Paint Calculator
Estimate how many gallons of paint you need for walls and ceilings — including doors, windows, coats and waste.
Last updated: September 2026 · Coverage and opening math reviewed (not a brand warranty or exterior specification service)
Best for a single interior room: walls, optional ceiling, and typical door/window openings. Enter coverage from your paint can. This tool does not size trim, door slabs, cabinets, or exterior siding — and it does not verify brand warranties. Use label rates for the most accurate gallon count.
From your wall (or ceiling) paint can. Used for all paint coats.
Used only when “Include primer” is checked.
Quick Answer: Paint Gallons Formula
Wall area ≈ 2 × (length + width) × height − doors − windows.
Paint gallons ≈ (wall area [+ ceiling] × coats × (1 + waste%)) ÷ coverage from the can.
- Default openings: ~20 sq ft per door, ~15 sq ft per window
- Typical interior coverage: ~350–400 sq ft per gallon (check the label)
- Most walls need 2 coats; primer is separate when the surface needs it
How to Use This Paint Calculator
- Enter the room length, width and ceiling height
- Optionally check Include ceiling if you will paint it
- Enter number of doors and windows (they are subtracted from wall area)
- Choose coats (usually 2) and coverage rate from your paint can
- Add waste (10% is typical) and optional primer / price
- Click Calculate Paint for gallons needed
How Much Paint for a Room? (Examples)
Quick reference for typical rooms. Assumes 1 door (~20 sq ft) + 2 windows (~15 sq ft each). Re-calculate for your exact openings and ceiling option.
| Room Size | Walls Only | Walls + Ceiling | With Primer (walls only) |
|---|---|---|---|
| 10 × 12 ft | ~2.0 gal | ~2.75 gal | ~3.0 gal paint + primer |
| 12 × 15 ft | ~2.5 gal | ~3.75 gal | ~3.75 gal paint + primer |
| 15 × 20 ft | ~3.25 gal | ~5.25 gal | ~5.0 gal paint + primer |
| 20 × 20 ft | ~3.75 gal | ~6.25 gal | ~5.75 gal paint + primer |
Figures are approximate and rounded up. Always verify with your paint’s labeled coverage rate.
Worked example (12 × 15 ft, 8 ft ceiling, walls only)
- Wall area = 2 × (12 + 15) × 8 = 432 sq ft
- − 1 door (20) − 2 windows (30) = 382 sq ft
- × 2 coats × 1.10 waste = 840.4 sq ft of film to buy
- ÷ 350 sq ft/gal ≈ 2.40 → round up by quart ≈ ~2.5 gal
Include the ceiling to add length × width (180 sq ft here) into the paintable area before coats and waste. Confirm room area with the square footage calculator if needed.
Typical Paint Coverage
| Surface / Product | Approx. Coverage per Gallon |
|---|---|
| Smooth interior walls | 350 – 400 sq ft |
| Textured or porous walls | 250 – 350 sq ft |
| Ceiling (flat paint) | 300 – 400 sq ft |
| Primer | 300 – 400 sq ft |
| Exterior siding | 250 – 350 sq ft |
Reference only. Enter the coverage from each product’s label. This calculator uses Paint coverage for all paint coats (walls and optional ceiling) and Primer coverage separately when primer is included. If ceiling paint has a different rate than wall paint, use that product’s number in Paint coverage or estimate the ceiling as a separate run.
Interior Walls, Ceilings & Primer
Most interior jobs are walls only or walls plus ceiling. New drywall, stains, and large color changes usually need primer before finish coats.
When to use primer
- New drywall or joint compound — seals the surface so paint does not flash
- Stains, water marks, or smoke — stain-blocking primer
- Big color shifts (dark to light or bold changes) — improves hide
- Bare wood or patched areas — even absorption
After hanging or finishing drywall, estimate sheets and mud with the drywall calculator, then return here for primer and paint gallons.
Ceilings
Ceiling paint is often flat and may list a different coverage than wall paint. Check Include ceiling in the calculator, and use the ceiling product’s coverage when it differs from the wall can.
What is not included
- Door slabs, casings, baseboards, and other trim (openings only remove wall area)
- Exterior siding and trim exposure
- Cabinets, furniture, and multi-room batching in one run
After floors are finished, estimate flooring materials with the flooring calculator.
Formulas Used
Wall area = 2 × (Length + Width) × Height − (Doors × ~20 sq ft) − (Windows × ~15 sq ft)
Ceiling area = Length × Width (if included)
Paint gallons = (Paintable area × Coats × (1 + Waste%)) ÷ Paint coverage, then rounded up to the nearest quart
Primer gallons (optional) = (Paintable area × (1 + Waste%)) ÷ Primer coverage, one coat, rounded up to the nearest quart
Paint coverage and primer coverage are separate inputs so wall paint and primer can use different label rates.
Painting Tips
- Two coats are standard for a durable, even finish
- Use primer on new drywall, stains, or big color changes — see the guide above and the drywall calculator for new walls
- Door & window sizes: Defaults are ~20 sq ft per door and ~15 sq ft per window. Large patio doors or picture windows need a higher count or a manual area adjustment.
- Buy from the same batch when possible to avoid shade variation
- Keep leftover paint for touch-ups (label the lid with room and date)
- Cut in edges first, then roll walls for a professional look
Important: This calculator estimates paint from room dimensions, typical door/window sizes, coats, and the coverage rate you enter. It does not account for texture depth, patching, trim color changes, or exterior exposure. Always check the coverage rate on your paint can before buying.
Check Paint Brand Warranties
Gallons and coverage are only part of a lasting job. Many brands offer limited warranties on adhesion, fade, or peel — but those promises usually depend on correct surface prep, primer, and application. Read the label and the brand’s written terms before you buy.
- What warranties often cover: Premature peeling, blistering, or fade when the product was applied as directed on a suitable surface.
- What they often exclude: Poor prep, moisture problems, structural movement, wrong product for the surface (e.g. interior paint outside), or skipping required primer.
- Prep & primer: New drywall, stained areas, glossy paint, and dark-to-light changes frequently need primer for the finish coat — and for warranty eligibility.
- Coats and coverage: Applying too thin (stretching coverage beyond the label) can void performance claims. Use the coverage rate on your can, not a generic average alone.
- Batch & records: Keep receipts, product names/codes, and lot numbers. Same-batch paint helps color match and claim support.
- Interior vs exterior: Exterior warranties may require specific primers, temperature ranges, and recoat windows. This calculator does not verify climate or substrate rules.
This tool estimates quantity only. It does not interpret or guarantee any manufacturer warranty. Confirm terms with the brand and follow the product data sheet for your surface.
Frequently Asked Questions
Related measurements
Room floor area pairs well with the square footage calculator. After painting, estimate flooring with the flooring calculator or wall finish with the drywall calculator. Browse all construction calculators.