Enter your room dimensions to instantly calculate the gallons of paint needed.
How the Paint Calculator Works
Calculate Wall Perimeter × Height
Subtract Doors & Windows
Multiply by Number of Coats
Divide by Coverage per Gallon
What is a Paint Calculator?
A Paint Calculator estimates how many gallons of paint you need to cover a room's walls, based on the room's dimensions, the number of coats you plan to apply, and your specific paint's coverage rate. Paint is sold by the gallon (or quart for small projects), and buying the right amount upfront saves you from an inconvenient mid-project store run or the frustration of ending up with several nearly-full leftover cans in different rooms.
The Paint Calculation Formula
Wall Area (ft²) = (2 × (Length + Width)) × Ceiling Height − Openings Area
Gallons Needed = (Wall Area × Number of Coats) ÷ Coverage per Gallon
The calculator first finds your room's total wall perimeter, multiplies it by ceiling height to get total wall area, then subtracts the area of doors and windows (which don't need paint). This net paintable area is then multiplied by your number of coats and divided by your paint's coverage rate — most standard paints cover approximately 350-400 square feet per gallon on smooth surfaces.
Typical Paint Coverage Rates
| Surface Type | Approx. Coverage per Gallon |
|---|---|
| Smooth drywall (standard paint) | 350 – 400 sq ft |
| Textured or rough walls | 250 – 300 sq ft |
| Previously unpainted drywall/primer | 200 – 300 sq ft (more absorbent) |
| Masonry or brick | 150 – 250 sq ft (highly absorbent) |
How Many Coats Do You Need?
- 1 coat: Only sufficient when applying the same color over a similar existing color in good condition.
- 2 coats (recommended for most projects): Provides even, opaque coverage and is the industry-standard recommendation for most color changes.
- 3 coats: Often necessary for dramatic color changes (like painting over a dark color with a light one) or when using certain premium/deep-tone colors that require extra coverage.
Tips for Accurate Paint Estimating
- Round up to the nearest full gallon or 5-gallon bucket: Partial gallon purchases aren't usually available, and running short mid-wall creates visible touch-up lines.
- Check your specific paint can's coverage rating: Coverage varies by brand and paint type (matte, eggshell, semi-gloss), so check the label for the most accurate figure.
- Buy a little extra for touch-ups: Keeping a small amount of matching paint on hand makes future touch-ups seamless.
- Consider ceiling and trim separately: This calculator estimates wall paint only — ceilings and trim typically need separate paint and calculations.
Frequently Asked Questions
How many gallons of paint do I need for an average bedroom?
A typical 12x12 foot bedroom with 9-foot ceilings and two coats usually requires about 1.5 to 2 gallons — enter your exact dimensions above for a precise figure.
Should I subtract closet doors and windows?
Yes, subtracting the area of all doors, windows, and other openings gives a more accurate paint estimate since those areas won't be painted with wall paint.
Do I need primer in addition to paint?
Primer is recommended when painting over a drastically different color, bare drywall, or a stained surface, and should be calculated and purchased separately from your topcoat paint.
Why does my actual paint usage differ from the calculator?
Application method (roller vs sprayer), wall texture, and paint brand can all affect real-world coverage, so treat this as a solid starting estimate and round up when purchasing.