Room Paint Calculator
Gallons for Walls, Ceiling, and Trim
Calculate exactly how many gallons of paint you need for a full room. Enter length, width, and ceiling height — the calculator returns separate gallon totals for walls and ceiling with door and window deductions applied automatically.
Measuring a room? Enter the dimensions below — walls and ceiling gallons calculate instantly with PCA-standard door and window deductions built in.
Interior measurements in feet. Standard ceiling heights: 8, 9, or 10 ft.
Standard interior door ≈ 20 sq ft including casing (PCA Standard P1)
Standard double-hung window ≈ 15 sq ft (PCA Standard P1)
Ceiling paint is calculated separately at 400 sq ft/gal (high-hide flat formulation)
Standard — new surfaces, colour change, or bare substrate
Enter room dimensions above
to see gallons needed
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Room measurement formula
Calculating wall area for a rectangular room is a two-step process: first compute the gross perimeter area, then subtract openings. The formula is universally applied by professional estimators and is the basis for every major paint calculator including Sherwin-Williams and Behr's own tools.
Wall Area Formula
Gross Wall Area = (2 × Length + 2 × Width) × Ceiling Height
For a 12×10 room with 9 ft ceilings: (2×12 + 2×10) × 9 = 44 × 9 = 396 sq ft gross. Deduct 1 door (20 sq ft) and 2 windows (30 sq ft) = 346 sq ft net. At 350 sq ft/gal for 2 coats: 346 ÷ 350 × 2 = 1.98 → order 2 gallons for walls. Use our paint gallon calculator if you already have the net sq ft and want to skip the room geometry step.
For non-rectangular rooms — L-shaped layouts, rooms with alcoves, or open-plan spaces — break the perimeter into individual wall segments, calculate each wall's area (width × height), sum them, and subtract openings. This approach is more accurate than the perimeter shortcut for irregular footprints. Our guide to calculating paint for a room covers complex layouts in detail.
Door and window deduction standards
PCA Standard P1 and major paint manufacturer calculators (Sherwin-Williams, Behr) apply fixed deduction values per opening for material estimation purposes. These are not architectural dimensions — they are standardised estimation units that account for the door leaf plus casing, and the window glazing plus surrounding trim.
| Opening Type | Deduction (sq ft) | Basis | Notes |
|---|---|---|---|
| Standard interior door | 20 sq ft | 3×6'8" door + casing | Both sides of door opening count as one deduction |
| Standard exterior door | 20 sq ft | 3×6'8" entry door | Same unit as interior; frame included |
| Small window | 6 sq ft | ~2×3 ft opening | Bathroom or narrow casement windows |
| Standard double-hung window | 15 sq ft | ~3×4 ft opening | Most common residential window |
| Large picture window | 20 sq ft | ~4×5 ft opening | Same deduction unit as a door |
This calculator uses 15 sq ft per window — the standard double-hung benchmark — and 20 sq ft per door. If your room has an unusually large picture window or sliding glass door, use the paint coverage calculator and enter the exact net wall area after your own deductions, rather than relying on the standard per-opening value.
Ceiling vs walls — different products, same session
Walls and ceilings should never be painted with the same product, even if the goal is the same colour. Ceiling paint is a distinct product category engineered for a specific optical problem: any sheen on a horizontal surface overhead reflects light directly into the eye, making every roller mark, lap line, and stipple texture immediately visible. Ceiling paint uses high concentrations of flatting agents to produce a dead-flat finish that absorbs light rather than reflecting it.
Standard wall paints — even flat-finish wall paints — contain more binder than ceiling formulations. On a ceiling, the slightly higher sheen of a wall paint creates a wet-look halo effect under overhead lighting that cannot be corrected without stripping and repainting with ceiling-specific product. The coverage rate for ceiling paint is 350–400 sq ft/gal — identical to wall paint — but the product cannot be interchanged. This calculator applies 400 sq ft/gal for ceiling coverage, reflecting the high-hide flat formulations standard in this category. For textured or popcorn ceilings, see our ceiling paint calculator which accounts for the reduced spread rate on acoustic surfaces.
