Paint calculator
How much paint do I need? Calculate gallons of paint, primer, and estimated cost based on room dimensions, surface type, and number of coats.
Drop calc results into projects and quotes, invoice through your own Stripe, get paid. Built by a contractor.
Try TradeMaster Pro →How much paint do I need?
The short version: surface area × number of coats ÷ coverage rate. One gallon of standard interior paint covers approximately 350 square feet. So a typical 12′×14′ room with 8′ ceilings — about 416 sq ft of wall area after door/window deductions — needs roughly 2.4 gallons for 2 coats. Round up to 3.
The longer version is what makes the difference between a clean two-coat job and running back to the store at 4pm because you ran out: surface type, coverage rate by paint product, and how to deduct openings correctly.
Coverage rates by surface
Coverage rate is the single biggest variable in the calculation. Manufacturer-claimed coverage assumes ideal conditions (smooth, primed, properly prepped surface). Real-world coverage rates:
- Smooth interior drywall: 350 sq ft/gal (manufacturer standard)
- Textured or knockdown drywall: 280-300 sq ft/gal (15-20% reduction)
- Popcorn ceilings: 250-280 sq ft/gal (20-30% reduction)
- Smooth exterior siding: 250-300 sq ft/gal (lower than interior due to surface absorption and weather exposure)
- Rough exterior (stucco, T1-11): 150-200 sq ft/gal
- Trim and doors (smooth): 400 sq ft/gal (paint goes farther on smaller, smoother surfaces)
- Bare wood: 250 sq ft/gal first coat (high absorption), 350 sq ft/gal subsequent coats
How to deduct doors and windows
Don’t paint over the door — deduct it from the wall area. Standard openings:
- Standard interior door:3′ × 7′ = 21 sq ft
- Exterior door:3′ × 7′ or 3′ × 8′ = 21-24 sq ft
- Average single window:3′ × 5′ = 15 sq ft
- Picture window or large window:4′ × 6′ = 24 sq ft
- Sliding glass door:6′ × 7′ = 42 sq ft
For ballpark estimates, the calculator assumes 21 sq ft per door and 15 sq ft per window. For precise estimates on a real job, measure each opening.
How many coats?
Industry standard for almost any paint job is 2 coats. A single coat over previously painted, well-prepped, similar-color walls is acceptable for touch-ups and refresh jobs but doesn’t give the depth or durability of two coats. Three coats are needed for:
- Very dark color over a light wall (or vice versa)
- Bold accent colors that lack pigment density (some reds, deep teals, oranges)
- Surfaces that absorb paint unevenly (mismatched repairs, water-stained drywall)
Don’t try to lay heavy coats to skip a coat. It leads to drips, runs, uneven sheen across the wall, and longer drying time. Two thin even coats always beats one thick coat.
When to use primer
Primer is necessary for:
- Bare drywall:drywall absorbs topcoat unevenly without primer; you’ll see flat spots and mottled finish.
- Dramatic color changes: dark over light, light over dark, or any saturated color change. Tinted primer is a good middle step.
- Repaired surfaces: joint compound and spackle absorb paint differently than surrounding paint. Without primer, repairs telegraph through.
- Bare wood: wood absorbs and tannin-bleeds; primer prevents both.
- Glossy or chalky surfaces: primer creates a bonding layer for the new paint.
- Stained surfaces: water stains, smoke damage, or old varnish need a stain-blocking primer (oil-based or shellac).
For repainting walls in a similar color over previously painted, well-prepped surfaces, primer is usually optional. Modern self-priming paints (most premium products) are designed to perform well in this scenario, though purists still prefer a separate primer coat.
Estimating paint cost
Paint pricing varies dramatically. Rough ranges as of 2026:
- Builder-grade interior: $25-35/gal (Sherwin-Williams ProMar 200, Benjamin Moore Eco Spec)
- Mid-range interior: $40-60/gal (Sherwin-Williams ProClassic, Benjamin Moore Regal)
- Premium interior: $65-85/gal (Sherwin-Williams Emerald, Benjamin Moore Aura)
- Premium exterior: $55-90/gal (paint is more durable on exterior; products are formulated differently)
- Specialty (cabinet, marine, epoxy): $80-150/gal+
For the calculator default, $45/gallon is a reasonable mid-range interior price. Adjust to your actual purchasing.
Related calculators
Doing the room math first? Drywall calculator uses the same room dimensions to estimate sheet count and joint compound. For exterior projects, also check roof area calculator (for paint estimating on roof trim) and board foot calculator (for trim material before painting).
Looking for the contract that covers a paint job? Free painting contract template covers surface prep, paint product specs, color approval, and EPA RRP lead-paint compliance for pre-1978 homes.
FAQ
How much paint do I need for a 12×14 room?
A 12′×14′ room with 8′ ceilings has roughly 416 sq ft of wall area (perimeter 52′ × 8′ height = 416 sq ft, minus typical door/window openings). At 350 sq ft per gallon coverage with 2 coats, you need about 2.4 gallons — round up to 3 gallons. Add another gallon if you’re priming first.
How much paint per square foot?
One gallon of paint covers approximately 350 square feet on smooth interior walls. Reduce by 15-20% for textured walls, popcorn ceilings, or rough exterior surfaces. Premium paints often achieve 400+ sq ft/gal on properly primed surfaces.
Do I need primer?
Primer is necessary for bare drywall, dramatic color changes, repaired surfaces, bare wood, glossy or chalky existing paint, and stained surfaces. For repainting walls in a similar color over previously painted surfaces in good condition, primer is usually not needed.
How many coats of paint do I need?
Two coats is the industry standard for most situations. One coat is acceptable for touch-ups over similar-color, well-prepped surfaces. Three coats may be needed for very dark over light (or vice versa), deep accent colors, or unevenly absorbing surfaces.
Should I add extra for waste?
Most pros add 10-15% extra. Waste comes from paint left in the can, tray drying, cleanup, drips, and touch-ups. Better to have an extra gallon than to drive back to the store mid-job. Unopened gallons can be returned at most retailers.