Calculate exactly how many gallons of paint to buy for any room or multiple rooms. Accounts for doors, windows, number of coats, and paint coverage.
add a room to start
Coats of paint
Coverage (sq ft/gallon)
Full breakdown (click to copy)
Before You Buy
Round up, don't round down
Paint is sold in fixed can sizes. Running short mid-wall means a return trip and a risk of slight colour/batch variation — always round up to the next full gallon or quart.
Walls only, not ceiling
This estimate covers wall area only. If you're also painting the ceiling, calculate that separately (room length × width) and add it to your total.
Check the can label for your specific paint
Coverage varies by product and surface — the default 400 sq ft/gallon is a common average, but your paint's actual coverage (printed on the can) may be higher or lower, especially on textured or porous surfaces.
How to Use the Paint Calculator
Add rooms — click 'Add Room' for each room you are painting. Enter length, width, and ceiling height for each.
Enter doors and windows — the calculator subtracts door area (approximately 21 sq ft each) and window area (approximately 15 sq ft each) from the paintable wall area.
Set number of coats — 2 coats is standard. Going from dark to light colour or painting bare drywall may require 3 coats.
Set coverage rate — standard latex paint covers 350–400 sq ft per gallon. Premium paints: 400–450 sq ft. Budget paints: 300–350 sq ft. Check the paint can label for the specific product's coverage.
Read gallons needed — the calculator shows gallons required and recommends rounding up to the nearest whole gallon, plus 10% extra for touch-ups and overages.
🎨 Primer consideration: If painting bare drywall, new plaster, or making a dramatic colour change (dark to light), primer is needed as a separate step with its own gallon calculation. Good primer improves coverage, saves paint, and produces a more durable finish. One coat of primer + one coat of paint often looks better than two coats of paint without primer.
Understanding Paint Types and Coverage
🎨 Paint Finish Types
Flat/Matte: no sheen, hides imperfections, not washable — ideal for ceilings and low-traffic walls. Eggshell: slight sheen, slightly washable — most popular for living rooms and bedrooms. Satin: moderate sheen, washable — good for family rooms, hallways. Semi-gloss: noticeable sheen, very washable — ideal for trim, doors, kitchens, bathrooms. Gloss: high sheen, most durable — trim, cabinets, high-traffic surfaces.
📊 Coverage Variables
Rough or textured surfaces absorb more paint — add 10–20% to coverage estimate. Painting over dark colours requires more coats. New/unpainted surfaces (drywall, wood) require more paint than repaint. High humidity increases drying time and may affect coverage. Multiple colours require calculating each colour separately.
💧 Oil vs Latex Paint
Latex (water-based): easy clean-up, fast drying (1–4 hours), low VOC, flexible. Modern latex has matched or exceeded oil paint in durability for most applications. Oil-based: longer dry time (8–24 hours), solvent clean-up, better adhesion on glossy surfaces, still preferred for some trim applications. Most interior residential painting uses latex exclusively.
🔢 Paint Calculation Formula
Wall area = 2 × (length + width) × height. Subtract door area (21 sq ft × count) and window area (15 sq ft × count). Divide by coverage per gallon. Multiply by number of coats. Add 10% for waste and future touch-ups. Round up to nearest gallon or half-gallon.
🌿 Low VOC and Zero VOC
VOC (Volatile Organic Compounds) in paint evaporate as it dries, causing strong smell and health concerns. Low VOC: <50g/L. Zero VOC: <5g/L. Most major brands (Benjamin Moore, Sherwin-Williams, Behr) offer low/zero VOC lines in all colours. Recommended for bedrooms, nurseries, and spaces with limited ventilation.
💰 Paint Quality vs Cost
Premium paints ($50–$80/gallon) typically provide better coverage (fewer coats needed), better colour retention, better washability, and longer life than budget paints ($20–$35/gallon). The extra cost per gallon is often offset by needing fewer gallons and having better long-term results. For high-traffic or high-visibility areas, premium paint is cost-effective over a 5–10 year horizon.
