How to Calculate Paint for a Room: A Practical Guide

Running out of paint halfway through a coat creates visible lap marks and requires another trip to the hardware store—often resulting in slight tint variations between batches. On the other hand, buying two extra gallons leaves expensive paint sitting in your garage until it dries out. Calculating your paint needs accurately takes less than ten minutes and saves money and frustration.

The foundational formula for paint estimation is straightforward: **Total Wall Area** = Room Perimeter × Ceiling Height **Net Paintable Area** = Total Wall Area − (Total Area of Doors & Windows) **Total Paint Area** = Net Paintable Area × Number of Coats **Gallons Needed** = Total Paint Area ÷ Paint Coverage Rate (typically 350 to 400 sq ft per gallon) In metric terms, 1 liter of quality interior acrylic paint covers approximately 9 to 11 square meters on smooth, primed drywall.

If you want a fast estimate without measuring every trim molding: - **Standard Interior Door**: 3 ft × 7 ft = 21 sq ft (approx. 2.0 sq meters) - **Standard Window**: 3 ft × 4 ft = 12 sq ft to 15 sq ft (approx. 1.1 to 1.4 sq meters) - **Sliding Glass Patio Door**: 6 ft × 7 ft = 42 sq ft (approx. 3.9 sq meters) When painting doors or window trim the same color as the walls, do not deduct them.

The standard coverage rate of 350–400 sq ft per gallon applies to previously painted, smooth drywall. Consider these adjustments: - **Unprimed Drywall or Bare Plaster**: Absorbs significantly more paint; budget 250–300 sq ft per gallon or apply a dedicated PVA primer first. - **Heavily Textured Walls (Orange Peel or Knockdown)**: Increases surface area by 15% to 25%. - **Drastic Color Changes (Dark to Light)**: Always requires two coats of topcoat plus a high-hiding primer coat.

Consider a 12 ft by 14 ft living room with standard 9-foot ceilings, containing two standard doors and two windows: 1. Perimeter = (12 + 14) × 2 = 52 ft 2. Gross wall area = 52 ft × 9 ft = 468 sq ft 3. Deductions = 2 doors (42 sq ft) + 2 windows (30 sq ft) = 72 sq ft 4. Net wall area = 468 − 72 = 396 sq ft 5. For 2 coats = 396 × 2 = 792 sq ft 6. Gallons required = 792 ÷ 350 sq ft/gal = 2.26 gallons Recommendation: Purchase two 1-gallon cans and one 1-quart can, or three 1-gallon cans to keep leftover paint for future touch-ups.