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Paint Calculator — Gallons, Cost & DIY-vs-Pro (2026)

Drop your room dimensions, door + window count, coats, and finish — get gallons needed for walls + ceiling + trim, retail DIY cost, pro install cost, and DIY time estimate. Sherwin-Williams + Benjamin Moore coverage tables.

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Reviewed by CalcBold Editorial · Sources: Sherwin-Williams + Benjamin Moore 2026 coverage guides + EPA VOC regulationsLast verified Methodology

Paint Calculator

Longest wall of the room in feet. For non-rectangular rooms, sum the bounding wall lengths.

Shorter wall in feet. Together with length, drives wall + ceiling area calculation.

Standard residential 8-9 ft; cathedral / vaulted ceilings 10-20 ft. For sloped ceilings, average the high + low points.

Standard 3'×7' door = 21 sq ft subtracted from wall area per door.

Standard 3'×5' window = 15 sq ft subtracted per window. For unusually large picture windows, count them as 2-3.

1 coat for similar-color repaint, 2 for standard, 3 for dark-to-light or light-to-dark color change.

Coverage varies by finish — flat/matte 375 sq ft/gal · eggshell + satin + semi-gloss 400 sq ft/gal · primer 250 sq ft/gal.

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How much paint do I need? — short answer first

For a typical 12' × 14' bedroom with 9' ceilings, 1 door, 2 windows, and 2 coats of eggshell: about 3 gallons total (2 wall + 1 trim gallon — ceilings are usually a separate gallon of flat). DIY material cost $170-$275 with supplies; pro install runs $600-1,050. Time: ~1-2 weekend days solo. The calculator above runs your specific room dimensions through Sherwin-Williams + Benjamin Moore coverage tables and surfaces the DIY-vs-pro economics for your scope.

What This Calculator Does

Drop your room dimensions (length × width × ceiling height), door + window count, number of coats, and finish — get gallons needed for walls + ceiling + trim, DIY retail material cost ($35-65/gal + supplies), pro install cost ($200-350/gal installed), and a DIY time estimate in days. Coverage rates come from Sherwin-Williams + Benjamin Moore published guides; both manufacturers agree within ±5% on per-finish rates.

The Math / Formulas Used

Coverage rates by finish (sq ft per gallon, on smooth primed walls): flat / matte = 375 · eggshell + satin + semi-gloss = 400 · primer = 250. Textured walls (orange peel, knockdown) reduce these by 15-25%; bump your gallon count up by 1 if the walls are textured. Color-change repaints (dark to light or vice versa) typically need 3 coats vs the standard 2.

Standard door = 21 sq ft (3' × 7'), standard window = 15 sq ft (3' × 5'). The calculator subtracts these from gross wall area. For unusually large picture windows or floor-to-ceiling glass, count them as 2-3 windows or measure manually and adjust the gross area downward.

How to Use This Calculator

  1. Measure the room. Length × width × ceiling height. For non-rectangular rooms, sum the bounding-wall lengths.
  2. Count openings. Doors + windows are subtracted from wall area (21 + 15 sq ft each, respectively).
  3. Pick coats. 2 standard. 1 for similar-color touch-up. 3 for major color change.
  4. Pick finish. Eggshell is the standard living-area choice. Flat for ceilings + low-traffic. Semi-gloss for trim + bathrooms.
  5. Read the verdict. Total gallons, DIY cost, pro cost, and time estimate. Buy 1 extra gallon for touch-ups (unopened gallons return easily).

Three Worked Examples

Example 1 — Small bedroom: 10' × 12', 8' ceilings, 1 door, 1 window

Wall area: (10+12) × 2 × 8 − 21 − 15 = 352 − 36 = 316 sq ft. 2 coats eggshell at 400 sq ft/gal = ceil(632/400) = 2 wall gallons. Ceiling: 120 sq ft × 2 / 375 = ceil(0.64) = 1 gallon. Trim: ~88 ft / 200 = 1 gallon. Total 4 gallons ($140-260 retail + $80 supplies = $220-340 DIY · $800-1,400 pro). Weekend solo project.

Example 2 — Living room: 16' × 20', 10' ceilings, 2 doors, 4 windows

Wall area: (16+20) × 2 × 10 − 42 − 60 = 720 − 102 = 618 sq ft. 2 coats eggshell: ceil(1,236/400) = 4 wall gallons. Ceiling: 320 sq ft × 2 / 375 = 2 gallons. Trim: ~140 ft / 200 = 1 gallon. Total 7 gallons($245-455 + $80 = $325-535 DIY · $1,400-2,450 pro). DIY is feasible (3-4 days) but pro saves ~3 days at 4× the cost.

