How this paint calculator works
Quick answer: the calculator adds the four wall surfaces and the ceiling, subtracts the doors and windows you enter, multiplies the paintable area by the number of coats and waste allowance, then divides by the coverage printed on the paint label. The main result is litres for metric inputs or gallons for imperial inputs.
The formula behind the estimate
- Wall area = 2 × (room length + room width) × wall height
- Opening area = doors × door width × door height + windows × window width × window height
- Ceiling area = room length × room width
- Adjusted area = (wall area − openings + ceiling area) × coats × (1 + waste percentage ÷ 100)
- Paint quantity = adjusted area ÷ label coverage
Use metres with m² per litre, or feet with square feet per gallon. Mixing unit systems will produce a misleading answer.
How to estimate paint step by step
- Measure the room length, width, and wall height at finished surfaces.
- Select metric or imperial before interpreting the result.
- Enter the finish paint’s stated coverage. Product-specific coverage is more reliable than a generic average.
- Choose the number of finish coats and set a transparent waste allowance.
- Enter door and window counts plus their typical dimensions if you do not plan to paint those openings.
- Round the result up to a practical can size and confirm the instructions on the product label.
The estimate includes all four walls and one full ceiling. It assumes every entered door has the same dimensions and every entered window has the same dimensions. Trim, cabinets, built-ins, primer, and separate accent colors are not included.
Why label coverage matters
Coverage varies by product, surface texture, porosity, application method, and color change. As a broad reference, Sherwin-Williams says one gallon typically covers about 350-400 square feet, but the selected product's label and technical data remain the controlling source. This calculator therefore lets you replace the default with the exact spreading rate for your paint.
A rough or newly repaired surface may absorb more paint than a smooth, sealed wall. A major color change can also affect the coating system. Treat the calculated quantity as a purchasing baseline, not a promise that every substrate will use paint at the same rate.
How coats, openings, and waste change the result
Coats
The calculator multiplies the paintable area by the number of coats. Enter finish coats only. If the project needs primer, estimate and purchase primer separately because its coverage can differ from the finish paint.
Doors and windows
Openings reduce the calculated wall area. Leave the counts at zero if you prefer a conservative estimate, if openings vary substantially in size, or if some of those surfaces will be painted with the same product.
Waste allowance
The default planning allowance is 10%, and you can change it from 0% to 30%. Extra material can cover cutting-in, roller and tray losses, small measurement errors, and later touch-ups. Complex layouts may justify more; a simple room measured carefully may need less.
What this calculator does not estimate
- Primer, stain blocker, or specialty undercoats
- Trim, doors, cabinets, floors, or exterior siding
- Different colors on separate walls
- Product loss caused by spray application
- Repairs, surface preparation, drying time, or labor
For better inputs, see how to measure a room for paint, how to choose a coat count, and the paint preparation checklist.
Frequently asked questions
Does this paint calculator include the ceiling?
Yes. It automatically adds one ceiling equal to room length multiplied by room width, as well as all four walls.
How much area does one gallon of paint cover?
A typical reference is about 350-400 square feet per gallon, but coverage varies. Use the exact figure printed on the selected product's label whenever possible.
Should I subtract doors and windows?
Subtract them when they will not receive the same paint. Leave them at zero for a more conservative estimate or when opening sizes vary too much for one typical dimension.
Can I use both metric and imperial measurements?
Yes, but use only one system per calculation. Metric expects metres and square metres per litre; imperial expects feet and square feet per gallon.