Paint Calculator turns a room's floor plan into a shopping list. Enter the length, width and ceiling height and it works out the wall area as perimeter times height, subtracts a standard allowance for each door and window, multiplies by the number of coats, and divides by the coverage rate to give litres, gallons and whole cans. The ceiling can be included, in which case the floor footprint is added on top. Coverage defaults to about 350 square feet per gallon, the figure manufacturers quote for one coat on a primed smooth wall, but it stays editable because bare drywall, textured surfaces and dark-to-light colour changes all drink noticeably more.
Features
Wall area from room length, width and ceiling height
Standard deductions for doors and windows
Optional ceiling coverage from the room footprint
Any number of coats, applied to the paintable area
Results in gallons, litres and whole cans to buy
Editable coverage rate for textured or porous surfaces
US and metric units throughout
Optional cost estimate from a price per can
How to use the Paint Calculator
Enter the room length, width and ceiling height
Set how many doors and windows you are painting around
Choose the number of coats and whether the ceiling is included
Read the litres, gallons and can count, and add a price for a cost estimate
Example
Input
12 × 10 ft room, 8 ft ceiling, 1 door, 2 windows, 2 coats
Output
Paintable 275 sq ft · 550 sq ft with coats · 1.57 gal · 2 cans
Two coats on the walls of an average bedroom, with one door and two windows deducted.
Common errors & troubleshooting
The estimate seems low for bare or previously dark walls. — Lower the coverage figure. Unprimed drywall and strong colour changes can cut a gallon's reach by a third, so 250 sq ft per gallon is a safer assumption than 350 for those jobs.
The ceiling area is not being counted. — Turn on the ceiling toggle. Without it only the walls are measured, since many rooms get a different paint on the ceiling and are bought separately.
You have oddly shaped walls that a length-by-width room does not describe. — Measure the total wall length yourself and enter it as half the perimeter — for example a 44 ft perimeter can be entered as a 12 × 10 room, since 2 × (12 + 10) is 44.
The can count seems to round up aggressively. — Paint is sold in whole containers, so a 1.1 gallon requirement means buying two. The exact gallon figure is shown alongside if you would rather buy a gallon plus a quart.
Frequently asked questions
How much paint does one gallon actually cover?
About 350 square feet for a single coat on a primed, smooth surface. Rough render, bare drywall and textured walls can drop that to 250 square feet or less, which is why the coverage figure is editable rather than fixed.
Should I include doors and windows in the paint calculation?
Not in the wall area, since you are not painting the glass or the door face with wall paint. Each door subtracts about 21 square feet and each window about 12, which is what this calculator deducts automatically.
How many coats of paint does a wall need?
Two is the normal answer for a colour change or bare surface. One coat can be enough for a refresh in the same colour over a sound existing finish, and three may be needed going from a dark colour to a pale one without primer.
Does the Paint Calculator include primer?
No, it sizes the topcoat. If you are priming, run the calculation again with one coat at the primer's own coverage rate, which is usually similar to the topcoat's.
Are my room measurements sent anywhere?
No. Every figure is used only for arithmetic in your browser and is never uploaded or stored.
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