If you’re serious about shopping for furniture, don’t set foot into a showroom without first spending some quality time with your tape measure. While you can impulse buy things like the occasional accent table, larger purchases require careful forethought. Here’s how to measure a room for different kinds of furniture so you can get it right the first time.

Measuring the Space

In addition to measuring the length and width of a room, don’t skip wall height. It will come into play for taller pieces like bookshelves and armoires. Measure openings for doorways and windows, as well as radiators, fireplaces and other features. If you don’t mind getting crafty, draw a blueprint and play with furniture cutouts in different shapes and sizes. It can help you get a better sense of scale so the room will be cohesive.

Sizing for a Sofa

In this case, you might want to work backward with some of the standard sizes for sofas. A 72-inch sofa is considered “apartment-size” and can fit two people, but you won’t want to nap on it. If your room is big enough, you can find choices that go beyond 100 inches. And depth is crucial. Some sofas are just 18 inches deep, and are more for perching than relaxing. If you want something you can sink into, go for 33 inches.

Choosing a Dining Table

More homeowners are making their dining rooms multipurpose, to accommodate homework, office space, projects, and more. Reflect on what you need in a table that will be truly versatile. In general, measure the length and width of the area, and subtract at least six feet from those numbers to allow for chairs. If you prefer something smaller, make sure it doesn’t get dwarfed by the size of the room.

Choosing Dining Chairs

If your dining table doesn’t come as a set, keep your tape measure handy in furniture stores. There should be about 10 inches between the seat and the underside of the table, and they need to slide easily under. Allow at least 24 inches between the center of one chair and the next. Don’t cram in too many chairs, especially if you want something comfortably large. Keep at least six inches between chairs.

Check the Entryways

Even once you’ve properly learned how to measure a room for different kinds of furniture, it will all come down to one question: Can you get it in the room? Pay attention to tight corners the furniture will have to pass—especially on the stairs—and the measurements inside the door frame: height, width, and diagonal. Trying to squeeze something in could destroy the furniture and the walls.