Paint Calculator

How to Use the Paint Calculator

Add each rectangular room, enter dimensions and opening counts, choose whether to include that room’s ceiling, then set coats, coverage, and optional allowance or price. Calculate to see room-by-room surfaces and total paint quantity.

How to Calculate Wall Area

For a rectangular room, wall area is:

Wall area = 2 × (Length + Width) × Height

All dimensions are converted to meters before multiplying so mixed US customary and metric inputs stay consistent.

Accounting for Doors and Windows

Each door and window count is multiplied by an assumed opening area, then subtracted from gross wall area. Defaults are labeled planning assumptions:

• Door: about 3 ft × 7 ft (21 ft²) unless you edit the door area field.
• Window: about 3 ft × 5 ft (15 ft²) unless you edit the window area field.

If openings would exceed wall area, the calculator rejects the input instead of inventing a negative surface.

Should You Include Ceilings?

Ceiling area is length × width. Include it only for rooms you plan to paint. You can include walls globally and toggle ceilings per room.

How Many Coats of Paint Do You Need?

Coat-adjusted area equals base paintable area multiplied by the number of coats (1–4; default 2). Dark color changes, new drywall, or product instructions may justify different coat counts—use the value that matches your project plan.

Understanding Paint Coverage

Coverage is the area one volume unit of paint can cover for a single coat, typically listed on the product label. Enter coverage as ft² per US gallon or m² per liter. Dividing coat-adjusted area by coverage produces theoretical paint volume before allowance.

Gallons vs Liters

This calculator uses the US liquid gallon definition: 1 US gallon = 3.785411784 liters. Results always show both gallons and liters.

Estimating Paint Material Cost

If you enter a price per gallon or liter, cost is quantity including allowance × your price. CostRafter does not invent market prices. Primer, tape, labor, tax, and delivery are not included. Currency for this version is USD.

Worked Examples

Examples below were computed with CostRafter’s paint_coverage engine (v1.0.0).

Imperial bedroom, walls only

Room 12 ft × 10 ft × 8 ft, 1 door (21 ft²), 1 window (15 ft²), walls on, ceiling off, 2 coats, coverage 350 ft²/gal, waste 10%:

• Gross walls: 352 ft²; openings: 36 ft²; net walls: 316 ft²
• Coat-adjusted area: 632 ft²
• Paint before allowance: 1.8057 gal (6.8354 L)
• Paint with 10% allowance: 1.9863 gal (7.5189 L)
• At a user-entered $35/gal: about $69.52

Metric room with ceiling

Room 4 m × 3 m × 2.5 m, no openings, walls and ceiling, 1 coat, coverage 10 m²/L, 0% waste:

• Walls 35 m² + ceiling 12 m² = 47 m² paintable
• Paint required: 4.7 L (about 1.2417 gal)

Common Painting Mistakes

• Using catalog coverage without checking the product label for your surface and sheen.
• Forgetting openings—or assuming every door/window matches the default sizes without editing them.
• Treating theoretical volume as exact can counts without rounding up for packaging.
• Skipping a second coat when coverage or color change requires it.
• Including ceilings accidentally when only walls will be painted.

Assumptions and Limitations

• Rooms are rectangular; irregular spaces need separate takeoffs.
• Opening sizes are uniform editable assumptions applied to each opening count.
• Texture, porosity, primer systems, and brand variation are not modeled.
• Waste percentage is optional purchasing guidance, not a universal requirement.
• Results are theoretical paint volume before commercial container rounding.

Confirm final purchase quantities against product labels and your installer or supplier guidance.