Two trips to the paint store mid-project is the most common avoidable mistake on a DIY repaint. Buying way too much paint is the second. This post is the math that fixes both — coverage rates by paint tier, the formula for any wall area, and worked examples for the three typical Massachusetts home sizes.

The basic coverage formula

Premium interior paint (Benjamin Moore Aura, Sherwin-Williams Emerald) covers about 350 sqft per gallon on a properly prepped, primed surface. Mid-tier paint (BM Regal Select, SW Cashmere) covers similarly. Builder-grade paint covers closer to 270 sqft per gallon because the formulation is thinner — you need more of it for the same hide.

Exterior paint covers less — roughly 300 sqft per gallon on smooth substrate, dropping to 200 sqft per gallon on rough cedar shingle or textured stucco. Two coats are standard for both interior and exterior. So plan the math as:

Gallons needed = (wall sqft × number of coats) ÷ coverage rate.

Then add 10% buffer for cut-ins, touch-ups, and the unavoidable bit you spill. Once the total passes about 4.5 gallons, a 5-gallon pail usually makes more sense than single cans: it typically costs less per gallon than the same paint in 1-gallon cans, and all of the paint comes from one batch, so there's no slight shade difference between cans.

Always check the coverage figure on the product's own label or technical data sheet — the numbers above are typical, and some lines are rated higher or lower.

Calculating wall area for a room or house

For a room: (2 × length + 2 × width) × ceiling height. Subtract 21 sqft per door and 15 sqft per window. Don't deduct outlets, switches, or trim — they round out in the wash.

For a whole house exterior: measure the perimeter at ground level, multiply by average wall height to the eaves, and subtract major windows + doors. Or use our paint coverage calculator — drop in dimensions, get the answer.

Worked examples by Massachusetts home size

1,500 sqft 1-bedroom condo or starter home (walls only)

Roughly 600 sqft of wall surface (8-ft ceilings, average rooms). Two coats premium paint: 3.4 gallons. Buy 4 × 1-gallon cans, save the leftover quart for touch-ups.

2,500 sqft 3-bedroom standard colonial (walls + ceilings)

Walls: ~1,000 sqft. Ceilings: ~1,000 sqft (separate flat ceiling paint). Two coats walls = 5.7 gallons. Two coats ceiling = 5.7 gallons. Total: 11.4 gallons. Buy a 5-gal pail of wall color + 5-gal pail of ceiling flat + 1-gal extra.

3,500 sqft 5-bedroom colonial (whole-home with trim)

Walls: ~1,500 sqft. Ceilings: ~1,500 sqft. Trim: ~400 linear ft × 0.5 sqft = 200 sqft of trim surface. Two coats everything: 9 gallons wall, 9 gallons ceiling, 1.5 gallons trim (Benjamin Moore Advance for trim). Total: ~20 gallons. Two 5-gal pails wall + two 5-gal pails ceiling + 1-gallon trim.

2,500 sqft home exterior (vinyl siding)

~2,800 sqft of paintable wall surface (2-story, 1,500 sqft per floor minus windows + doors). Two coats Sherwin-Williams Duration: 18.7 gallons. Buy three 5-gal pails + 1 extra gallon.

Where DIY paint estimates go wrong

  • Forgetting the second coat. Single-coat coverage is a myth on color change or any wall over 5 years old. Plan for 2.
  • Ignoring vaulted ceilings. A 10-ft or vaulted ceiling adds 25–40% more wall area than the 8-ft standard.
  • Underestimating cut-ins. Detail work eats more paint than open rolling. The 10% buffer above covers this.
  • Buying gallons instead of pails over 5 gallons total. Pails are usually cheaper per gallon than the same paint bought one can at a time, and one batch means one consistent color.
  • Not accounting for the primer coat. If you're going dark→light or covering stains, primer is a third coat. Roughly half the paint volume since primer is rolled lighter.

When to call a pro instead

If your math says 12+ gallons and the home is occupied, it's worth getting a professional estimate before you buy anything. A job that size also means protecting floors and furniture, patching and sanding, and somewhere around 40–80 hours of your weekends. Our guide to what drives painting cost in Massachusetts walks through what a professional quote should cover. If you'd like a written estimate for your own house, AJ Painting offers a free on-site visit — call (508) 258-4325 or request one online.