The calculator above does this math automatically, but here's the method behind it — useful if you want to sanity-check a number or figure it out by hand.
Measure your room's perimeter and ceiling height
Add up the length of all four walls (or however many walls the room has) to get the perimeter, then measure from floor to ceiling for the height. For a 12×15 ft room with a 9 ft ceiling, the perimeter is 2×(12+15) = 54 ft and the wall area is 54×9 = 486 sqft.
Divide the perimeter by your wallpaper's roll width
Wallpaper is hung in vertical strips called drops, each as wide as the roll. A 54 ft (648 in) perimeter with a 27 in wide roll needs 648 ÷ 27 = 24 drops to go all the way around the room.
Figure out how many drops fit on one roll
Divide the roll's length by the drop length (your ceiling height, rounded up to the nearest full pattern repeat — more on that below). A 33 ft roll cut into 9 ft drops yields 3 drops per roll, since a 4th drop would need 36 ft.
Divide total drops by drops per roll, then round up
24 drops ÷ 3 drops per roll = 8 rolls exactly. Since you can't buy a fraction of a roll, round up to the next whole roll if it doesn't divide evenly.
Add one extra roll as a buffer
Cutting mistakes, matching errors, and future repairs happen — one spare roll (9 total for this example) covers it without a second trip to the store.
Worked example: 12×15 ft room, 9 ft ceiling, 27 in × 33 ft roll
486 sqft of wall → 24 drops needed → 3 drops per roll → 8 rolls minimum → 9 rolls recommended with the buffer roll included.