Example 3 — Bathroom: 8' × 10', 8' ceiling, 1 door, 1 small window, semi-gloss

Wall area: (8+10) × 2 × 8 − 21 − 15 = 288 − 36 = 252 sq ft. 2 coats semi-gloss at 400 sq ft/gal = ceil(504/400) = 2 wall gallons. Ceiling: 80 sq ft, treat as flat: 1 gallon. Trim: ~72 ft = 1 gallon.Total 4 gallons ($140-260 + $80 = $220-340 DIY). Semi-gloss is mandatory for the humidity; 1 weekend solo. Pre-prime any new drywall patches with separate primer can.

Common Mistakes

  • Buying exactly the calculator’s gallon count. Always buy 1 extra gallon as touch-up reserve. Unopened gallons return easily at Home Depot, Lowe’s, and Sherwin-Williams. Running out at 95% complete on a Saturday night when the store’s closed is brutal — the partial wall dries differently than the rest.
  • Using flat finish in bathrooms or kitchens. Flat paint is hardest to clean and shows moisture damage. Semi-gloss is mandatory in bathrooms (humidity + cleaning), satin works in kitchens (cooking grease wipes off), eggshell is the standard living-area choice.
  • Skipping primer on new drywall or color change. New drywall is highly absorbent; primer seals it and prevents the first finish coat from soaking in (wasting paint). Color-change repaints (dark-to-light especially) without primer show through 2 finish coats — you’ll need 3+ finish coats vs primer + 2 finish.
  • Painting over textured walls without adjusting gallons. Orange peel + knockdown texture reduces coverage 15-25%. The calculator assumes smooth walls; add 1 gallon to your purchase if the walls are textured.
  • Cheap brushes + rollers. $5 brushes shed bristles into wet paint (you’ll pick them out for hours); $5 rollers leave lint. Spend $15-25 per brush + $20-30 per roller — they last years and the line quality is night-and-day. Purdy + Wooster are the pro-standard brands.
  • Painting over lead-based paint without testing (pre-1978 homes). EPA RRP rule mandates lead-safe practices (HEPA vacuum, plastic containment, respirator) when sanding or scraping. Free EPA lead test kits or $25 at hardware stores. Skipping this on a pre-1978 home is a serious health risk + EPA fine territory.

Methodology & Sources

Per-finish coverage rates: Sherwin-Williams 2026 Coverage Guide + Benjamin Moore Product Specifications— both manufacturers publish nearly identical rates (within ±5%), validating the calculator’s per-gallon coverage as industry consensus. Standard door + window sizes: 3' × 7' door (21 sq ft) + 3' × 5' window (15 sq ft) — industry-standard residential dimensions. Cost benchmarks: 2026 US-average retail ($35-65/gal mid-tier paint + ~$80 supplies budget) and pro install ($200-350/gal installed = labor + material + overhead + warranty). Lead-safe practices reference: EPA Renovation, Repair, and Painting (RRP) Rule — federal regulation for pre-1978 homes.

How to Read the Verdict

  1. 1-3 gallons — easy weekend DIY. Solo project, 1-2 days. Buy 1 extra gallon for touch-ups. Pro install rarely makes sense at this scope unless time is the constraint.
  2. 4-6 gallons — solid DIY (2-day project). Standard living room or master bedroom. DIY saves $300-500 vs hiring. Pro install finishes in 1 day if time matters; otherwise solo DIY with one helper for cut-in.
  3. 7-10 gallons — time-vs-money trade. Multi-room scope. DIY is 3-5 days solo; pro finishes in 1-2 days. The pro premium ($1,000-2,000 above DIY) buys you 3-4 days back + cleaner cut-lines. Decide based on what your time is worth.
  4. 10+ gallons — whole-house repaint, hire a pro. The labor savings on DIY don’t scale linearly — fatigue + cut-line quality compounds across rooms. Pro install at this scope finishes a whole house in 3-5 days vs your 2-3 weeks. Get 3 quotes.
  5. Pre-1978 home with chipped/peeling paint. Test for lead before any prep work. If positive, hire an EPA-certified lead-safe contractor — DIY lead remediation without proper containment is a serious health risk.

Planning more of the room or full project? If the walls need patching, run the drywall calc for sheet + mud + tape first. If the paint is part of a bathroom remodel, the bathroom renovation cost calc gives the full project budget. For exterior projects, the deck cost calc handles deck stain budgeting — same coats-and-coverage thinking, different chemistry.

Sources & Methodology

The formulas, thresholds, and benchmarks behind this calculator are anchored to the primary sources below. Where a study or agency document is the underlying authority, we link straight to it — not a summary or republished version.

