Paint Calculator
Paint calculator: how many gallons of paint you need, with doors and windows subtracted, coats and primer counted. Free, no sign-up, and it runs in your browser.
Dedicated page: /en/paint-calculator
Default checked 10 Nov 2026: $38 per gallon (Behr Premium Plus 1 gal at Home Depot, $37.98). Leave it, or enter your exact price if you know it. Check the current value here.
Quick answer
A paint calculator works out how many gallons of paint a room needs. It measures the wall area from the room size, subtracts doors and windows, multiplies by the number of coats, divides by the coverage per gallon, and rounds up to the nearest quart.
About this Paint Calculator
Paint calculator: how many gallons of paint you need, with doors and windows subtracted, coats and primer counted. It is one of the Home and DIY on ToolUstad, and like every tool here it runs entirely in your browser: nothing you type, paste or open is uploaded to a server, and there is no sign-up or usage limit.
What is a paint calculator?
A paint calculator answers the question every decorator asks in the paint aisle: how many gallons do I actually need? Buying too little means a half-finished wall and a second trip, and the new batch may not match the first. Buying too much means gallons of paint sitting in the garage for years. The maths is simple in principle: measure the walls, subtract what is not wall, multiply by the coats, and divide by the coverage. In practice people forget the doors, guess the coverage, and buy for one coat when two are needed, so the calculator does each step properly.
The standard coverage is about 350 square feet per gallon on smooth interior walls, 400 on very smooth primed walls, and 300 or less on rough or textured surfaces. A standard door is about 21 square feet and a standard window about 15. The calculator uses those figures, counts coats and primer separately, adds the ceiling and trim as options, and rounds the result up to the nearest quart, because paint is sold in whole gallons and quarts, and running out mid-wall is the worst possible outcome.
How does paint maths work?
A 12 by 12 foot room with 8 foot ceilings has 384 square feet of wall: two walls of 12 by 8 and two more of 12 by 8, which is 2 times the length plus the width, times the height. Subtract one door at 21 square feet and two windows at 15 each, and 333 square feet remain. Two coats double that to 666. At 350 square feet per gallon, that is 1.9 gallons, rounded up to 2 gallons. Add a primer coat over the same 333 square feet and you need another gallon of primer. The ceiling, if painted, adds the floor area of 144 square feet for one coat.
When and why to use a paint calculator
- Before going to the store: arrive with a number instead of guessing, and buy all the paint from one batch so the colour matches.
- Budgeting a whole house: run each room separately, because ceiling heights and window counts change the answer more than floor area does.
- Colour changes: a dark to light change needs the primer coat this tool adds, and the maths shows why.
Best practices
- Measure every wall, not just the floor: a room with a vaulted ceiling has far more wall than the floor plan suggests.
- Buy all the paint at once from the same batch, and keep a little spare for touch-ups in a labelled jar.
Common mistakes to avoid
- Planning one coat for a colour change: the old colour shows through, and the second coat costs more than the paint saved.
- Using the coverage on the tin as a promise: it assumes perfect smooth walls and a skilled painter, so rough or patched walls need the lower figure.
Who is the paint calculator for?
Homeowners painting a room themselves, landlords refreshing a rental between tenants, and anyone getting a quote from a painter who wants to check the materials. It is also used by small decorating businesses for quick estimates on site. The tool is an estimator: real coverage depends on the paint, the surface and the painter, so buy a little extra rather than a little less.
Current value, checked 10 Nov 2026: the default paint price is $38 per gallon (Behr Premium Plus 1 gal at Home Depot, $37.98). The price box is an optional override: leave the default, or enter your exact price and the cost uses it.
Learn: how to use the Paint Calculator
- Measure the room in feet: length, width and wall height. A 12 by 12 foot room with 8 foot ceilings is the default.
- Count the doors and windows so their area is subtracted, then pick the number of coats and the paint coverage.
- Read the gallons to buy, rounded up to the nearest quart, plus primer if you asked for it, and the estimated cost.
Features
- Wall area from room dimensions, with doors at 21 sq ft and windows at 15 sq ft subtracted
- One, two or three coats, with 300, 350 or 400 sq ft per gallon coverage
- Primer coat calculated separately at its own coverage
- Ceiling painting and trim allowance included as options
- Cost estimate from your price per gallon, rounded up to whole quarts
Worked example
A 12 by 12 foot room with 8 foot ceilings, 1 door and 2 windows, painted in 2 coats at 350 sq ft per gallon: wall area is 384 sq ft, minus 51 sq ft of openings, times 2 coats is 666 sq ft, so buy 2 gallons (1.9 gallons rounded up to the nearest quart).
Frequently asked questions
How many square feet does a gallon of paint cover?
About 350 square feet per gallon on smooth interior walls, 400 on very smooth primed walls, and 300 or less on rough or textured surfaces. This tool defaults to 350 and lets you change it.
Do I really need two coats?
Almost always for a colour change, and always over bare drywall or a dark colour. One coat is only enough for a same-colour refresh on a wall that is already in good shape.
Should I paint the ceiling the same colour?
Ceilings are usually painted flat white, not wall colour, because white reflects light and hides imperfections. This tool adds the ceiling area as one coat when you tick the option.
How much primer do I need?
One coat over the same area as the paint, at the same coverage. Primer matters most on bare drywall, patched walls and dark colours. On a previously painted wall in good shape you can usually skip it.
How much paint do I need for a 12 by 12 room?
About 2 gallons for two coats on the walls, plus 1 gallon of primer if the walls are bare or dark. The ceiling adds about half a gallon if you paint it too.
What is the best paint finish for walls?
Eggshell or satin for living rooms and bedrooms, because they hide imperfections and wipe clean. Semi-gloss for kitchens, bathrooms and trim, because it resists moisture and scrubbing.
Can I paint over dark colours without primer?
You can, but it takes three coats instead of two, and the result is often patchy. One coat of tinted primer is cheaper and faster than the extra coat of top paint.
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