Nanocoils: The smallest mattress coils so far It used to be that the shortest mattress coils were the 4" Bonnell coils in a sofabed mattress. The reason they were so short, when most coils were 7 or 8 inches high, was so the mattress could be folded to fit inside the sofa under the seat cushions. With the introduction of pocket coils, the trend was for innerspring coils, at least the wrapped coils, to become taller, up to 9 or 10 inches. This allowed greater conformabiity and more...