Standard gallons by room size
The following benchmarks represent typical gallon requirements for standard residential rooms at 8–9 ft ceiling height with one door and one to two windows, using 2 coats at 350 sq ft/gal for walls and 400 sq ft/gal for ceiling.
| Room Size | Wall Gallons (2 coats) | Ceiling Gallons (2 coats) | Total (walls + ceiling) |
|---|---|---|---|
| Small bedroom (10×10, 8 ft) | 2 gal | 1 gal | 3 gal |
| Standard bedroom (12×10, 8 ft) | 2 gal | 1 gal | 3 gal |
| Large bedroom (14×12, 8 ft) | 3 gal | 1 gal | 4 gal |
| Living room (16×14, 9 ft) | 4 gal | 1 gal | 5 gal |
| Large living room (20×16, 9 ft) | 5 gal | 2 gal | 7 gal |
| Master bedroom (16×14, 9 ft) | 4 gal | 1 gal | 5 gal |
These figures are starting points — actual requirements vary with ceiling height, number of openings, and surface condition. A 9 or 10 ft ceiling adds 12.5–25% more wall area than the 8 ft standard, bumping most rooms up by one full gallon. If your room has an accent wall in a contrasting colour, calculate it separately using the wall paint calculator and purchase it as a distinct colour in the correct quantity.
Frequently asked questions
How do I calculate paint for a full room?
Calculate wall area with: (2 × Length + 2 × Width) × Ceiling Height. Subtract 20 sq ft per door and 15 sq ft per window to get net wall area. Divide by 350 sq ft/gal, multiply by coats, and round up to whole gallons for walls. Add ceiling separately: Length × Width ÷ 400 sq ft/gal × coats, rounded up. Total both for your combined shopping list.
Should I paint walls and ceiling the same colour?
Painting walls and ceiling the same colour is a legitimate design choice that creates a cocooning, enveloping effect — it also makes small rooms appear larger by eliminating the visual break at the ceiling line. However, walls and ceilings always require different products regardless of colour. Ceiling paint uses flatting agents that prevent light reflection on horizontal surfaces; wall paint on a ceiling will show every roller mark under direct overhead lighting.
How many gallons do I need for a 12x12 room?
A 12×12 room at 8 ft ceilings has 384 sq ft gross wall area. After deducting one door (20 sq ft) and one window (15 sq ft), net area is 349 sq ft. At 350 sq ft/gal for 2 coats: 349 ÷ 350 × 2 = 1.99 gallons — order 2 gallons of wall paint. For the ceiling: 12×12 = 144 sq ft ÷ 400 sq ft/gal × 2 coats = 0.72 gallons — order 1 gallon of ceiling paint. Total: 3 gallons across two products.
Does ceiling paint cover more area than wall paint?
Ceiling paint covers 350–400 sq ft per gallon — the same range as smooth wall paint. The difference is formulation, not coverage rate. Ceiling paint uses high concentrations of flatting agents to produce a dead-flat finish that hides roller stipple on horizontal surfaces. Wall paint on a ceiling will show visible roller marks because it lacks the flatting agent concentration required for overhead application.
How much extra paint should I buy for a full room job?
Add 10% to your calculated total for smooth walls and 15% for textured surfaces. For touch-up storage, always keep at least one extra quart per colour from the original mixing session — custom-tinted colours cannot be reliably reproduced in a second batch due to batch-to-batch pigment variation. Store sealed cans upside down to prevent skin formation inside the lid.
References
Painting Contractors Association. (2023). PCA Standard P1 — Touch Up Painting and Damage Repair, and Definition of a Properly Painted Surface. PCA Industry Standards.
Master Painters Institute. (2025). MPI Maintenance Repainting Manual (RSM). MPI Publications.
Angi. (2025). 2025 State of Home Spending Pulse Report. Angi Research.
Sherwin-Williams. (2025). Paint Calculator and Estimating Guide. Sherwin-Williams Company.