The Paint Calculation Formula
The formula for calculating interior wall paint is straightforward: (Room Perimeter × Ceiling Height) − Deductions = Paintable Wall Area. Divide the result by 400 (the standard coverage rate per gallon) to get gallons per coat. Multiply by your number of coats for the total.
Standard deductions are 20 square feet per standard door and 15 square feet per medium window. Large windows and double doors need larger deductions — use 30 sq ft for a patio door or picture window.
For example: a 12×14 room with 8ft ceilings has a perimeter of 52ft. 52 × 8 = 416 sq ft gross. Subtract one door (20 sq ft) and two windows (30 sq ft) = 366 sq ft net. Divide by 400 = 0.92 gallons per coat. For two coats: 1.84 gallons — buy 2 gallons.
Paint Calculation — Step by Step
Step 1: Measure your room. Measure the length and width of the room in feet. Add all four wall lengths together to get the perimeter. Multiply by the ceiling height.
Step 2: Deduct doors and windows. Subtract 20 sq ft for each standard door and 15 sq ft for each medium window. Skip deductions if you want a conservative (slightly generous) estimate.
Step 3: Divide by coverage rate. Standard paint covers 350–400 sq ft per gallon. Divide your net wall area by 400 for a standard estimate. Use 350 for textured walls or porous surfaces.
Step 4: Multiply by coats. Two coats are standard. Multiply your single-coat result by 2 for the total gallons needed.
Step 5: Round up. Always round up to the nearest half gallon. Having slightly too much paint is far better than running short mid-wall.
Paint Calculation Reference Table
| Room Size | Wall Area | 1 Coat | 2 Coats |
|---|---|---|---|
| 10×10 | ~285 sq ft | 0.75 gal | 1.4 gal |
| 12×14 | ~366 sq ft | 0.92 gal | 1.84 gal |
| 14×16 | ~430 sq ft | 1.1 gal | 2.2 gal |
| 15×20 | ~575 sq ft | 1.44 gal | 2.9 gal |
Factors That Change Your Paint Calculation
Ceiling height. Standard calculations assume 8ft ceilings. For 9ft ceilings add 12.5% more paint. For 10ft ceilings add 25%.
Surface texture. Textured walls absorb more paint. Use 300 sq ft per gallon instead of 400 for orange-peel or knockdown texture.
Colour change. Going from a very light to very dark colour (or vice versa) typically requires a third coat for full opacity. Add 50% to your estimate.
Paint quality. Budget paints cover less — around 300–350 sq ft per gallon. Premium paints can cover 400–450 sq ft per gallon. Always check the label for the manufacturer's stated coverage rate.
Related Paint Calculators
- How Much Paint for a Bedroom? →
- How Much Paint for a Living Room? →
- How Much Paint to Cover 500 Sq Ft? →
- How Many Gallons of Paint for a Room? →
- Paint Coverage Calculator →
- Two Coat Paint Calculator →
- Free Paint Calculator →
Frequently Asked Questions
What is the formula for calculating paint?
(Perimeter × Ceiling Height) − (20 sq ft per door + 15 sq ft per window) = Paintable Area. Divide by 400 for gallons per coat. Multiply by number of coats.
How do I calculate paint for an irregular room?
Measure each wall individually, multiply each wall's length by the ceiling height, add all walls together, then subtract doors and windows. Use the calculator above for accurate results.
Does ceiling height affect how much paint I need?
Yes significantly. A room with 9ft ceilings needs 12.5% more wall paint than the same footprint with 8ft ceilings. Always use your actual ceiling height in calculations.
Should I round up my paint calculation?
Always round up to the nearest half gallon. Running out of paint mid-wall forces a second store trip and risks a slight colour mismatch between batches.
How accurate is the paint calculator?
The calculator is accurate for standard rooms with flat walls. It accounts for doors, windows, ceiling height, number of coats, and surface condition for a precise estimate.
How much extra paint should I buy?
Add 10% to your calculated total for touch-ups. Keep leftover paint sealed and labelled — it stays usable for 2–5 years and is invaluable for future spot repairs.