  1. Sherwin-Williams — 2026 Coverage Guide & Product Data· Sherwin-Williams Company

    Manufacturer-published per-finish coverage rates (350-400 sq ft/gal for finish coats, 250 for primer) used in the calculator. Sherwin-Williams is one of the two largest US residential paint manufacturers.

    Accessed

  2. Benjamin Moore — Product Specifications & Coverage Calculator· Benjamin Moore & Co.

    Cross-reference for per-finish coverage + multi-coat guidance. Benjamin Moore's published coverage matches the Sherwin-Williams reference within ±5%, validating the calculator's per-gallon rates as industry consensus.

    Accessed

  3. EPA — Lead RRP Rule (Renovation, Repair & Painting)· U.S. Environmental Protection Agency

    Federal rule requiring lead-safe practices in homes built before 1978. Referenced in the calculator's lead-paint red-flag guidance — testing + containment + HEPA vacuum required when sanding or scraping older paint.

    Accessed

Frequently Asked Questions

The most common questions we get about this calculator — each answer is kept under 60 words so you can scan.

  • How accurate is the paint calculator?
    Within ±1 gallon when room measurements + door/window count are accurate — it uses Sherwin-Williams + Benjamin Moore published coverage tables (350-400 sq ft per gallon for finish coats, 250 for primer). Adjustments may be needed for textured walls (orange peel + knockdown reduce coverage 15-25%), dark-over-light color changes (often need 3 coats), or unusually rough or absorbent surfaces.
  • How many square feet does a gallon of paint cover?
    350-400 sq ft for finish coats (flat = 375 sq ft/gal, eggshell + satin + semi-gloss = 400 sq ft/gal in the calculator). Primer is thicker and covers less: ~250 sq ft/gal. These are manufacturer-published rates on smooth, primed surfaces; textured walls reduce coverage 15-25%. The calculator already rounds UP because you can't buy partial gallons.
  • How many coats of paint do I need?
    2 coats is standard for most repaints. 1 coat works only when repainting the same or very similar color (touch-up jobs). 3 coats is required for major color changes (dark-to-light or light-to-dark — the underlying color bleeds through if you only do 2). New drywall requires 1 coat of primer + 2 coats of finish. The calculator's 'coats' input maps directly to this — bump to 3 if you're changing color significantly.
  • What's the difference between flat, eggshell, satin, and semi-gloss?
    Flat (matte) hides wall imperfections best but is least washable — good for ceilings + low-traffic walls. Eggshell is the standard living-area finish — mild sheen, washable, hides most imperfections. Satin is more durable + slightly shinier — hallways + kid rooms. Semi-gloss is high-sheen + very washable — bathrooms, trim, kitchens (resists humidity + cooking grease). Gloss is reserved for high-impact accent trim. Higher-sheen finishes show wall imperfections more, so prep matters more.
  • Should I DIY or hire a pro?
    DIY makes sense for small-to-medium rooms (1-5 gallons total) — you save 60-70% vs pro pricing. Hire a pro for: time-constrained projects, ceiling work on cathedral/vaulted heights, color-change repaints where cuts matter, or whole-house projects. Pro labor is fast (2-3× DIY speed) and cleaner on cut-lines, but the cost premium is real — pros typically charge $200-350 per gallon installed (vs your $35-65/gal retail).
  • How long will a DIY paint job take?
    Roughly 8 hours per 200 sq ft per coat for a solo first-timer; 6 hours for experienced; 4 hours for fast. Add taping + cutting in time (1-2 hours per room) and drying between coats (2-4 hours minimum). A 12'×14' bedroom with 2 coats typically takes a solo painter 6-10 hours total (one weekend day). The calculator estimates DIY time in days based on this pace.
  • What supplies do I need beyond paint?
    Drop cloths ($15-30), painter's tape ($10-20 per roll, you'll use 2-3 per room), 2-3 brushes (1.5" angled sash for cutting + 2.5" for trim, ~$15-25 each), 1-2 rollers + covers ($20-30), roller tray + liners ($10-15), and a 5-in-1 painter's tool ($8). Total ~$80 supplies budget per project. Sandpaper + spackling if you need to repair walls first.
  • What's a 'cut in' and why does it take so long?
    Cutting in is hand-brushing the perimeter of a wall — corners, around trim, around the ceiling line — before rolling the main field. It's the slowest part of painting (1-2 hours per room) and the part that distinguishes pro work from DIY. Pros use a 1.5-2.5" angled sash brush + freehand technique; DIY-ers use painter's tape (slower but more forgiving). Skipping the cut-in means you can't roll close to the edges = paint splatter on trim + ceiling.