gallons
4
Add 10% waste/buffer
With waste = total x (1 + 10%)
= 1.97 x 1.10
= 2.17 gal -> order 3 gallons
Note: Always round up at the paint store - partial gallons aren't available in all colors.
Formulas
Paint coverage formula — how the painting estimate calculator works
Figuring paint coverage with this paint coverage calculator comes down to dividing your paintable area by the coverage rate of the product you are using.
For a room: Gross Wall Area = 2 x (Length + Width) x Ceiling Height Net Area = Gross Area - (Doors x 21 sq ft) - (Windows x 15 sq ft)
Gallons per coat = Net Area / Coverage (sq ft/gal) Total gallons = Gallons per coat x Number of coats Order = Total gallons x (1 + Waste %) - round up to nearest gallon
Reference Table
Paint coverage by surface type
Coverage rate varies widely by surface texture and porosity. Always use the lower end of the range for rough, porous, or bare surfaces when figuring paint coverage.
Surface type
Coverage (sq ft / gallon)
Smooth, primed drywall
400
Standard drywall / plaster
350-400
Lightly textured drywall
300-350
Heavily textured drywall
200-300
New / bare wood
250-300
Rough wood siding
200-250
Masonry / concrete / brick
150-250
Previously painted (same color)
350-400
These figures apply to a single coat. Multiply by your number of coats for total project paint. Actual coverage may vary by brand.
Context
Understanding paint quantity and coverage
Why does surface texture matter so much?
Rough or porous surfaces have far more actual physical area than a flat surface. A textured ceiling can use 40-50% more paint than a smooth one of the same size.
Primer vs. paint coats
Primer seals the surface and improves adhesion, allowing finish paint to go further. When painting over a dark color, priming first is usually more effective.
Deck stain and exterior estimating tips
Most solid deck stains cover 200-300 sq ft/gal; semi-transparent stains cover 250-400 sq ft/gal on bare wood.
Examples
Worked examples
Example 1 - Bedroom
A 12 ft x 10 ft bedroom with 9 ft ceilings: gross wall area = 2 x (12 + 10) x 9 = 396 sq ft. Subtract 1 door (21 sq ft) + 2 windows (30 sq ft) = 51 sq ft. Net = 345 sq ft. At 350 sq ft/gal x 2 coats with 10% waste: 345 / 350 x 2 x 1.1 = 2.17 gal. Order 3 gallons.
Example 2 - House exterior
A house with a 140 ft perimeter and 10 ft wall height: 140 x 10 = 1,400 sq ft gross. Subtract 4 doors (84 sq ft) + 8 windows (120 sq ft) = 204 sq ft. Net = 1,196 sq ft. At 375 sq ft/gal x 2 coats with 10% waste: 1,196 / 375 x 2 x 1.1 = 7.03 gal. Order 8 gallons.
Mistakes
Common mistakes when calculating paint
Not deducting doors and windows
On a typical room, these represent 10-20% of the gross wall area. Skipping this step leads to significant over-ordering.
Ignoring texture
Using the 400 sq ft/gal coverage number for a rough textured wall will lead to short delivery.
Forgetting the ceiling
This calculator focuses on walls. If you're painting the ceiling too, re-run the calculator with the floor area as your custom area.
FAQ
Frequently asked questions
Q
How do I calculate how much paint I need?
Measure the total wall area to be painted: for a room, add up all four walls (perimeter x ceiling height) then subtract door and window areas. Divide the net area by the paint's coverage rate (typically 350-400 sq ft per gallon for most quality paints) to get gallons per coat. Multiply by the number of coats. Round up to the nearest quart or gallon.
Q
How many square feet does a gallon of paint cover?
Most quality interior and exterior paints cover 350-400 sq ft per gallon for one coat on a smooth, primed surface. Rough or porous surfaces (bare wood, brick, textured drywall) may reduce coverage to 200-300 sq ft per gallon. Always check the coverage spec on your specific paint can, as premium paints vary.
Q
How many coats of paint do I need?
Most interior paint jobs require 2 coats for full, even coverage - especially when changing color significantly or painting over dark colors. A coat of primer followed by one finish coat can sometimes match 2 coats of paint. For ceilings or painting over similar color, 1 coat may suffice. Exterior painting usually requires 2 coats for weather protection.
Q
Do I need to add extra paint for waste?
Yes. Factor in 10% extra for touch-ups, roller nap absorption, overspray, and spills. For rooms with lots of corners, intricate trim, or ceiling-wall transitions, budget 15% extra. It is always better to have leftover paint for future touch-ups than to run short mid-project with a discontinued color.
Q
How much paint do I need for a house exterior?
Measure the perimeter of your house (in feet) and multiply by the wall height to get total gross area. Subtract large windows and doors (typically 15-20 sq ft each). Divide by 350-375 sq ft/gallon. A typical 2,000 sq ft home may have 1,200-1,500 sq ft of paintable exterior wall and require 4-5 gallons per coat - so 8-10 gallons for 2 coats.
