📐 Room Dimensions
🚪 Subtract Openings
⚙️ Options
Gallons to Buy
—
gallons
Paintable Area
—
sq ft
With Waste (10%)
—
sq ft
Est. Paint Cost
—
at $45/gal avg
⚠️ Don't get caught short
Paint from different batches can vary in color — even with the same name and code. Buy everything you need in one trip, and keep one sealed can for touch-ups.
🛒 Supplies Checklist
- Roller frame (9-inch)
- Roller covers — get 2–3 (one per coat, plus a spare)
- Extension pole (fits roller frame)
- Paint tray + tray liners
- 2-inch angled brush (for cutting in)
- Painter's tape (2-inch, blue or green)
- Drop cloths or plastic sheeting
- Small can of spackling for nail holes
- 120-grit sandpaper or sanding block
- Primer (if painting over dark colors or new drywall)
- Paint mixing sticks
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How to Calculate How Much Paint You Need
The standard formula for estimating paint is simple: calculate the total square footage of your paintable surfaces, divide by the coverage rate of your paint, and multiply by the number of coats.
// Basic formula
Wall area = 2 × (length + width) × ceiling height
Subtract: doors (~21 sq ft each), windows (~15 sq ft each)
Gallons = (paintable area × coats) ÷ coverage per gallon
// Always add 10% for waste and touch-ups
Wall area = 2 × (length + width) × ceiling height
Subtract: doors (~21 sq ft each), windows (~15 sq ft each)
Gallons = (paintable area × coats) ÷ coverage per gallon
// Always add 10% for waste and touch-ups
Most standard interior latex paints cover 350–400 square feet per gallon on smooth, previously painted surfaces. Rough or porous surfaces (like bare drywall, brick, or heavily textured walls) will need more — budget closer to 300 sq ft per gallon.
Why You Should Always Buy a Little Extra
Buying one extra quart or gallon protects you against:
- Touch-ups — Scuffs, nicks, and marks happen. You'll want to match paint within the first year.
- Dye lot variation — Paint mixed on different days can have subtle color shifts. One trip means one batch.
- Underestimation — Rollers waste more paint than a brush. Porous surfaces absorb more than expected.
1 Coat vs 2 Coats: When Do You Need Two?
Almost always go with two coats. One coat is only sufficient if:
- You're painting the same color with the same sheen (minor touch-up scenario)
- You've primed first and are applying a solid medium-tone color
You definitely need two coats (or primer + one coat) when:
- Covering a dark color with a lighter one
- Painting bare, new, or repaired drywall
- Switching sheens (e.g., flat to satin)
- Using a deep or saturated color
Paint Coverage by Surface Type
| Surface Type | Coverage (per gal) | Notes |
|---|---|---|
| Smooth interior wall (previously painted) | 350–400 sq ft | Standard for most repaint jobs |
| New drywall (primed) | 300–350 sq ft | Primer helps; drywall is porous |
| Textured or orange peel wall | 250–325 sq ft | More surface area to cover |
| Bare wood (trim, doors) | 200–300 sq ft | Very absorbent; primer strongly recommended |
| Ceiling (flat paint) | 350–400 sq ft | Similar to walls; usually only needs 1 coat if repainting same color |
| Exterior siding (lap) | 200–300 sq ft | Weathered wood absorbs heavily |
| Masonry / brick / block | 100–200 sq ft | Very porous; use masonry primer |
Paint Calculator — Frequently Asked Questions
How many gallons of paint do I need for a 12×12 room?
For a 12×12 room with 8-foot ceilings, your wall area is approximately 2 × (12+12) × 8 = 384 sq ft. Subtract one door (~21 sq ft) and two windows (~30 sq ft) and you have about 333 sq ft. At 350 sq ft/gallon, that's roughly 1 gallon per coat. For two coats, buy 2 gallons — or 3 if you want a comfortable buffer. This calculator handles all of that automatically.
Should I paint the ceiling the same color as the walls?
It's a popular trend in contemporary design and can make a room feel cozy and intentional. Standard practice is to use the same color at a lighter tint, or use flat white ceiling paint. If you match exactly, buy extra — ceilings are notoriously hard to cut in cleanly against walls.
Does the calculator account for doors and windows?
Yes. Enter the number of doors and windows in the calculator and it will subtract approximately 21 sq ft per door and 15 sq ft per window from your total paintable area. These are standard averages; if your doors or windows are unusually large, add extra paint to be safe.
How much does a gallon of interior paint cost?
In 2025, interior paint typically runs $25–35/gallon for budget brands, $40–55 for mid-range (Behr, Valspar, Glidden), and $65–90+ for premium paints (Benjamin Moore, Sherwin-Williams Emerald). The cost estimator uses $45/gallon as a reasonable mid-range average. Premium paints often cover better and last longer — the per-coat cost difference is less than it appears.
What's the difference between flat, eggshell, and satin paint?
Flat (matte) paint has no sheen and hides surface imperfections well, but is harder to clean — good for ceilings and low-traffic areas. Eggshell has a slight sheen and is washable — the standard choice for living rooms and bedrooms. Satin is more durable and scrubbable — ideal for kitchens, bathrooms, hallways, and trim. Semi-gloss and gloss are used for trim, cabinets, and doors.
Can I return unused paint?
Most stores will not accept returns on custom-mixed paint. Buy what you need plus 10%, and store the remainder sealed in a cool, dry place. Custom-mixed paint is worthless once the store closes that color — keep your leftovers for at least 2–3 years for touch-ups.