Measurement

Paint Calculator

Calculate interior wall and ceiling paint, doors and window deductions, coats, gallons and quarts to buy, and primer.

Enter room length, width, ceiling height, and count of doors and windows. The calculator figures net surface area, deductions, coats, gallons and quarts to purchase, and primer.

Calculation and worked example reviewed 2026-11-14. See our calculation methodology.

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Measurement

Enter your measurements

US UNITS

Room length

Room width

Ceiling / wall height

Number of doors

Number of windows

Number of coatsInclude ceiling?

Optional: Material Price ($ / unit)

YOUR ESTIMATE

3 gallons to order (2 coats)365 sq ft net · 730 sq ft coverage

Wall & ceiling surface: 416 sq ft gross minus 51 sq ft openings (1 doors, 2 windows).

Packaging recommendation: 2 gal + 1 qt (730 sq ft at 350 sq ft/gal) · ~2 gallon(s) primer.

Formula and example

How this calculation works

This estimator converts your entered jobsite inputs (room length, room width, ceiling / wall height, number of doors, number of windows, number of coats, include ceiling?) into consistent US trade units, computes the exact geometric baseline first, and then applies any selected waste or order allowance so you can verify both numbers before purchasing materials.

How does the paint calculator formula work?

Gross wall area = 2 × (length + width) × height; ceiling area = length × width (if toggled); deductions = (doors × 21 sq ft) + (windows × 15 sq ft); net area = max(0, gross wall + ceiling - deductions); total paint area = net area × coats; gallons = ceil(total paint area ÷ coverage sq ft).

(Paint Calculator)

where:

Room length – Entered in ft (default reference: $14 ft$ );
Room width – Entered in ft (default reference: $12 ft$ );
Ceiling / wall height – Entered in ft (default reference: $8 ft$ );
Number of doors – Entered in ea (default reference: $1 ea$ );
Number of windows – Entered in ea (default reference: $2 ea$ );
Number of coats – Entered in selected option (default reference: $2$ );
Include ceiling? – Entered in selected option (default reference: $no$ );

The formula

Inputs to result

Worked example

Check the math

A 14 ft × 12 ft room with 8 ft ceilings, 1 door, 2 windows, and 2 coats has 365 sq ft net area. Two coats equal 730 sq ft total coverage, requiring 3 gallons of paint (at 350 sq ft/gal) and 2 gallons of primer for bare drywall.

Quick lookup chart

Interior Room Paint Coverage Reference (8 ft Ceilings, 2 Coats)

Estimates deduct 1 door (21 sq ft) and 2 windows (15 sq ft each) at 350 sq ft per gallon.

Room Dimensions	Net Wall Area	Paint Needed (2 Coats)	Primer Needed
10 ft × 10 ft small bedroom	269 sq ft	2 gallons (538 sq ft cov)	1 gallon
12 ft × 14 ft master bedroom	365 sq ft	3 gallons (730 sq ft cov)	2 gallons
14 ft × 18 ft living room	461 sq ft	3 gallons (922 sq ft cov)	2 gallons
16 ft × 20 ft great room	525 sq ft	3 to 4 gal (1,050 sq ft cov)	2 gallons

Limits and field checks

Before you use this result

Assumes standard smooth drywall or plaster walls. Heavily textured walls (knockdown, orange peel, popcorn, masonry) or raw unprimed drywall absorb 20% to 50% more paint. Dark to light color transformations require dedicated high-hide primer.

Wash walls with TSP (trisodium phosphate) or a mild detergent to eliminate grease, fingerprints, and dust prior to applying primer or topcoat.
Patch small nail holes and gouges with lightweight spackle; sand flush with 220-grit sandpaper and prime patched spots to avoid dull 'flashing'.
Cut in ceiling edges, baseboards, and window frames with a 2-1/2 inch angled sash brush before rolling the open wall fields.
Maintain a wet edge while rolling; work in 3-foot by 3-foot sections rolling in a 'W' or 'N' pattern, then lightly backroll vertically from ceiling to floor.
Allow at least 2 to 4 hours of drying time between latex coats depending on ambient room temperature and relative humidity.

Questions answered

Calculator FAQ

How many square feet does one gallon of paint cover?+

A standard gallon of interior acrylic latex paint covers 350 to 400 square feet on smooth, primed drywall. Highly porous or rough textured surfaces (such as raw masonry, heavy orange peel, or acoustic plaster) yield 200 to 250 square feet per gallon.

How much area do doors and windows deduct from wall paint?+

Professional painters standardly deduct 21 square feet per standard 3 ft × 7 ft interior door (or 20 sq ft) and 15 square feet per standard 3 ft × 5 ft window. Double doors or picture windows should be counted as 2 units.

Do I really need two coats of paint?+

Yes. Two coats provide uniform color saturation, proper sheen development (matte, eggshell, satin, or semi-gloss), and wipeable durability. A single coat is only sufficient when refreshing the exact same color and finish over clean, undamaged walls.

When should I use a separate primer coat?+

Apply a dedicated primer over bare drywall, fresh joint compound patches, glossy enamel finishes, water or smoke stains, and when making dramatic color transitions (e.g. painting white over navy blue or dark red).

Method references: PDCA: Painting and Decorating Contractors of America estimating guide