Paint Calculator
Enter the room and get the paintable area, the gallons required and the exact combination of cans to buy — doors and windows subtracted, coats and spread rate under your control.
How to use this calculator
- Measure the perimeter and the height. The tool does perimeter × height for you from length and width; for an irregular room, switch to "I already know the wall area" and add the walls up yourself.
- Count doors and windows. They are deducted at 21 and 15 sq ft — the standard figures for a 3 × 7 ft door and an average window. If your windows are large picture units, use the wall-area mode and subtract their real size.
- Use the spread rate on the can, not the default. 350 sq ft per gallon is typical for a smooth, previously painted wall. Textured or porous surfaces drop to 250–300, and a deep colour over a light one may need three coats rather than two.
- Read the "cans to buy" line. Paint comes in gallons and quarts, so 2.3 gallons means two gallons plus two quarts, not three gallons — which saves both money and leftover storage.
The formula
Two steps: work out what is actually being painted, then divide by the spread rate.
wall area = 2 × (length + width) × height
paintable area = wall area − (doors × 21) − (windows × 15) + ceiling
gallons = paintable area × coats ÷ coverage per gallon
| Surface | Typical coverage per gallon |
|---|---|
| Smooth, previously painted drywall | 350–400 sq ft |
| Lightly textured drywall | 300–350 sq ft |
| Heavy texture, knockdown, popcorn | 250–300 sq ft |
| Bare drywall or plaster (prime first) | 200–250 sq ft |
| Rough masonry, block, stucco | 150–250 sq ft |
Worked example
A 14 × 12 ft bedroom with 8 ft walls, one door, two windows, two coats, walls only:
- Perimeter: 2 × (14 + 12) = 52 ft
- Wall surface: 52 × 8 = 416 sq ft
- Deduct openings: 21 + (2 × 15) = 51 → 416 − 51 = 365 sq ft
- Two coats: 365 × 2 = 730 sq ft of coverage needed
- At 350 sq ft/gal: 730 ÷ 350 = 2.09 gallons
- Buy 2 gallons + 1 quart
Rounding that up to three gallons would leave you storing most of a gallon you paid full price for. The quart is also the can you want left over for touch-ups.
Rules of thumb worth knowing
- One gallon of trim paint covers roughly 600 linear feet of standard baseboard or 25 average doors and windows' casings — trim is usually a quart or two, not gallons.
- Ceilings need their own paint. Flat ceiling white hides roller lap marks that a wall sheen will show. Ceiling area is simply length × width.
- Dark over light means primer, ideally tinted grey toward the final colour. It is cheaper than a third and fourth coat of expensive colour.
- Buy all of a colour in one trip. Tinted batches vary slightly; mixing cans from different days in the same room can show on a long wall.
Frequently asked questions
How much does a gallon of paint cover?
About 350 to 400 square feet in one coat on smooth, previously painted drywall. Lightly textured walls drop to roughly 300 to 350, heavy texture to 250 to 300, and bare drywall or masonry can drink 150 to 250 square feet per gallon. The spread rate on the can is the number to use.
How many gallons do I need for a 12x12 room?
A 12 by 12 foot room with 8 foot ceilings has 384 square feet of wall. Take off about 50 square feet for a door and a window and you have roughly 334 square feet, so two coats need about 1.9 gallons — buy two gallons. Add another 1.2 gallons if you are painting the ceiling as well.
Do I subtract doors and windows?
Yes for anything meaningful. The standard deductions are 21 square feet per door and 15 per window. Skipping the deduction on a room with a patio slider can overstate the paint by half a gallon, though a small allowance for error is not a bad thing.
How many coats of paint do I need?
Two on almost any repaint. One coat only works when you are refreshing the same colour on a sound, clean surface with a premium paint. Plan on primer plus two coats for bare drywall, patched repairs, stains, or a dramatic colour change such as white over deep red.
Should I buy gallons or quarts?
Buy whole gallons for the bulk of the work and make up the remainder in quarts — four quarts make a gallon but cost more per unit, so they are for the last fraction and for touch-up stock, not the main volume. The calculator shows the cheapest combination.
Guides
- How Much Does a Gallon of Paint Cover?
- How Many Coats of Paint Do You Need?
- How Much Primer Do I Need — and Do You Even Need It?
- Paint Sheen Guide: Flat, Eggshell, Satin, Semi-Gloss
- How Much Paint Do I Need for a House Exterior?