Paint Calculator - Gallons, Litres, Coats & Room Cost

Paint Coverage Quick Reference

Standard interior latex coverage is 350-400 sq ft per US gallon (8-10 m²/L) for one coat on a primed, average-texture wall. Use this for the back-of-envelope number; the calculator handles your real coverage value, coats, and openings.

For a primer coat, drop coverage to 250-300 sq ft per gallon - primer soaks into porous and patched surfaces more than topcoat does.

What Determines How Many Gallons

The formula is simple: paint volume = (net wall area x number of coats) divided by coverage. Net area is the painted surface minus doors, windows, large vents, and anything else that does not get paint. Coverage is the number on the can label - usually 350-400 sq ft per US gallon, or 8-12 m² per litre. Coats are almost always 2 over a tinted primer or one previous colour; 3 coats if you are going from dark to light or covering bold colour.

What kills budgets is texture and absorbency. New drywall sucks the first coat in. Knockdown texture has more surface area per square foot than a flat wall. Stucco eats paint. Smooth, previously-painted walls in matching colour drink almost nothing - one coat may even be enough. The 350 sq ft/gallon number is the supplier's polite average; your real coverage on a textured wall can be 250-300.

Primer - When To Use, When To Skip

Primer is a separate gallon, not a discount on the topcoat. It is its own coat with its own coverage rate (250-300 sq ft/gal).

• New drywall: always prime. The paper face absorbs paint unevenly without primer. A drywall-specific PVA primer works fine.
• Bare wood: prime with a stain-blocking primer (Zinsser BIN, Kilz Premium). Knots bleed through topcoat otherwise.
• Going from dark to light: 1 primer coat plus 2 topcoats, or use a tinted primer in the new colour family.
• Repainting same-colour latex: usually skip primer. Touch up patches, then 2 topcoats.
• Over oil-based paint with latex: always prime with a bonding primer. Latex peels off cured oil within a year if you skip this.
• Bathrooms, kitchens, stained ceilings: use a moisture-resistant or stain-blocking primer. The cheap version saves $20 and costs you the whole job.

Sheen - The Other Decision Nobody Plans For

Sheen changes how the paint looks, cleans, and shows imperfections. Same colour, different sheen, totally different room.

Subtracting Openings - Where People Lose Money

The calculator subtracts an openings area you supply. Standard planning values:

• Door: 20 sq ft (1.86 m²) - a 3 ft x 6 ft 8 in opening.
• Window: 15 sq ft (1.4 m²) - average residential. Picture windows and patio sliders are bigger; measure those.
• Built-in cabinetry: measure the actual wall area covered. A typical kitchen wall has 40-60 sq ft hidden behind cabinets.
• Wall openings (pass-throughs, half walls): count both faces if both get painted.

The rule that catches DIYers: subtracting too aggressively. If you take out every electrical outlet (each is about 0.1 sq ft) you spend more time measuring than you save in paint. The 350 sq ft/gallon coverage already assumes some material loss in cutting in around outlets, switches, and tight corners.

Cost Estimates Around The World

2026 retail prices for mid-tier interior latex / acrylic emulsion. Top-tier (zero-VOC, premium washable, designer colours) and exterior weatherproof lines cost 30-80% more.

Brand reference: US - Sherwin-Williams, Benjamin Moore, Behr (Home Depot), Valspar (Lowe's); UK - Dulux, Crown, Farrow & Ball, Little Greene; India - Asian Paints, Berger, Nerolac, Dulux, Indigo; Australia - Dulux, Taubmans, Wattyl; Philippines - Boysen, Davies, Rain or Shine. Bigger tins always cost less per litre - the 20 L Indian bucket is roughly 40% cheaper per litre than the 1 L can.

Working Order - The Day-Of Plan

1. Patch and sand. Filler dries in 30-60 minutes; sanded patches need spot priming.
2. Mask and protect. Painter's tape on trim, drop cloths on floor, plastic on furniture. A roll of 1.5 in tape per average room.
3. Prime patches, repairs, and new substrates. Let dry 1-4 hours per the label.
4. Cut in with a 2.5 in angled sash brush - corners, edges, around trim.
5. Roll the field while the cut-in is still wet. Use a 9 in roller with 3/8 in nap for smooth walls, 1/2 in for light texture, 3/4 in for heavy texture.
6. Second coat after the first cures - 2 to 4 hours for latex at 70°F. Cold or damp rooms push that to overnight.
7. Pull tape while the last coat is just-dry-to-the-touch, not fully cured. Tape pulled off cured paint takes the paint with it.

Common Mistakes

• Skipping primer on new drywall - the colour goes patchy and the second coat does not save it.
• Treating the coverage number as a hard floor instead of the manufacturer's polite average.
• Using flat paint in a bathroom or kitchen - the surface absorbs steam and stains permanently.
• Not subtracting large openings - a single 4 x 8 ft window is 32 sq ft you do not paint.
• Buying one gallon for a job that needs three, then driving back twice and getting two slightly-different mixes.
• Storing leftover paint without sealing the can - skin forms on top and the next touch-up looks lumpy.
• Painting over wallpaper without sealing - paint reactivates the paste and the wallpaper bubbles.
• Pulling tape after the paint fully cures - the cured film tears.

Related Construction Calculators

For wallpaper rolls and drops, see the Wallpaper Calculator. New construction or repair drywall lives in the Drywall Calculator. Wall area cleanup starts in the Square Footage Calculator. For exterior planning, the Roof Area Calculator handles roof surface. More tools live on the Construction Calculators hub.