Paint Calculator – How Much Paint Do I Need?

"Gallons to buy" is rounded up to the next quarter gallon because paint is sold in quarts and gallons, not by the exact square foot. Buying a single larger can rather than several quarts also keeps the color batch consistent, which matters for uniform sheen and tone across a wall.

The primer line only appears when you enter primer coats. Primer is calculated over the same paintable area using the same coverage-per-gallon figure, since most primers cover similarly to paint on a comparable surface.

Measure the length of each wall and the ceiling height. If walls differ, enter an average length and the number of walls — the calculator multiplies them together for total wall area.

Subtract openings by entering how many doors and windows the room has. The defaults match a standard 3 ft × 7 ft door and a 3 ft × 5 ft window, but you can enter your own areas.

Two coats is the normal expectation for a quality finish, and more when covering a dark colour with a light one. Add primer coats for bare drywall, patched repairs, stains or glossy existing surfaces. Coverage per gallon is printed on the can and drops on textured or porous walls.

Wall area = average wall length × wall height × number of walls

Paintable area = wall area − (doors × door area) − (windows × window area)

Gallons = paintable area × coats ÷ coverage per gallon

The result is rounded up to the next quarter gallon, since paint is sold in quarts and gallons. Primer is calculated the same way using the number of primer coats.

A 14 ft × 14 ft room with 9 ft ceilings, two doors and two windows:

Wall area = 14 × 9 × 4 = 504 sq ft

Openings = (2 × 21) + (2 × 15) = 72 sq ft → paintable = 432 sq ft

Two coats means 864 sq ft of coated surface. At 375 sq ft per gallon that is 2.3 gallons, rounded up to 2.5. At $45 per gallon the paint costs about $113 — and buying three one-gallon cans is usually simpler than mixing gallons and quarts.

Surface porosity

Bare drywall, new plaster, brick and stucco soak up more paint than a previously painted, sealed wall, reducing effective coverage per gallon.

Texture

Orange-peel, knockdown or heavily textured surfaces have more surface area than their flat square footage suggests, and roll less efficiently, both increasing paint use.

Sheen

Flat and matte finishes generally spread and hide slightly better per coat than high-gloss or semi-gloss, which are typically reserved for trim and high-wear areas.

Color change

Moving from a dark base color to a light one — or the reverse — reduces one-coat hiding power and often requires a tinted primer or an extra coat.

Cut-in vs. roller area

Edges, corners and trim lines painted by brush ('cutting in') use paint less efficiently than open wall area rolled with a roller, which matters more in small, cut-up rooms.

Drywall vs. masonry

Masonry, brick and concrete are far more porous than drywall and can need a dedicated masonry primer plus additional paint volume to achieve full coverage.

• Ignoring surface porosity

  Using the can's stated coverage on an unprimed or textured wall usually means running out of paint mid-job.

• Skipping primer to save money

  Skipping primer on bare drywall or a big color change often costs more in extra topcoats than the primer itself would have.

• Forgetting trim and ceiling paint

  Trim, baseboards and ceilings use different products and sheens and are not covered by a wall paint estimate.

• Buying paint in odd quart amounts

  Mixing several small quart cans from different batches can produce visible color and sheen variation across a wall; a single gallon or matched batch avoids this.

• Not deducting large openings

  Skipping the doors-and-windows deduction on a room with several large windows or French doors can meaningfully overstate the paint needed.

• Buy one extra quart for touch-ups; color batches can vary slightly between purchases months apart.
• Use a tinted primer close to your final color when covering a strong existing color.
• Box (mix) multiple gallons of the same color together in a larger container if a wall will take more than one can, to avoid subtle batch differences.
• Budget separate paint and sheen for trim, doors and ceilings rather than assuming the wall paint estimate covers them.
• Check the label's coverage claim against your wall condition — treat it as a best case on a smooth, sealed surface.

Actual paint use depends on your rolling technique, nap thickness, how many coats you actually apply for full hiding, and how porous or textured the real wall is compared to the can's test surface. Treat this estimate as a shopping-list starting point and keep the receipt in case you need another partial can.

What this calculator assumes

• Walls are flat and the surface is in normal condition.
• Coverage per gallon reflects the manufacturer's stated figure.
• Every coat uses the same amount of paint.
• The ceiling, when included, is square with sides equal to the wall length entered.

What it does not account for

• Trim, baseboards, doors and window frames, which need separate paint.
• Texture, porosity and colour change, which can raise consumption substantially.
• Brushes, rollers, tape, drop cloths, patching compound and sundries.
• Professional labor costs.

• Paint Quality Institute — coverage and number of coats — Industry guidance on the 350-400 sq ft per gallon spread rate used here.
• EPA — Renovation, Repair and Painting (RRP) rule — Lead-safe work practices required when disturbing paint in pre-1978 homes.
• ASTM D2486 — Scrub resistance of wall paints — Standard behind durability differences between flat, eggshell and semi-gloss sheens.
• MPI — Master Painters Institute standards — Primer and surface-preparation requirements for drywall, masonry and previously painted surfaces.