1. Dining Room Table

A 36-inch-wide rectangular table is perfect for conversation. A round table with a diameter of 48 inches seats six; a 60-inch round will handle eight standard dining chairs or 10 ballroom chairs.
—Charlotte Moss

2. Light Fixtures

How big should an overhead light fixture be? Just add the length and width of the room in feet, and whatever number you come up with is, in inches, your guide for the fixture’s diameter. So a 15-by-20-foot room would need a 35-inch-wide chandelier.
—Bunny Williams

3. Paint Coverage

One gallon of paint will cover about 400 square feet of wall.
—Alexa Hampton

4. Curtain Height

Mount curtains as high as possible to give the room more height, and let them break 1½ inches on the floor.
—Miles Redd

5. Kitchen Island

A kitchen island should be about 38 inches high—a little taller than the countertops—to be comfortable for prep.
—Thomas O’Brien

6. Window Treatment Width

Curtains should be 2½ to 3 times the width of the window. So if you’re doing two panels, each should be 1¼ to 1½ times the window width. Buy a rod that’s 20 inches wider than your window so it extends 10 inches on either side. Your window will seem much wider than it really is.
—Libby Langdon

7. Dining-Room Chandelier

The bottom of a dining-room chandelier should hang 36 inches above the table.
—Thom Filicia

8. Light Switches

Install light switches 36 inches above the floor and 1½ to 2 inches to the side of the door trim.
—Gil Schafer

9. Fabric for a Sofa

For a standard 84-inch sofa with exposed legs and a tight back, you’ll need 14 yards of plain 54-inch-wide fabric. Add two yards for a skirt.
—Madeline Stuart

10. The Golden Ratio

From classical times to today, the golden ratio has always been the perfect proportion: 1 to 1.62.
—Eric Cohler