Running out of paint halfway through a wall is frustrating, and buying too much wastes money. Working out how much paint you need takes just three numbers: the wall area, the number of coats and the coverage per gallon. This guide walks you through the formula, a quick chart and real room examples.
The Paint Formula Step by Step
Step 1: Find the Wall Area
Add the lengths of all walls to get the perimeter, then multiply by the wall height. Wall area = perimeter x height.
Step 2: Subtract Openings
Subtract about 21 sq ft for a standard door (3 x 7 ft) and about 15 sq ft for a typical window (3 x 5 ft), or measure your actual sizes.
Step 3: Multiply by Coats
Use 2 coats for a new color or new drywall, 1 coat for a same-color refresh.
Step 4: Divide by Coverage
Gallons = paintable area x coats / coverage per gallon. Use the coverage on the can, often 350 to 400 sq ft. Round up.
Paint Gallon Chart (350 sq ft per Gallon)
| Paintable Area | 1 Coat | 2 Coats |
|---|---|---|
| 250 sq ft | 1 gallon | 2 gallons |
| 500 sq ft | 2 gallons | 3 gallons |
| 1,000 sq ft | 3 gallons | 6 gallons |
| 2,000 sq ft | 6 gallons | 12 gallons |
| 3,000 sq ft | 9 gallons | 18 gallons |
Amounts are rounded up to whole gallons. A 5-gallon bucket usually costs less per gallon on bigger jobs.
Worked Example: 12 x 14 Bedroom
| Step | Calculation | Result |
|---|---|---|
| Perimeter | 12 + 14 + 12 + 14 | 52 ft |
| Wall area | 52 x 8 ft | 416 sq ft |
| Openings | 1 door (21) + 2 windows (15 each) | -51 sq ft |
| Paintable area | 416 – 51 | 365 sq ft |
| Two coats | 365 x 2 | 730 sq ft |
| Gallons | 730 / 350 | 2.1 gallons |
Buy 2 gallons plus a quart, or 3 gallons if you want leftover paint for touch-ups. The ceiling is extra: 12 x 14 = 168 sq ft, about 1 gallon for two coats of ceiling paint.
Exterior Walls and Ceilings
For a house exterior, measure each wall’s width x height, add gable triangles (base x height / 2) and subtract large openings. For example, 1,500 sq ft of siding with two coats is 3,000 / 350 = 8.6, so about 9 gallons. Rough siding, stucco and masonry soak up more paint, so coverage may drop to 200 to 300 sq ft per gallon.
What Changes Paint Coverage
| Factor | Effect |
|---|---|
| Bare drywall or wood | Absorbs more; prime first |
| Dark to light color change | May need an extra coat or tinted primer |
| Textured walls | Lower coverage per gallon |
| Spraying vs rolling | Spraying usually uses more paint |
| Paint quality | Better paint often hides in fewer coats |
Common Paint Mistakes
- Not subtracting doors and windows on big jobs, or subtracting too much on small ones.
- Forgetting the second coat.
- Using can coverage for rough surfaces.
- Skipping primer on new drywall.
- Mixing batches mid-wall; box them together first. Painting contractors can speed this up with painting estimating software.
Frequently Asked Questions
How much paint do I need for a 12×12 room?
A 12×12 room with 8 ft walls has about 384 sq ft of wall. After a door and two windows, about 330 sq ft remains. Two coats need about 1.9 gallons, so buy 2 gallons.
How many square feet does a gallon of paint cover?
Most interior paints cover about 350 to 400 square feet per gallon per coat on smooth, primed walls. Rough or porous surfaces cover less.
How many coats of paint do I need?
Two coats is standard for a new color or fresh drywall. One coat can work when repainting the same color over a sound surface.
How much primer do I need?
Primer often covers about 250 to 300 square feet per gallon. New drywall, stains and big color changes usually need one coat of primer.
Should I buy extra paint?
Yes. Keep a little extra, often a quart, for touch-ups and to avoid color differences between batches.
Painting Contractor? Bid Faster
Mega Estimating measures walls, ceilings, trim and openings from your plans and prices paint, primer and labor so you can send accurate painting bids fast. Mega Estimating is a U.S. construction estimating company that prepares quantity takeoffs and detailed cost estimates for residential, commercial and industrial contractors.
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