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Measure the length and width of your mattress. Calculate the drop length. Determine the comforter's width. Calculate the comforter's length.

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Measure from the point where the mattress touches the box spring to the top of the mattress. Add 3 inches (7.6 cm) to that number. A 12-inch (30.5-cm) deep mattress will have a drop length of 15 inches (31 cm). Multiply the drop length by 2. A drop length of 15 inches (31 cm) gives you 30 inches (76. 2 cm). Add the sum to the width of the mattress. A 54-inch (137-cm) wide mattress results in 84 inches (213.4 cm). Multiply the drop length by 2. A drop length of 15 inches (31 cm) gives you 30 inches (76.2 cm). Add the sum to the mattress length. If your mattress is 75 inches (109.5 cm) long, your total is 105 inches (266.7 cm).