Paint Calculator
Calculate how much paint you need for any room by entering wall dimensions, doors, windows, ceiling area, coats, and coverage.
Quick shows a simplified estimate of your detailed room — switch back to Detailed for exact numbers.
Room dimensions
Openings
Walls
Ceiling
Uses the room rectangle derived from your walls.
Trim (optional)
Trim paint is separate from wall paint. Per trimmed wall it uses the full wall width × the selected trim heights, painted with the room's coat count.
Elevations
Each elevation is one face of the house — add one per side, enter width and height, then add its doors and windows.
Surface
Coverage varies by surface. Adjust the coverage value below.
Paint
Defaults to 300 sq ft/gal for exterior surfaces — edit it to match your paint's label.
Trim (optional)
Exterior trim is estimated approximately — enter the trim surface area you plan to paint (e.g. window and door surrounds, fascia). The result is labeled approximate.
Paint settings
Preview your room
View the room in 3D
Drag to orbit · scroll to zoom. Walls show your paint color.
3D preview is simplified for detailed/exterior estimates.
Paint estimate
Round up to whole gallons. Excludes primer.
How to estimate paint for a room
Before you buy paint, you want a number you can trust — not a guess that leaves you short mid-wall or with three leftover gallons. Estimating paint is simple arithmetic, and getting it right saves money and a second trip to the store. A paint calculator turns your room measurements into a shopping list: paintable wall area, gallons and liters for the number of coats you plan, and a cost estimate if you enter a price.
You can calculate paint by room dimensions or square footage. Enter your wall dimensions and the calculator estimates the paintable wall area in square feet after doors and windows, then converts it to gallons and liters.
1. Measure the walls
For each wall, multiply height by width. A 12 ft × 9 ft wall is 108 sq ft. Add all walls together. In a rectangular room, that is 2 × (length + width) × height. Our calculator does this the moment you type. For a wall paint calculation, enter the room's length, width, and wall height — wall area, after doors and windows are deducted, is what the calculator works from. Measuring an odd-shaped room? Add each wall individually in Detailed mode instead of relying on the rectangle.
2. Subtract doors and windows
These surfaces are not painted. A standard door is about 21 sq ft; a typical window around 15 sq ft. Subtract the total opening area from your wall area to get the true paintable surface. The calculator deducts doors and windows automatically — or use custom opening sizes in Detailed mode. For example, a 12 × 14 ft room with 9 ft walls has roughly 468 sq ft of wall, but one door and two windows bring the paintable surface down to about 417 sq ft.
3. Count the coats
One coat rarely covers. Two coats is standard over primer; three if you are going from a dark color to a light one. Multiply your paintable area by the number of coats to get the true paint volume. The coat count applies to walls and ceiling alike in the calculator.
4. Divide by coverage
A gallon of interior paint covers roughly 350–400 sq ft per coat on smooth, primed walls. Divide your total area by that number. The result is your gallons — round up, and keep one spare for touch-ups. The tool also shows liters for metric users. Coverage depends on the surface: rough exterior siding and stucco absorb more, which is why the exterior estimate defaults to 300 sq ft/gal and lets you adjust it.
5. Don't forget the ceiling
Need ceiling paint too? Enable "Include ceiling" to add the ceiling surface (length × width) to the estimate. Ceilings usually take a single coat, and the result separates wall and ceiling totals so you can buy accordingly. Primer is not included in any estimate — add it separately if your walls are bare or patched.
The formula in one line
Paint needed (gallons) = (wall area − openings) × coats ÷ coverage. For a room, wall area is 2 × (length + width) × wall height. That is the entire calculation behind this tool — you just type the numbers and it does the arithmetic for walls, ceiling, and exterior elevations.
Room presets
Use the room presets for common spaces such as bedrooms, living rooms, kitchens, bathrooms, dining rooms, offices, and garages, or enter your own dimensions. The presets set sensible starting sizes and openings, which you can edit at any time.
Exterior painting
For exterior projects, switch to Detailed → Exterior to calculate paint across multiple elevations, doors, windows, and optional trim. Exterior coverage is separately configurable — it defaults to 300 sq ft/gal for siding, stucco, and brick, and you can adjust it to match your paint's label.
Example: a 12 × 14 ft room
The amount of paint depends on wall area, ceiling area, openings, coats, and coverage — not just room floor size. For a 12 × 14 ft room with 9 ft walls and no openings, the walls are roughly 468 sq ft (2 × (12 + 14) × 9), so two coats need about 2.7 gallons at 350 sq ft/gal. Enter 12 ft length, 14 ft width, and your wall height in the calculatorand adjust doors, windows, and coats for your exact room.
This page is a neutral calculator — no brand, no upsell. Just the math, fast.
Frequently asked questions
How do I calculate how much paint I need for a room?
Add up your wall area (height × width for each wall), subtract the area of doors and windows, multiply by the number of coats, then divide by the paint's coverage rate (usually 350–400 sq ft per gallon). Our calculator does this instantly and shows both gallons and liters.
How do I calculate paint by square feet?
Measure each wall (height × width), add them together, subtract doors and windows, multiply by the number of coats, then divide by your paint's coverage rate (350–400 sq ft per gallon). This calculator does all of that from your room dimensions — the result shows the paintable wall area in square feet before converting to gallons and liters.
How many square feet does a gallon of paint cover?
Most interior paints cover about 350–400 square feet per gallon per coat on primed, smooth walls. Flat finishes and darker colors may need more. Use 350 as a safe default; our tool lets you edit the coverage rate.
Does the calculator subtract doors and windows?
Yes. Enter the number of doors and windows (standard or custom sizes in Detailed mode) and their combined area is deducted from the wall area before the paint volume is calculated.
Do I need to paint the ceiling too?
If you toggle 'Include ceiling,' we add the floor area (length × width) to the paintable surface. Ceilings usually take one coat. The result block separates wall and ceiling totals so you can buy accordingly.
Can I calculate exterior paint?
Yes. In Detailed Estimate, switch to Exterior and add one elevation per side of the house with its width, height, doors, and windows. Exterior coverage defaults to 300 sq ft/gal and is separately editable; optional trim is estimated from the area you enter.
How many coats of paint should I use?
Two coats is the standard for most rooms over primer. Use three when painting over a dark color or a porous surface. The calculator multiplies the paintable area by the coat count you select.
Should I round up the gallons?
Yes. Paint is sold by the gallon or litre, so always round up to the next whole unit and buy one extra for touch-ups. Our estimator shows the raw figure — round up at the store.