Paint Calculator
Calculate paint gallons required for wall square footage.
This paint calculator estimates how many gallons of paint you need for a room, based on wall dimensions and the number of coats, after subtracting the area taken up by doors and windows. It helps avoid both running short mid-project and over-buying paint you won't use.
Enter your room's wall dimensions, door and window counts, and desired number of coats, and the calculator returns the gallons of paint needed.
Worked Calculation Examples
| Scenario | Result | Calculation Step |
|---|---|---|
| 40 ft perimeter, 8 ft walls, 2 doors + 2 windows, 2 coats | 2 gallons | Wall area = 40 × 8 = 320 sq ft. Openings = (2×21) + (2×15) = 72 sq ft. Net area = 320 − 72 = 248 sq ft. Gallons = (248 ÷ 350) × 2 ≈ 1.42, rounded up to 2 gallons. |
| 60 ft perimeter, 9 ft walls, 1 door + 3 windows, 1 coat | 2 gallons | Wall area = 60 × 9 = 540 sq ft. Openings = 21 + (3×15) = 66 sq ft. Net area = 540 − 66 = 474 sq ft. Gallons = (474 ÷ 400) × 1 ≈ 1.19, rounded up to 2 gallons. |
How Paint Coverage Is Calculated
Most standard paint covers roughly 350-400 square feet per gallon for one coat. The calculator finds total wall area (perimeter x height), subtracts a standard allowance for doors and windows, then divides by coverage per gallon and multiplies by the number of coats you plan to apply. Standard paint coverage figures (commonly 350-400 square feet per gallon) come from manufacturer testing on primed, smooth drywall under controlled conditions, which is why porous, textured, or unprimed surfaces typically require more paint than the label's coverage estimate suggests.
Why Two Coats Are Usually Recommended
A single coat often leaves uneven coverage or lets the previous wall color show through, especially with a dramatic color change. Two coats is the standard recommendation for most interior painting projects, which roughly doubles the paint quantity needed compared to a single-coat estimate.
How to Use the Paint Calculator
- Open the Paint Calculator and enter the values requested in the input fields.
- Check the units, percentages, dates, or time periods before reading the answer.
- Review the instant result and adjust any value to compare another scenario.
- Use the formula, example, and FAQs below to understand how the paint calculator works.
Frequently Asked Questions
How much paint do I need for a room?
Calculate total wall square footage, subtract area for doors and windows, then divide by your paint's coverage rate (typically 350-400 sq ft per gallon) and multiply by the number of coats.
How many square feet does a gallon of paint cover?
Most standard interior paint covers 350-400 square feet per gallon per coat, though coverage varies by paint brand, sheen, and the surface being painted.
Do I need to subtract doors and windows from wall area?
Yes - excluding the area taken up by doors and windows prevents over-estimating how much paint you need, since those areas aren't painted with wall paint.
How do I use this paint calculator?
Enter the known values, review the units or settings, and the calculator updates the result instantly. The formula and example on this page show how the answer is produced.
What does the Paint Calculator calculate?
Calculate paint gallons required for wall square footage. It is designed for fast browser-based calculations without sign-up, downloads, or manual spreadsheet setup.
What formula does this paint use?
The formula is: Gallons = [(Wall Area − Door/Window Area) ÷ Coverage per Gallon] × Coats. Wall Area is perimeter × height. Coverage per Gallon is the manufacturer's stated square-foot coverage (commonly 350-400 sq ft), and Coats is how many layers you plan to apply. Dividing gives gallons for one coat; multiplying by the coat count scales it up, since each additional coat needs the full area covered again.
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Last updated: September 29, 2026.