OptiDrop

Paint Calculator — How Much Paint Do I Need?

Enter your room dimensions, number of doors and windows, and coats of paint to calculate exactly how many gallons you need. 100% free, no sign-up.

Free Paint Calculator Online — Estimate Paint Gallons Needed

OptiDrop's Paint Calculator helps you estimate exactly how many gallons of paint you need for your room. Enter the room dimensions, number of doors and windows, and the number of coats. The calculator uses a standard coverage rate of 350 square feet per gallon and gives you an accurate estimate so you can buy the right amount of paint.

How Does the Paint Calculator Work?

The calculator first computes the total wall area by multiplying the room perimeter (2 times length plus 2 times width) by the room height. It then subtracts the area of doors (21 sq ft each) and windows (12 sq ft each) to get the paintable area. This area is multiplied by the number of coats, then divided by 350 sq ft per gallon to determine the gallons needed.

Tips for Accurate Paint Estimation

Always round up to the nearest gallon when purchasing paint. If your walls are textured or porous, consider adding 10-20% more paint. For dramatic color changes (dark to light or vice versa), use a tinted primer first and plan for 2-3 coats. Buy all your paint from the same batch to ensure consistent color throughout the room.

Frequently Asked Questions

To calculate paint needed, measure the room's wall area (perimeter times height), subtract the area of doors and windows, then divide by the paint coverage per gallon (typically 350 sq ft). Multiply by the number of coats. This calculator does it automatically.
One gallon of paint typically covers about 350 square feet (32.5 square meters) of smooth, primed surface. Coverage varies based on paint type, surface texture, and application method. Textured or porous surfaces may require more paint.
Most walls need 2 coats of paint for even coverage and true color. If painting over a dark color with a lighter one, or using a similar shade, you may need a primer plus 2 coats. Painting a similar color over existing paint may only need 1 coat.
Yes, you should subtract the area of doors and windows from your total wall area for a more accurate estimate. A standard door is about 21 sq ft and a standard window is about 12 sq ft. This calculator lets you enter the number of each and deducts them automatically.

Last updated: June 2026