Paint Calculator & Coverage Estimator
Find out how much paint a room needs. It measures the wall area, subtracts doors and windows, adjusts for surface texture and the number of coats, adds a waste margin, and tells you what to buy in gallons and quarts.
Written by Suhaib Hassan. Spread rates are typical manufacturer figures; your can label rules. Our methodology.
Net wall area × coats ÷ spread rate × 1.10 gives the gallons, which are rounded up to whole cans.
Buying too little means a second trip and colour mismatch. This shows exactly what to buy and what it costs.
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Paint sheen guide
Typical spread rates and where each finish is used. Gloss ranges are approximate and vary by brand.
| Sheen | Spread (sq ft/gal) | Gloss at 60° | Best used for | Durability |
|---|---|---|---|---|
| Flat / matte | 350 – 400 | 0 – 5 | Ceilings, low-traffic bedrooms | Hides flaws, hard to scrub |
| Eggshell | 350 – 400 | 10 – 25 | Living rooms, dining rooms, offices | Moderate scrubbing |
| Satin | 350 – 400 | 25 – 35 | Hallways, kids' rooms, laundry | Good wipe resistance |
| Semi-gloss | 350 – 400 | 35 – 70 | Bathrooms, kitchens, trim and doors | Very washable |
| High-gloss | 300 – 350 | 70+ | Cabinets, entry doors, railings | Hardest, shows every flaw |
How the paint is calculated
Net wall area
A = 2 × (L + W) × H − doors × 21 − windows × 15 − otherGallons to buy
gal = ⌈ A × coats ÷ spread × 1.10 ⌉Ceiling
A = L × WWhat is included
Doors are taken as 3 × 7 ft (21 sq ft) and windows as 3 × 5 ft (15 sq ft). Use the custom field for larger openings such as sliding doors.
Waste of 10% covers spills, roller loss and touch-ups. Ceilings use the same number of coats. Trim is estimated at 6 in baseboards and 3.5 in casings, painted at 350 sq ft per gallon.
Very deep colour changes and rough textures may need extra coats or primer.
Worked examples
Illustrative rooms, each checked by hand. The first is the default setup.
18 × 14 ft room, 9 ft ceiling
2 doors, 2 windows, smooth drywall, 2 coats.
openings = 42 + 30 = 72
net = 504 sq ft
504 × 2 ÷ 400 × 1.10 = 2.77 gal → 3 gal
Ceiling of the same room
18 × 14 ft, 2 coats, smooth.
252 × 2 ÷ 400 × 1.10 = 1.39 gal
buy 2 gal (or 1 gal + 2 qt)
Textured walls
Same room, orange-peel texture at 350 sq ft per gallon.
buy 3 gal + 1 qt
How much paint do I need?
- Measure the room length, width and ceiling height.
- Count doors and windows so their area comes off the walls.
- Choose the surface texture, which changes how far a gallon goes, and the number of coats.
- Add ceiling, trim and primer if you are painting those too, then read the shopping list.
Buy all the paint for one room at the same time and mix cans together (boxing) so the colour matches from wall to wall.
By Suhaib Hassan · Reviewed 29 September 2026 · How CalculatePilot verifies formulas →
Sources
- Master Painters Institute (MPI), gloss and sheen level definitions for architectural coatings.
- Paint manufacturers' product data sheets, which state coverage of about 350 to 400 square feet per gallon on smooth, primed surfaces.
Frequently asked questions
How much does a gallon of paint cover?
Manufacturers usually state 350 to 400 square feet per gallon on smooth, primed walls. Textured, porous or unprimed surfaces soak up more paint and can drop coverage to 250 to 300 square feet per gallon.
Do I need primer?
Use primer on new drywall, bare wood, patched areas, stains, and when moving from a glossy or oil-based paint to latex. A good paint-and-primer can replace it on already-painted, clean walls.
How many coats are needed?
Two coats give a durable, even finish for most jobs. One coat can work for touch-ups in the same colour, and three may be needed for a big colour change such as dark to light.
How do I measure a vaulted ceiling?
For a sloped ceiling, find the sloped length with the Pythagorean theorem (√(rise² + run²)) and multiply by the room length. Add the triangular gable wall, which is ½ × base × height.
How should I store leftover paint?
Seal the lid tightly, store the can upside down or on its side if the manufacturer allows, and keep it somewhere cool and dry above freezing. Label it with the colour, room and date.
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