Paint Calculator
Calculate how many gallons of paint you need for a room based on dimensions, doors and windows.
How to Use the Paint Calculator
Estimate exactly how much paint you need for any room. This calculator accounts for doors, windows and multiple coats so you can buy the right amount without waste.
- Enter room dimensions — length, width and ceiling height in feet.
- Specify doors and windows — the calculator uses standard sizes (door: 3'x7' = 21 sq ft, window: 3'x5' = 15 sq ft) to subtract unpaintable area.
- Set paint coverage — most interior paints cover 350-400 sq ft per gallon. The default is 350 sq ft. Check your paint can label for the exact figure.
- Choose number of coats — 2 coats is standard for most walls. Use 3 when making a dramatic colour change.
The results show total gallons needed (rounded up), total wall area, paintable area after subtracting openings, and a detailed step-by-step breakdown.
Frequently Asked Questions
How many square feet does a gallon of paint cover?
Most interior latex paints cover 350-400 square feet per gallon on a smooth surface. Rough or textured walls may reduce coverage to 250-300 sq ft. The default value of 350 sq ft is a safe general estimate.
Why does the calculator subtract area for doors and windows?
Doors and windows are not painted with wall paint, so their area is subtracted to give you an accurate estimate. The tool uses standard sizes: 21 sq ft for a door (3' x 7') and 15 sq ft for a window (3' x 5'). If your openings differ significantly, adjust your door or window count accordingly.
Should I round up when buying paint?
Yes, always round up to the nearest whole gallon. It is better to have a little extra paint for touch-ups than to run out mid-project. The calculator automatically rounds up for you.
Do I really need two coats of paint?
Two coats are recommended for nearly all interior painting projects. The first coat may look streaky or uneven, and the second coat ensures full opacity, even colour and durability. If painting over a similar colour with premium paint, one coat may suffice.
Does this calculator work for exterior walls?
You can use this calculator for exterior walls too, but exterior paint often has lower coverage (200-300 sq ft per gallon) due to surface texture. Adjust the coverage-per-gallon value accordingly.
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