Paint Calculator

What Does This Paint Calculator Do?

This paint calculator estimates how many gallons of paint you need for a room. You can either enter the total wall square footage directly, or switch to "Room Dimensions" mode and enter the room's length, width, and wall height — the calculator will work out the wall area for you. Add the number of coats you plan to apply, the paint's coverage rate, and an optional deduction for windows and doors to get an accurate gallon estimate.

How Paint Coverage Is Calculated

Paint cans list a coverage rate in square feet per gallon — commonly around 350 sq ft per gallon, though this varies by brand, paint type, and surface texture. The calculation works in three steps:

Worked Example: 12 ft x 12 ft Room, 8 ft Walls, 2 Coats

Let's calculate paint for a standard bedroom-sized room:

• Length = 12 ft, Width = 12 ft, Wall Height = 8 ft
• Wall Area = 2 x (12 + 12) x 8 = 2 x 24 x 8 = 384 sq ft
• No deduction for windows/doors in this example, so Paintable Area = 384 sq ft
• Total Area to Cover (2 coats) = 384 x 2 = 768 sq ft
• At 350 sq ft/gallon coverage: 768 / 350 = ~2.2 gallons, typically rounded up to 3 gallons since paint is sold in whole-gallon or quart increments

Why Two Coats?

Most professional painters recommend two coats of paint for even color, full opacity, and durability — especially when covering a darker existing color, bare drywall, or a stain. A single coat often looks streaky or lets the underlying color or primer show through in spots.

Tips for Buying the Right Amount of Paint

1. Round up, not down: Paint is sold in gallon and quart increments, so always round your gallon estimate up rather than trying to buy the exact fractional amount.
2. Buy a little extra: Keep a partial gallon on hand for touch-ups later — paint colors can be hard to match exactly if you run out and need to remix.
3. Check the can's actual coverage rate: Different paint lines and sheens (flat, eggshell, satin, semi-gloss) can have meaningfully different coverage rates — check the label rather than assuming 350 sq ft/gallon for every product.
4. Account for texture: Textured walls, popcorn ceilings, and unprimed drywall soak up more paint than smooth, previously painted surfaces.
5. Don't forget the ceiling: This calculator estimates wall paint only — ceilings need to be calculated separately using the ceiling's square footage.
6. Use a deduction for rooms with lots of windows: If a room has multiple large windows or doors, subtracting 5-15% from the wall area avoids over-ordering paint.

Planning a bigger renovation? Check out our concrete calculator and square footage calculator for more DIY project planning tools.