Measure 30 inches out from the house or railing to provide room to swing backward.
Measure directly upward from the porch floor to the support beams where you will be hanging the swing.
Subtract 18 inches from this measurement to find the length from the support to the bottom of the swing where the chains usually attach. For example, if your porch is 88 inches from floor to support, the distance from the support to the bottom of the swing will be 70 inches.
Measure the width of the swing and add an inch to either side. For example, if you have a swing 48 inches wide, the distance between the chains will be 50 inches.