Paint Calculator — Gallons Needed for Walls & Ceilings
Calculate exactly how much paint you need for any room. Enter wall dimensions, doors, windows, and number of coats. Supports walls, ceilings, trims, and exterior surfaces with waste factor.
What Is the Paint Calculator — Gallons Needed for Walls & Ceilings?
This calculator estimates how much paint you need for any room by computing the paintable wall area, subtracting doors and windows, then dividing by the coverage rate of your paint. It supports walls, ceilings, and trim in both imperial and metric units.
- ›Multi-room mode — add several rooms and get a combined total, saving multiple trips to the hardware store.
- ›Separate surface calculations — walls, ceiling, and trim/baseboard are calculated independently so you can buy different paints for each.
- ›Cost estimate — enter a price per gallon to see the total material cost for your project.
- ›Paint quality tiers — economy, standard, and premium paints have different coverage rates; the calculator adjusts when you choose a tier.
- ›Primer toggle — optionally add a primer coat with its own coverage rate for new drywall or colour-change projects.
Formula
Wall Paint Area
Wall Area = 2 × (Length + Width) × Height
Paintable Area = Wall Area − Door Deductions − Window Deductions
Paint Needed
Paint (gal) = Paintable Area × Coats × (1 + Waste%) ÷ Coverage (sq ft/gal)
Default Deductions
Door ≈ 21 sq ft each | Window ≈ 15 sq ft each
How to Use
- 1Enter room dimensions: Type the length, width, and ceiling height. Toggle between imperial (ft) and metric (m) as needed.
- 2Add deductions: Enter the number of standard doors and windows, or add a manual custom deduction in square feet.
- 3Set number of coats: Choose 1, 2, or 3 coats. Two coats is the standard for most interior projects.
- 4Choose coverage rate: The default is 350 sq ft per gallon. Adjust for your specific paint or choose Economy/Standard/Premium quality tier.
- 5Toggle optional surfaces: Enable Ceiling and/or Trim calculations if you are painting those surfaces too.
- 6Add more rooms: Click "Add Room" to include additional rooms. Each room can have its own dimensions and deductions. Totals are summed across all rooms.
- 7Add primer: Toggle "Include Primer" to add a primer coat calculation with its own separate coverage rate.
Example Calculation
12 × 14 ft bedroom, 9 ft ceiling, 2 doors, 2 windows, 2 coats
Room: 12 ft × 14 ft, ceiling 9 ft
Wall perimeter: (12 + 14) × 2 = 52 ft
Gross wall area: 52 × 9 = 468 sq ft
Doors (×2): 2 × 21 = 42 sq ft
Windows (×2): 2 × 15 = 30 sq ft
Net wall area: 468 − 42 − 30 = 396 sq ft
2 coats × 396 = 792 sq ft total coverage needed
+ 10% waste: 792 × 1.1 = 871 sq ft
At 350 sq ft/gal: 871 ÷ 350 = 2.49 gal
Buy: 3 gallons (round up)
Disclaimer — estimates are approximate
Paint coverage estimates depend on surface texture, porosity, application method, and colour change. These calculations provide a close starting estimate, but professional measurement is recommended for large or complex projects. Always buy a small amount extra as a buffer for touch-ups.
Understanding Paint — Gallons Needed for Walls & Ceilings
How Paint Coverage Works
Paint coverage is expressed as the area one gallon covers in a single coat — typically 350–400 sq ft per gallon for interior flat or eggshell paints. Premium paints with higher solids content often cover slightly more per gallon. Economy paints and textured surfaces may cover less. The calculator uses 350 sq ft/gallon as the default, which is conservative enough to avoid running short.
Walls vs Ceiling vs Trim — Why Buy Separately
- ›Wall paint — typically flat, eggshell, or satin finish. Eggshell is the most common for living areas; semi-gloss for kitchens and bathrooms.
- ›Ceiling paint — formulated to be thicker and to reduce spatter. Often flat white. Ceiling area = length × width for a rectangular room.
- ›Trim and baseboard — usually semi-gloss or gloss for durability. The calculator estimates trim area as perimeter × 0.5 ft (representing standard 3–6 inch baseboards).
How Many Coats Do You Actually Need?
- ›One coat: touching up the same colour with the same paint, or using a paint+primer on a freshly primed surface
- ›Two coats: the standard for most interior repaints. Required for significant colour changes
- ›Three coats: painting over a very dark colour with a light colour, or painting bare drywall without primer
- ›Always prime new drywall or major repairs — it dramatically reduces the number of finish coats needed
The 10% Waste Factor
Every estimate includes a default 10% waste factor to account for roller loading, brush tips, drips, and minor touch-ups. You can adjust this in the calculator. For heavily textured walls or rollers with thick nap, consider increasing it to 15%.
When to Buy Quarts vs Gallons
Paint is typically sold in quarts (1 qt = ¼ gallon) and gallons. If your calculation calls for 1.3 gallons, it is more economical to buy 1 gallon + 1 quart than 2 gallons. The calculator shows the most economical can combination (gallons + quarts) for your exact requirement.
Disclaimer — estimates are approximate
All paint quantity estimates are approximate. Actual coverage varies with surface porosity, texture, application method, paint brand, and colour changes. These estimates are a close guide for planning and shopping, but professional measurement is recommended for large, multi-room, or exterior projects. Always purchase a small buffer for future touch-ups.
Frequently Asked Questions
How much wall area should I subtract for doors and windows?
- ›Standard interior door: ~21 sq ft (6.75 ft tall × 3 ft wide)
- ›Standard window: ~15 sq ft (3 ft × 5 ft)
- ›Sliding glass door or patio door: use custom deduction (~40 sq ft)
- ›Decorative arch or half-window: measure actual dimensions
The calculator uses these defaults but lets you override with a custom deduction.
How many gallons do I need for a standard 12×14 bedroom?
- ›Walls: ~2.5 gallons for two coats (buy 3 gallons or 2 gallons + 2 quarts)
- ›Ceiling (12×14 = 168 sq ft): ~0.5 gallons for one coat
- ›Trim/baseboard: ~1 quart for one coat
- ›Total: roughly 3–4 gallons for a complete room repaint
What is the coverage rate for standard interior paint?
- ›Economy paint: 250–300 sq ft/gallon
- ›Standard interior paint: 350–400 sq ft/gallon
- ›Premium paint: 400–450 sq ft/gallon
- ›Textured surfaces or new drywall: reduce coverage by 15–20%
Do I need primer, and how much?
- ›New drywall: one coat of drywall primer, then two finish coats
- ›Stain blocking (water stains, smoke): use shellac-based primer
- ›Dark to light colour change: tinted primer saves one finish coat
- ›Paint+primer products: usually fine for same-colour repaints over clean surfaces
How do I calculate paint for a room with a vaulted or angled ceiling?
- ›Measure the actual slope length, not just the horizontal span
- ›Vaulted ceiling area > floor area by a factor of 1/cos(pitch angle)
- ›For a standard 6-in-12 pitch (26.6°): area × 1.12
- ›Use the manual square footage entry in the calculator for non-rectangular rooms
Are the estimates accurate enough to avoid buying too much paint?
- ›Standard rooms: accurate within ±5-10%
- ›Textured or rough surfaces: add 15% above the calculated amount
- ›Dye-lot matching: always buy slightly more than needed in a single batch
- ›Leftover paint: store sealed in a cool, dry place for touch-ups
Does the calculator save my room dimensions?
- ›All rooms, dimensions, and settings are saved to localStorage
- ›Data is restored automatically on your next visit
- ›No server communication — fully browser-local
- ›Click Reset All to clear the saved state