Paint Calculator
This free paint calculator estimates how many gallons of paint a room needs from its length, width, and wall height, minus standard door and window sizes, times the number of coats you plan to apply. Nothing you enter is sent anywhere or stored.
How to Use It
- Enter the room length, width, and wall height.
- Enter the number of doors and windows, and how many coats you plan to apply.
- Adjust the coverage per gallon if your paint label lists a different figure.
- The paintable area and gallons needed update instantly, no submit button.
How It Works
This calculator finds the total wall area from the room's length, width, and wall height, then subtracts a standard area for each door and window since those parts of the wall do not get painted. Multiplying the remaining paintable area by the number of coats gives the total area of paint that actually needs to go on the wall, which is then divided by the paint's coverage rate to find how many gallons that takes.
Formulas
wall area = 2 × (length + width) × height
paintable area = wall area − (doors × 20) − (windows × 15)
gallons needed = (paintable area × coats) ÷ coverage per gallon
A worked example: a 12 ft by 10 ft room with 8 ft walls has 352 square feet of total wall area. One standard door (about 20 square feet) and two standard windows (about 15 square feet each) remove 50 square feet, leaving 302 square feet of paintable wall. Two coats at that size works out to 604 square feet of paint, which needs about 1.73 gallons at a coverage rate of 350 square feet per gallon, so the practical answer is to buy 2 gallons.
Everything runs client-side in your browser. Nothing you enter is sent anywhere or stored.
Frequently Asked Questions
How many coats of paint do most rooms need?
Two coats is the standard recommendation for most interior painting jobs, since a single coat rarely covers evenly and often shows the old color or roller marks underneath. One coat can be enough when the new color is very close to the existing wall color and the surface is already in good condition.
Why does paint coverage vary so much between products?
A gallon's stated coverage (this calculator defaults to 350 square feet, a common figure for interior latex paint) is measured on a smooth, primed, similarly colored surface. Textured walls, unprimed drywall, and porous surfaces all soak up more paint and cover less area per gallon, so check the specific product's label for its own coverage rate.
What if I am painting over a much darker or more saturated color?
A big color change, especially going from a dark or bold color to a lighter one, often needs a third coat or a tinted primer first to fully hide the old color. Budget extra paint for that situation rather than assuming the standard two coats will be enough.
Why round up to the next whole gallon instead of buying the exact amount?
Paint is sold by the gallon, and separate gallons of the same product and color can still vary slightly between manufacturing batches. Buying enough to finish the job from as few separate gallons as possible avoids a visible seam or touch-up mismatch partway through a wall.
Does this calculator account for the ceiling or trim?
No, this calculator estimates wall paint only. Ceilings and trim use different paint types (often flat ceiling paint and semi-gloss trim paint) and are usually calculated and purchased separately from wall paint.
Can you give a real-world example?
A 12 ft by 10 ft room with 8 ft walls, one door, two windows, and two coats needs about 1.73 gallons at a coverage rate of 350 square feet per gallon, so buy 2 gallons. A larger 15 ft by 12 ft room with the same wall height, two doors, two windows, but only one coat needs about 1.03 gallons, also rounding up to 2 gallons.