Work out how much paint you need the way a painter does — wall area minus the doors and windows, times the coats you’re really doing, divided by real coverage. Interior or exterior, several rooms at once, in gallons and litres.
Units
Surface
Enter each room by its perimeter (add up all wall lengths) and wall height, or switch a room to wall-by-wall by typing a single wall length in the perimeter box. Length & width are only used when “ceiling” is ticked.
RoomPerimeter & height (ft)DoorsWindowsWall area
Two is normal. One only for a same-colour refresh.
Check your can. Textured or porous walls cover less.
Estimate only — applied to the painted area.
Coverage and opening sizes are editable examples, and price and labour fields are your own numbers, not a price table. Surface texture, colour change and application method all shift real coverage, so treat the result as a buy-list estimate, not a quote. Not affiliated with any paint brand.
[ Ad slot — replace with AdSense / Ezoic code ]
Decorating the same room another way?
Papering a feature wall instead of painting it? The Wallpaper Calculator turns the same wall area into rolls and handles the pattern repeat. Doing the floor too? Use the Flooring Calculator for carpet, laminate or vinyl, and the Tile Calculator for a tiled wall, splashback or floor.
What this paint calculator does — and its lane
This is a paint coverage tool, plain and simple: it takes wall and ceiling area, takes the doors and windows back out, multiplies by your coats and divides by coverage to answer how much paint do I need in gallons and litres. It is built for figuring interior and exterior house and wall paint — not the wallcovering or the floor. If you’re hanging paper, the wallpaper calculator handles rolls and pattern match; for the floor, the flooring calculator and tile calculator are separate tools with their own units.
The two things most paint calculators get wrong
Search the home-improvement forums and the same two gripes come up again and again: a paint estimator that hands back the full wall area without ever subtracting the doors and windows, and one that quietly figures a single coat so you run dry on the last wall. This calculator fixes both by default.
Openings are subtracted — a door defaults to 20 ft² and a window to 15 ft², both editable. Tell it how many each room has and the painted area drops before the gallons are figured.
Two coats by default — because nearly every job needs them, and manufacturers quote coverage per coat. Drop to one for a same-colour touch-up, or push to three for a hard colour change like red over white.
Real coverage, not the can's best case — the 350 ft²/gallon default is editable. Bare drywall, knock-down texture, masonry and spray all eat more, so field coverage is often nearer 300.
The same area drives both unit systems. In metric, one litre of wall paint covers roughly 10–12 m² per coat, so the calculator divides square metres by your litre-coverage figure and rounds up to whole litres or tins. Switch the surface to exterior and the method is identical — perimeter, height, openings, coats — but exterior coverage usually runs lower over rough siding or masonry, so adjust the coverage field down. Interior and exterior paints are formulated differently and should never be substituted for one another; this tool only sizes the quantity.
Item
Typical area
Note
One gallon (1 coat)
~350–400 ft²
Smooth, sealed wall
One litre (1 coat)
~10–12 m²
Metric equivalent
Standard door
~20 ft²
Subtracted, editable
Average window
~15 ft²
Subtracted, editable
Coverage figures around 350 ft² a gallon are the common Sherwin-Williams, Behr and Dulux label range and are facts of the product, not a brand price list. The price and labour fields are your own local numbers and stay empty until you fill them.
Several rooms, one shopping list
Painting more than one room in the same colour? Add a row per room and the calculator sums every wall and ceiling area into one total before working out the cans, so you buy paint once instead of running the numbers room by room. Each room keeps its own doors, windows and ceiling toggle, and the shareable link remembers the whole job.
[ Ad slot — replace with AdSense / Ezoic code ]
Frequently asked questions
How much paint do I need for a room?
Wall area = perimeter′ × height; subtract the doors and windows; multiply by coats; divide by coverage. A door is ~20 ft² and a window ~15 ft², and most interior latex covers ~350 ft² a gallon per coat on a smooth wall — less on texture or a colour change. This tool runs the whole chain live and rounds the gallons up.
Does this calculator subtract doors and windows?
Yes — the gap many estimators leave. It takes off 20 ft² per door and 15 ft² per window by default, both editable for oversized openings, before the gallons are figured. Enter how many each room has.
Why two coats by default?
Because most jobs need two and manufacturers quote coverage per coat. One coat rarely covers evenly, especially over a colour change, patch or fresh primer. Set coats to 1 for a same-colour refresh or 3 for a strong shift like red over white.
How many square feet does a gallon cover?
Roughly 350–400 ft² per coat on a smooth, sealed surface; about 10–12 m² per litre. Bare drywall, texture, masonry and spray all cover less — often nearer 300. The coverage field starts at 350 and is editable, so use your own can's figure.
How much paint for a 12x12 room?
48′ perimeter; at an 8′ ceiling that's 384 ft² of wall. Take off a door and a window (~35 ft²) for ~349 ft². Two coats is ~698 ft² of coverage, just under two gallons — so buy two. Add the 144 ft² ceiling and you're into a third can. Enter your own numbers above for the exact count.
Should I add the ceiling?
Only if you're painting it, and usually with its own flat product. Ceiling area = room length × width. Tick the ceiling box to fold it into the same total when using one paint throughout, or leave it off and size the ceiling separately. Trim and frames are bought separately.