Everyday Tools
Paint Calculator
Estimate how much paint a room needs from its wall length and height, the openings you will not paint, and the number of coats.
Calculation inputs
Add up the length of every wall you plan to paint. Coverage is printed on the can.
Results
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Complete the fields and select Calculate to show results here.
Estimates only. Check the coverage rate on your chosen product before buying.
About this calculator
This tool estimates how many gallons and quarts of paint a room will take, based on the walls you plan to cover, the openings you will skip and the number of coats. It helps you buy close to the right amount the first time instead of running short halfway through a wall.
How the calculation works
- Paintable area = wall length × height − (doors × 20) − (windows × 15)
- Gallons = paintable area × coats ÷ coverage per gallon
- Quarts = gallons × 4, rounded up
Notes and assumptions
Most interior paints list a coverage of roughly 250–400 sq ft per gallon; 350 is a common mid-point.
Ceilings and trim are not included — measure them separately if you plan to paint them.
How it works in plain English
Paint is sold by volume, but it is used by area, so the first job is to find the surface you will actually cover. The tool multiplies the combined length of all the walls by their height. That gives the gross wall area, as if the room had no doors or windows at all.
Openings are then taken away. A typical interior door is close to 20 square feet and an average window close to 15 square feet, so each one you enter removes that much from the total. What remains is the paintable area.
Every extra coat covers that same surface again, so the paintable area is multiplied by the number of coats. Finally, the result is divided by the coverage rate printed on the can, which tells you how many square feet one gallon spreads across a smooth, primed wall. The answer is shown in gallons and converted to quarts, rounded up, along with a sensible mix of gallon and quart cans to buy. Rough plaster, fresh drywall, bare wood and big colour changes all drink more paint, so treat the result as a minimum in those cases.
The formula
- Paintable area = (wall length × height) − (doors × 20) − (windows × 15)
- Gallons = paintable area × coats ÷ coverage per gallon
- Quarts = gallons × 4, rounded up
Worked example
Picture a 12 by 12 foot bedroom with 8 foot walls, one door and two windows, getting two coats of a paint rated at 350 square feet per gallon.
The four walls add up to 48 feet of length, and 48 × 8 gives 384 square feet of gross wall. The door removes 20 square feet and the two windows remove 30, leaving 334 square feet to paint.
Two coats double that to 668 square feet. Dividing by 350 gives about 1.91 gallons, which is just under 8 quarts. Since three quarts usually cost about the same as a full gallon, the practical purchase here is two gallon cans, with a small amount left over for touch-ups later.
Typical coverage rates by product
| Product | Approx. coverage per gallon | Notes |
|---|---|---|
| Interior latex wall paint | 250–400 sq ft | 350 sq ft is a common label figure |
| Paint-and-primer in one | 250–400 sq ft | Fewer coats on matching colours |
| Drywall primer | 200–300 sq ft | New drywall absorbs more |
| Exterior siding paint | 250–350 sq ft | Lower on rough or textured siding |
| Ceiling paint | 350–400 sq ft | Flat finish, measured separately |
Source: typical ranges printed on manufacturer can labels and product data sheets; always check the label on the product you buy.
Frequently asked questions
How many coats of paint does a wall usually need?
Two coats is the normal recommendation for an even, durable finish, especially when the new colour differs from the old one. A single coat can work when you are refreshing a wall in the same shade with a good-quality paint. Very dark or bright colours over a light base sometimes need a third coat or a tinted primer first.
Why do you subtract 20 square feet for a door and 15 for a window?
Those are rounded sizes for a standard interior door and an average window. A 30 by 80 inch door is about 17 square feet with trim, and many windows fall between 12 and 18 square feet. Using simple round figures keeps the estimate quick, and the small difference rarely changes how many cans you need to buy.
Does the result include the ceiling or trim?
No. The tool only counts the walls you describe. Ceilings are usually painted with a separate flat ceiling paint, so measure the room's length times width and run that area on its own. Baseboards, door frames and window trim take very little paint and are normally done with a quart of semi-gloss.
Should I buy extra paint beyond the estimate?
Rounding up to the next can already gives a small buffer in most rooms. If the surface is porous, textured or freshly patched, it can absorb noticeably more, so an extra quart is sensible. Keeping a little leftover paint from the same batch also makes future touch-ups blend in much better than a new mix.
Can I use this for exterior walls?
Yes, as long as you enter the total length and height of the siding you will paint and use the coverage rate from your exterior product. Rough surfaces such as stucco, brick or cedar shakes absorb more paint than smooth siding, so lowering the coverage figure gives a more realistic result for those materials.
Helpful next steps
- Flooring Calculator — Estimate flooring for the same room while you plan the project.
- Area Converter — Convert wall area between square feet and square meters to match the can label.
- Volume Converter — Convert gallons to liters when comparing paint sizes.
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