Whether you’re painting a powder room or the exterior of your house, the general rule of thumb is one gallon per 400 square feet. But that’s just a rough guideline: To get a more precise number, which you’ll definitely want for large projects, use a paint calculator like the ones provided by Benjamin Moore or Pratt & Lambert; they take into account window and door measurements. (And both assume two coats of paint per project.)

Planning on whitewashing a charcoal-gray wall? You’ll likely need additional paint when going from dark to light. On the other end of the spectrum, a deep color base tends to require more coats of paint than a lighter color, says Carolyn Noble, color marketing and design manager at Pratt & Lambert. She recommends applying a gray tinted primer to the surface before you paint your walls a saturated color to help reduce the number of applications. When it comes to finish, you may have heard the glossier it is, the higher the coverage rate, but it’s not enough of a difference to change the number of gallons you need to buy, says Minchew.

If you’re painting a highly textured surface rather than a smooth one, buy a little extra.Cabinets with complicated millwork require more paint, too; Suggests purchasing about 10 percent more than calculated.