Got the math? Now show your customer what it will actually look like. Paintviz generates realistic before-and-after mockups in real Sherwin-Williams and Benjamin Moore colors. 14-day free trial.
How to use this calculator
Three steps. First, pick the mode that matches the job: a single accent wall, a full room, the outside of the house, or a whole-house interior with multiple rooms. Second, punch in the dimensions off your tape measure, plus doors and windows so they're subtracted at standard sizes. Third, read the results: paintable square footage, gallons for your coat count, what to actually buy, and what it'll cost at your price per gallon. Adjust the coverage number down if your surface is textured or thirsty.
How paint coverage works
The math under the hood is simple: surface area, times coats, divided by coverage per gallon.
Area is length times height for each wall, minus 21 sq ft per standard door and 15 per window. A gallon of quality paint covers about 350 sq ft on smooth, previously painted drywall, so a 400 sq ft room at two coats is 800 sq ft of work, and 800 divided by 350 is 2.3 gallons: you buy 2 gallons and a quart, or round to 3 if you like margin. Every fancy estimating tool a contractor carries is doing this same arithmetic with better defaults.
Paint coverage rates by surface
| Surface | Realistic coverage (sq ft/gal) |
|---|---|
| Smooth drywall, previously painted | 350-400 |
| Textured drywall (orange peel, knockdown) | 250-300 |
| Bare / new drywall (first coat) | 250-300 |
| Wood or fiber cement siding | 300-350 |
| Stucco | 150-250 |
| Brick and masonry | 200-300 |
| Trim and doors (brushed) | 350-400 |
Porous and textured surfaces sit at the low end of their range on the first coat and recover toward the high end on the second, once the surface is sealed.
How many coats do you need?
Two coats is the professional default, and it's what paint warranties and color accuracy assume. One coat gets by only when you're repainting a sound surface in the same or a very close color with a premium line. Three coats enters the picture on drastic changes (bright red over white, or white over navy), where the smarter move is usually one coat of tinted primer followed by two of finish. Deep, saturated colors like reds and yellows have weaker hiding pigments and often want the primer route regardless of what's underneath.
When to add 10-15% extra
Add margin when the wall has heavy texture, when you're spraying (overspray is real), when the surface is patched or chalky, when the color change is dramatic, and on exteriors with gables, deep lap siding, or lots of trim. Painters carry this margin in every bid because a second trip to the paint store costs more than the extra quart, and touch-up paint from the same batch is worth having in the garage for years.
Should you hire a pro instead?
The calculator tells you the materials. It doesn't climb the ladder, fix the drywall, cut a straight line at the ceiling, or warranty the result. A room is a reasonable weekend for a homeowner with patience; a two-story exterior, a kitchen full of cabinets, or anything involving scaffolding is where a painting contractor earns the bid, and materials are typically only 15-20% of a professional job's price, so the paint math above is the small half of the decision.
If you're getting bids, here's the ace question to ask: can the painter show you what the color will look like on your actual walls before you commit? Painters who use a visualizer settle color decisions at the estimate instead of leaving you guessing from a chip. And if you're the painter reading this: that's the job Paintviz does, with the real Sherwin-Williams, Benjamin Moore, and Behr palettes, on interiors and exteriors alike.
This calculator is the first of a growing set: the free tools library is where the rest land.