The most common mistake when buying or bidding paint is guessing. Too much and you waste money. Too little and you’re making a second trip mid-job—or losing margin on a bid you underbid. Here’s how professionals calculate exactly how many gallons they need.
The Formula
Gallons = (Net paintable area × coats) ÷ coverage per gallon
Net paintable area = Total wall area − door area − window areaSimple in principle. The complications come in the details.
Step 1: Calculate Total Wall Area
For a rectangular room, measure each wall’s width and height, then:
Total wall area = 2 × (Length + Width) × HeightExample: A 12 × 14 ft room with 9 ft ceilings:
Total wall area = 2 × (12 + 14) × 9 = 2 × 26 × 9 = 468 sq ftFor rooms with alcoves, bump-outs, or angled walls, measure each wall individually and add them up.
Step 2: Subtract Doors and Windows
Standard deductions per opening:
| Opening | Approximate Area |
|---|---|
| Standard interior door (3 × 7 ft) | 21 sq ft |
| Standard window (3 × 5 ft) | 15 sq ft |
| Large window or patio door | 35–40 sq ft |
Example continued: 2 windows and 1 door:
Deductions = (2 × 15) + 21 = 51 sq ft
Net paintable area = 468 − 51 = 417 sq ftNote: Many professional painters skip window deductions for simpler math and to build in a material buffer. Both approaches are valid—just be consistent.
Step 3: Account for Coats
Most interior jobs require 2 coats for full, even coverage. One coat may work when refreshing the same color with a high-quality paint and primer combo. When changing colors—especially from dark to light or vice versa—plan for 2 full coats plus a primer.
| Scenario | Coats |
|---|---|
| Same color refresh, premium paint | 1 |
| Color change, medium depth | 2 |
| Dark to light or heavily patched | 2 + primer |
Example: 2 coats → 417 × 2 = 834 sq ft to cover
Step 4: Apply Paint Coverage
Check your paint can. Most quality interior paints list 350–400 sq ft per gallon under ideal conditions. Use the lower end for:
- Textured surfaces (orange peel, knockdown, popcorn)
- Highly saturated or deeply pigmented colors
- Porous, unprimed drywall
- First coat on any wall
Example at 350 sq ft/gallon with 10% waste buffer:
Gallons = (834 × 1.10) ÷ 350 = 917.4 ÷ 350 = 2.62 → round up to 3 gallonsApply the 10% waste buffer for tray drip, roller waste, and touch-ups before rounding up — that way you’re buying the minimum number of whole gallons that cover the waste, not rounding up twice. Result: buy 3 gallons.
What Changes Your Calculation
Textured walls reduce coverage by 20–30%. More surface area, more absorption.
Ceiling height matters more than floor area. Bump ceilings from 8 ft to 10 ft and you add 25% more wall area to every room.
Dark colors often need a gray-tinted primer coat. Budget a third coat when going from white to charcoal or navy.
Accent walls reduce your total—but run a separate calculation per color so you buy the right amount of each.
Trim and ceiling are always separate calculations. Ceiling paint typically has better hiding power (400–450 sq ft/gallon). Trim requires a different sheen and often a different formula.
Per-Wall Breakdown for Bidding
For homeowners buying paint, room-level math is fine. For contractors bidding jobs, you need it per wall—especially when:
- Different walls have different colors or sheens
- One wall is an accent
- Trim is a different product than the field coat
A per-wall breakdown lets you calculate exactly what each wall costs, what you’ll charge, and what margin you’re carrying into the job. It’s the difference between a tight bid and one that leaks money on material overages.
Use our Paint Calculator to run these numbers for any room without a spreadsheet.
If you’re bidding multiple rooms, SiteScope generates a per-wall breakdown automatically from a 3D scan—gallons by wall, openings deducted, cost and margin applied. You walk out with a takeoff before you leave the site.