Last updated October 13, 2026 · Report an error
Buying too little paint means an interrupted project and a possible color-match issue on a second can — buying too much wastes money on leftover paint.
This calculator finds exactly how many gallons you need for a room, based on its dimensions, number of coats, and paint coverage.
How it works
Enter the room's length, width, and wall height, along with the area of any doors and windows to exclude, the number of coats, and the paint's coverage per gallon. The calculator finds the total area to coat and divides it by the coverage rate to find the gallons needed.
- Enter room length (ft).
- Enter room width (ft).
- Enter wall height (ft).
- Enter doors/windows to exclude (sq ft).
- Enter number of coats.
- Enter coverage per gallon (sq ft).
- Click Calculate to see your results.
Examples
A 12×10 ft bedroom
A 12 ft by 10 ft room with 8 ft walls and 40 sq ft of doors/windows needs about 1.78 gallons for two coats at standard 350 sq ft/gallon coverage — round up to 2 gallon cans.
Who should use it
- Planning how much paint to buy for a single room repaint.
- Budgeting material costs for a painting project before starting.
Industry applications
- Residential painting and home improvement
- Contractor project estimating
Advantages
- Accounts for door/window deductions, multiple coats, and custom coverage rates.
- Rounds up to whole gallon cans, matching how paint is actually sold.
Limitations
- Calculates wall area only — ceilings and trim need to be calculated separately.
Common mistakes to avoid
- Forgetting to subtract door and window area, which can lead to buying slightly more paint than necessary.
- Assuming darker colors need fewer coats — dramatic color changes often need extra coats or a tinted primer regardless of the direction of the change.
Best practices
- Buy all the paint for a project in one batch when possible, since paint color can vary slightly between production batches even with the same product code.
Tips
- When in doubt, round up to the next full gallon — running short mid-coat on a wall creates a visible seam once the paint dries.