Professional Painting Techniques
Preparation is 80% of the job
Professional painters spend more time on preparation than painting. Fill holes and cracks with spackling compound; sand smooth when dry. Sand existing glossy paint for better adhesion. Clean walls with TSP solution or sugar soap to remove grease and dirt. Tape around trim, windows, and ceiling with painter's tape. Use drop cloths on all floors and furniture. Proper preparation eliminates paint failure, brush marks on adjacent surfaces, and touch-up work that doubles labour time.
Painting order for professional results
Paint in this sequence for clean results: 1) Ceiling first (roller, working edge to edge). 2) Walls second (cut in edges with brush, then roll). 3) Trim and doors last (semi-gloss or gloss, brush only). This sequence lets ceiling paint drips onto unpainted walls, and wall paint spills onto unpainted trim — each subsequent step covers previous drips from earlier steps. Cutting in (painting the edge with a brush before rolling) should be done in sections, keeping a wet edge to prevent lap marks.
How much paint to buy for future touch-ups
Always buy slightly more paint than calculated and save it in a labelled container with the date, colour name, and room it was used in. Scuffs, dings, and nail holes requiring touch-up are inevitable over years of living in a space. Paint that is even 1 year old may not perfectly match due to colour drift during storage. Record the exact paint colour code from the can (most retailers can re-mix from the code). For major renovations, consider mixing all same-room paint together in a larger container to ensure consistent colour throughout.
🎨 Paint buying guide: Calculate wall area correctly (subtract doors and windows). Add 10% for waste and touch-ups. One coat on previously painted same colour: standard calculation. New colour or new surface: add 25% and consider primer. Ceiling: calculate separately, usually 1 coat. Trim: approximately 1 quart per 150 linear feet. Buy same batch number (printed on can) if buying multiple cans — colour can vary slightly between batches.
Wall Area & Paint Formula
Wall Area = 2 × (Length + Width) × Height − (Doors × 21) − (Windows × 15). Gallons = Wall Area × Coats ÷ Coverage per Gallon.
Worked example
A 12ft × 10ft room with 8ft ceilings, 1 door, 2 windows, 2 coats, at 400 sq ft/gallon coverage: Wall area = 2 × (12+10) × 8 − 21 − 30 = 352 − 51 = 301 sq ft. Total to cover (2 coats) = 602 sq ft. Gallons = 602 ÷ 400 = 1.50 gallons, round up to 2 gallons (1.66 with 10% extra, still 2 gallons).
Avoid These Mistakes
Forgetting doors and windows entirely
Skipping the door/window count overestimates paint needed — but guessing high on door/window count can underestimate it. Count them accurately per room.
Assuming 1 coat is enough
Most jobs need 2 coats for even, full coverage — especially over a different colour. Budgeting for 1 coat often means a visible patchy finish and a second shopping trip.
Buying different batches for one room
Paint colour can vary slightly between manufacturing batches. Buy all cans for one room from the same batch number where possible, especially for larger walls.
Ignoring surface texture and porosity
Textured, unprimed, or very porous surfaces absorb more paint than the can's stated coverage assumes — budget extra for these surfaces rather than relying on the default coverage figure.
Frequently Asked Questions
How much paint do I need per room?
A standard bedroom (12×10×8ft) needs about 1.5 gallons for 2 coats. Calculate: 2×(L+W)×H = wall area, then subtract doors (21 sq ft each) and windows (15 sq ft each). Divide by coverage (usually 350-400 sq ft/gallon).
How many coats of paint do I need?
Most paint jobs require 2 coats for full coverage. Going from a dark colour to a light colour may need 3 coats. Premium paints sometimes cover in 1 coat. Always apply a primer when painting bare walls or drastic colour changes.
What is paint coverage?
Paint coverage (or spread rate) is how much area one gallon covers. Most latex paints cover 350-400 sq ft per gallon. Cheaper paints may cover less. The coverage is usually printed on the paint can label.
References
🎨 Paint can label
Your specific paint's actual coverage, recommended coat count, and dry/recoat times are printed on the can — always defer to this over the generic 400 sq ft/gallon default used here.
📄 Retailer colour-matching service
Most paint retailers can re-mix an exact colour from the code printed on your can — record this code when buying so future touch-ups match precisely.