Wall area overview
Room length, room width, and wall height are used to estimate the total wall surface before openings are removed.
Construction
Paint Calculator
Estimate how much paint a room needs from wall dimensions, doors, windows, coats, paint coverage, and waste factor. Use metric or imperial units, and include the ceiling when it is part of the job.
Paint Calculator
Unit system
Result
Ready
Guide
Paint estimate basics
This paint calculator works best for standard rooms. Enter the room size, openings, coats, coverage, and waste allowance to get a paint quantity. For rooms with several wall sections or unusual shapes, calculate the sections separately and add the amounts together.
Inputs
How to use the paint calculator
Room length and widthEnter the floor dimensions of the room. These are also used for ceiling area when the ceiling option is turned on.
Wall heightUse the height from floor to ceiling, not including trim or baseboards unless you plan to paint them as wall surface.
Doors and windowsEnter counts. The calculator subtracts standard opening areas for the selected unit system.
Number of coatsUse 1 for a single coat, 2 for most repainting jobs, or more when the surface needs extra coverage.
Paint coverageUse the product label when available. The default is 10 ㎡/L in metric mode or 350 ft²/gal in imperial mode.
Waste factorAdd a small percentage for roller loading, touch-ups, texture, and normal jobsite loss.
Paint ceilingCheck this only when the ceiling will be painted. It adds room length x room width.
Visual guide
What the calculator measures
These diagrams show the surfaces used by the estimate. They are simplified on purpose, so the numbers stay easy to follow.
Doors and windows
Openings are deducted from the wall area so the estimate is closer to the paintable surface.
Ceiling included
When Paint ceiling is checked, the ceiling adds length x width to the paintable area.
Calculation flow
The estimate moves from room dimensions to paint quantity after openings, coats, waste, and coverage are applied.
Method
How the paint estimate is calculated
- 1
Calculate the wall area from the room perimeter and wall height.
- 2
Subtract the estimated area taken up by doors and windows.
- 3
Add ceiling area if Paint ceiling is selected.
- 4
Multiply the net paintable area by the number of coats.
- 5
Apply the waste factor, then divide by paint coverage.
- 6
Round the recommended purchase quantity up to the next whole liter or gallon.
Accuracy
Is the estimate accurate?
It is an estimate, not an exact shopping list. Rough plaster, bare drywall, patched areas, porous surfaces, and strong color changes can all use more paint than a smooth wall in good condition.
The waste factor is there for the real parts of painting: roller loading, cutting in, touch-ups, small spills, and paint left in trays or cans. A modest allowance usually gives a more useful result than calculating only the bare surface area.
Ceiling
Should I include the ceiling?
Check Paint ceiling when the ceiling is part of the job. Leave it unchecked if you are painting walls only. The ceiling area is calculated as room length x room width and is included before coats, waste, and coverage are applied.
Formula
Formula steps
The calculation starts with paintable area. Coats, waste, and coverage turn that area into liters or gallons, then the purchase amount rounds up.
1
Wall area
2 × (Room length + Room width) × Wall height
2
Openings area
(Doors × Door area) + (Windows × Window area)
3
Ceiling area
Room length × Room width
4
Net paintable area
Wall area − Openings area + Ceiling area, if selected
5
Total area for all coats
Net paintable area × Number of coats
6
Waste-adjusted area
Total area for all coats × (1 + Waste factor / 100)
7
Paint required
Waste-adjusted area ÷ Paint coverage
Example
Example calculation
This example uses metric units: a 5 m by 4 m room, 2.8 m wall height, one door, two windows, two coats, 10 ㎡/L coverage, 10% waste, and the ceiling included.
Wall area2 × (5 + 4) × 2.8 = 50.4 ㎡
Openings(1 × 1.8) + (2 × 1.5) = 4.8 ㎡
Ceiling5 × 4 = 20 ㎡
Net paintable area50.4 − 4.8 + 20 = 65.6 ㎡
Two coats65.6 × 2 = 131.2 ㎡
Add 10% waste131.2 × 1.10 = 144.32 ㎡
Paint required144.32 ÷ 10 = 14.43 L, so buy 15 L
Note
For unusual spaces, split the room into simpler rectangles, run each part through the calculator, then add the results. This keeps the estimate easier to check.
FAQ
Paint calculator FAQ
How much paint do I need for one room?
Enter the room length, room width, wall height, openings, coats, coverage, and waste factor. The calculator returns the estimated paint required and rounds the purchase quantity up to the next whole liter or gallon.
Does this calculator include doors and windows?
Yes. Door and window deductions are included. The default metric deductions are 1.8 m2 per door and 1.5 m2 per window. Imperial mode uses 20 ft2 per door and 15 ft2 per window.
Should I include the ceiling?
Check Paint ceiling only if the ceiling will be painted. Ceiling area is calculated as room length x room width and added before coats, waste, and coverage are applied.
What paint coverage should I enter?
Use the coverage printed on the paint container when you have it. If you are still planning, 10 m2/L or 350 ft2/gal is a common starting point, but actual coverage varies by surface and paint type.
Why should I add a waste factor?
Waste factor gives room for roller loading, touch-ups, small mistakes, surface texture, and leftover paint in trays or cans. A small allowance is usually more useful than an exact bare-wall number.
How accurate is the estimate?
It is a planning estimate, not a purchase guarantee. Rough walls, patched areas, strong color changes, first coats on bare material, and detailed trim can increase paint use.
What if my room is not a simple rectangle?
Break the space into smaller rectangular sections, calculate each section, and add the results. For unusual shapes, a higher waste factor is usually sensible.
Sources
- Standard rectangular room wall-area estimate.
- Paint coverage should be checked against the product label when available.