Paint Calculator
Estimate how many gallons of paint you need for walls, ceilings, and trim. Accounts for doors, windows, and number of coats. Includes cost estimation.
Paint estimation is one of the most common DIY math problems and one of the easiest to get wrong. Buy too little and you have to make a second store trip mid-job (often with paint that doesn't exactly match if it's a custom tint). Buy too much and you have $40+ of paint sitting in the garage going stale. The math is straightforward once you know the right coverage numbers.
This calculator computes paint quantity for interior rooms: walls only, walls + ceiling, accounting for doors and windows, with multiple coats. The standard assumption is 350 square feet covered per gallon with one coat — slightly conservative compared to the 400 sq ft on the can label, but more realistic for the cutting-in, brush loss, and over-application that happens during real painting.
For most rooms, you'll need 1–2 gallons per coat for walls, plus an additional gallon for ceilings if included. Most interior paint jobs need two coats unless you're refreshing the same color or doing touch-ups.
Inputs
Results
Gallons to Buy
2
Paintable Area
344 sq ft
Estimated Cost
$80.00
Paint Breakdown
| Item | Area | Note |
|---|---|---|
| Wall Area (gross) | 416 sq ft | 52 ft perimeter x 8 ft height |
| Door Deduction | -42 sq ft | 2 door(s) x 21 sq ft |
| Window Deduction | -30 sq ft | 2 window(s) x 15 sq ft |
| Net Paintable Area | 344 sq ft | |
| Coverage (2 coats) | 688 sq ft | 350 sq ft/gallon coverage |
Formula
How to use this calculator
- Measure room length, width, and wall height. Round to the nearest half-foot.
- Enter number of doors and windows to subtract from wall area.
- Choose number of coats. Two is standard for most interior repaints. One can work for a same-color refresh; three for very dark-to-light color changes (use primer + 2 coats instead).
- Toggle the ceiling option if you're painting it (most projects do).
- Enter price per gallon. Interior latex runs $20–60 depending on quality; high-end paints (Benjamin Moore Aura, Sherwin-Williams Emerald) hit $80+.
- Round up to the next gallon (or quart for very small remainders) — buying short mid-job is the worst outcome.
Worked examples
Standard bedroom
14 × 12 × 8 ft bedroom, 2 doors, 2 windows, 2 coats, no ceiling. Wall area: 2 × 26 × 8 = 416 sq ft Net (after openings): 344 sq ft With 2 coats: 688 sq ft Gallons: 688 / 350 = 1.97 → 2 gallons At $40/gallon: $80 in paint, plus rollers, brushes, primer, and drop cloths if you don't already have them.
Whole-room project
Same bedroom + ceiling, 2 coats. Adding ceiling area: 168 sq ft × 2 coats = 336 sq ft Total: 688 + 336 = 1,024 sq ft Gallons: 1,024 / 350 = 2.93 → 3 gallons Ceiling paint is often a different (cheaper, flatter) formula than wall paint, so you may buy 1 gallon ceiling + 2 gallons wall.
When to use this calculator
Use this calculator for any interior paint job: bedrooms, living rooms, hallways, ceilings. The same math works for exterior siding (with adjusted coverage — exterior paint typically covers ~300 sq ft per gallon due to texture).
Add to gallons for: - Heavily textured walls (orange peel, knockdown): +10% - Porous surfaces (fresh drywall, raw wood): need primer first - Dark-to-light color change: use a primer + tinted to a midtone, then 2 coats of color - High-quality paint may need only 1 coat on similar-color jobs
Don't forget supplies: - Painter's tape (for crisp lines): 1 roll per 60–80 ft of trim - Drop cloths: 1 per room minimum - Rollers: 1 cover per gallon, or per ~350 sq ft - Brushes: 2–3 inch angled sash brush + 1 small detail brush - Primer: ½ gallon per 200 sq ft of bare/dark areas
For paint cost estimation including supplies, budget about 20–30% above the paint-only cost on a first-time job.
Common mistakes to avoid
- Trusting the 400 sq ft per gallon claim on the can. Real-world coverage is closer to 350 due to cutting-in, brush loss, and over-application.
- Forgetting primer for dark-to-light changes. Two coats of off-white over deep red will still bleed through; primer first saves a coat of finish paint.
- Not subtracting doors and windows on a small room. A 10×10 room with 2 doors and 2 windows has 87 sq ft less wall than gross dimensions suggest.
- Mixing different paint batches without intermixing. Even from the same store, different batches can be slightly different shades. Mix gallons together before starting.
- Skipping the primer on glossy surfaces. Latex won't adhere to oil-based glossy paint without an adhesion primer.
- Painting ceilings last. Always paint ceiling first, then walls, then trim — drips and overspray work top-down.
Frequently Asked Questions
Sources & further reading
- How much paint do I need? — Sherwin-Williams