Paint Calculator

Calculate gallons (or litres) of paint needed for walls and ceilings — with door and window deductions, coat count and optional cost.

Room details

ft
ft
ft
Standard ceiling: 8 ft (2.44 m); vaulted: measure to ridge.
Each deducts 21 ft² (1.95 m²) from the wall area.
Each deducts 15 ft² (1.39 m²) from the wall area.
ft²/gal
Default: 350 ft²/gal (9 m²/L). Check your paint can.
$

Calculation results

Paint needed

— gallons

Net paintable area

— ft²

Total surface area

— ft²

Results round up to whole gallons. Buy the quantity shown — leftover paint from a second can seals for storage up to 2 years.

Informational only. Coverage varies by paint type, sheen, surface porosity and applicator. Check the coverage on your specific paint can and add one extra gallon as a buffer for cut-ins and touch-ups.

Understanding the formula

Paint quantity starts with total surface area, deducts openings, then divides by the coverage rate and multiplies by the number of coats.

walls = 2 × (L + W) × H  ·  net = walls + ceiling − deductions  ·  gallons = ⌈ net × coats / coverage ⌉

Deductions use standard averages: a door is 21 ft² (32″ × 80″ plus frame trim), a window is 15 ft² (a typical 3 × 4 ft residential window). Override the coverage rate to match your paint’s label — premium paints often cover 400 ft²/gal.

Worked example

A 12 × 10 ft room, 9 ft ceiling, 1 door, 2 windows, 2 coats, 350 ft²/gal:

  • Walls = 2 × (12 + 10) × 9 = 396 ft²
  • Deductions = 1 × 21 + 2 × 15 = 51 ft²
  • Net = 396 − 51 = 345 ft²
  • Gallons = ⌈ 345 × 2 / 350 ⌉ = ⌈ 1.97 ⌉ = 2 gallons

Common mistakes & tips

  • Always buy at least one extra gallon. Touch-ups, cut-ins around trim and ceiling line, and differences between batches (lot numbers) can consume more than the formula predicts.
  • Match sheen to the room. Flat/matte for ceilings and bedrooms; eggshell or satin for living spaces; semi-gloss for kitchens, bathrooms and trim.
  • Two thin coats beat one thick coat. A thick single coat drips, takes longer to dry and hides less. Two thinner coats give more even coverage and a harder film.
  • Prime new drywall. Bare or freshly skim-coated drywall absorbs paint aggressively. A PVA primer coat reduces paint consumption by 20–30 %.
  • Check lot numbers. Mix paint from the same lot to avoid subtle colour shifts mid-wall, especially on large surfaces.

Frequently asked questions

How much paint do I need for a 12 × 12 room?
A 12 × 12 ft room at 9 ft ceiling height has 432 ft² of wall area. Deducting a door (21 ft²) and two windows (30 ft²) gives 381 ft². Two coats at 350 ft²/gal needs 3 gallons.
How many square feet does a gallon of paint cover?
Most interior paints state 350–400 ft² per gallon per coat. Flat paint on smooth, primed walls reaches 400 ft². Rough textures or unprimed surfaces can drop to 250–300 ft².
Do I need primer before painting?
Prime bare drywall, patched spots, stains (use shellac-based primer for water stains) and colour changes from dark to light. Properly painted walls that just need a fresh coat of the same colour rarely need primer.
Should I paint the ceiling the same colour as the walls?
Most rooms use white or off-white ceiling paint (higher reflectivity and sheen). Matching the ceiling to the walls can make a room feel taller and more dramatic — it works well in small rooms and alcoves.
How many coats of paint do I need?
Two coats are the professional standard for any colour change. Dark-to-light transitions may need three coats for full hide. One coat works for touch-ups and re-coating the same colour.
What coverage rate should I use?
Check the label on your paint can — it is listed as "coverage" or "spreading rate". Common values: 350 ft²/gal for most latex interior paints, 400 ft²/gal for premium paints, 250–300 ft²/gal for exterior paint on rough surfaces.