Paint Calculator: the logic (simple, but “real-world”)

Most paint calculators fail in one of two ways: they’re either too simplistic (“room size × coverage”), or they ask for so many details that nobody finishes the form. This tool is built in the middle: it stays fast, but it includes the factors that actually change what you buy at the store—coats, openings, coverage, and waste.

You can estimate area in two ways:

• Room mode: You enter room length, room width, and wall height. The calculator assumes a rectangular room and computes the wall area using perimeter × height.
• Custom area mode: You directly type the total paintable wall area (great for L-shaped rooms, hallways, stairwells, accent walls, and non-standard spaces).

In room mode, perimeter is computed as:
Perimeter = 2 × (Length + Width)
Then wall surface area is:
Wall Area = Perimeter × Wall Height

If you include the ceiling, the ceiling area is:
Ceiling Area = Length × Width
And total paintable area becomes:
Total Area = Wall Area + Ceiling Area

You usually don’t paint most doors and windows, so subtracting them prevents overbuying. To keep the form fast, we use typical default sizes:

• Standard door: 3 ft × 7 ft = 21 sq ft
• Standard window: 3 ft × 5 ft = 15 sq ft

Total openings area is:
Openings Area = (Doors × 21) + (Windows × 15) (in square feet)

If you’re in meters, the calculator automatically converts and uses the same logic internally.

Paint coverage is usually labeled per coat. If you do 2 coats, you effectively need to cover the whole area twice. So the coated area is:
Coated Area = (Total Area − Openings Area) × Coats

Real projects have roller waste, tray loss, extra coverage on repairs, and “oops” moments. So we add a buffer:
Final Area = Coated Area × (1 + Waste%/100)

If your paint covers C square feet per gallon, then:
Gallons Needed = Final Area ÷ C

We also show liters because many markets buy paint that way:
Liters Needed = Gallons Needed × 3.78541

Then the calculator recommends a buy quantity by rounding up to practical containers: gallons (and a “quarts” hint for small jobs). Rounding up matters because it’s better to have a little extra than to run out mid-wall.

Paint estimate examples (quick sanity checks)

Room: 12 ft × 10 ft, height 8 ft, 2 coats, 1 door, 2 windows, coverage 350 sq ft/gal, waste 10%.

• Perimeter = 2 × (12 + 10) = 44 ft
• Wall area = 44 × 8 = 352 sq ft
• Openings = (1 × 21) + (2 × 15) = 51 sq ft
• Paintable per coat = 352 − 51 = 301 sq ft
• 2 coats = 602 sq ft
• +10% waste = 662.2 sq ft
• Gallons = 662.2 ÷ 350 = 1.89 → buy 2 gallons

You measured one wall at 14 ft × 9 ft = 126 sq ft. No windows. 2 coats. Coverage 400 sq ft/gal. Waste 5%.

• Area = 126
• 2 coats = 252
• +5% = 264.6
• Gallons = 264.6 ÷ 400 = 0.66 → buy 1 gallon (or 3 quarts)

Same bedroom as Example 1, but you also paint the ceiling (one coat for ceiling is common—if you want that, you can run walls and ceiling as two separate calculations, or set coats to 1 for the combined job).

Pro move: run two estimates—(A) walls, 2 coats; (B) ceiling, 1 coat—then buy accordingly.

• Heavy texture (orange peel, knockdown, stucco) increases paint use.
• Fresh drywall or patched areas absorb more paint without primer.
• Color change from dark → light often needs extra coverage.
• Low-quality rollers waste paint; good rollers improve coverage.

The exact formulas this calculator uses

• Perimeter = 2 × (L + W)
• Wall Area = Perimeter × H
• Ceiling Area (optional) = L × W
• Total Area = Wall Area (+ Ceiling Area if selected)

• Door Area ≈ 21 sq ft each (3×7)
• Window Area ≈ 15 sq ft each (3×5)
• Openings Area = Doors×21 + Windows×15

• Net Area = max(0, Total Area − Openings Area)
• Coated Area = Net Area × Coats
• Final Area = Coated Area × (1 + Waste%/100)

• Gallons = Final Area ÷ Coverage
• Liters = Gallons × 3.78541
• Recommended buy = round up to whole gallons (and show quarts hint for small jobs)

Note: If you input meters, the calculator converts to an internal square-foot equivalent for coverage math, then converts output back to liters.

Frequently Asked Questions

• How accurate is this paint calculator?

  It’s a strong planning estimate for most indoor walls. The biggest real-world variables are wall texture, surface porosity, primer use, and paint brand. If your walls are textured or you’re changing color drastically, increase the waste buffer (10–20%).

• Should I include doors and windows?

  Yes, subtracting openings usually improves accuracy. If you plan to paint doors too, set doors to 0, or run a separate estimate for doors.

• What coverage should I use?

  Many interior paints advertise about 300–400 sq ft per gallon per coat. If you don’t know, 350 is a solid default. Always check the can label for the best number.

• Why does the calculator round up?

  Running out mid-project is a pain—and repaint batches can vary slightly. Rounding up reduces risk and makes buying simpler (whole cans).

• Can I use this for exterior painting?

  Yes, but increase your buffer. Exterior surfaces (wood, brick, stucco) often need more paint, and weather conditions can affect application. Measure carefully and consider primer.