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.
wall area = perimeterft × heightft − (doors × 20 + windows × 15)
+ ceiling = lengthft × widthft (if painting it)
gallons = ceil( area × coats ÷ coverage )
Gallons, litres, interior and exterior
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.