The pneumatic gain-in-weight filling system for free-flowing bulk solids conveys and weighs the material and discharges it into bulk bags. The filter receiver of the vacuum dilute-phases pneumatic conveying system is mounted on load cells, allowing the controller to weigh incoming material at high rates. Pressing a "fill" button on the control panel actuates a bag inflation blower to remove creases in the bag and opens a slide gate valve at the outlet of the filter receiver, allowing the weighed batch to gravity discharge into the bag at high rates. A port within the discharge spout vents displaced air and dust to a remote bag house. The controller then releases the bag straps and deflates the spout seal, allowing the operator to tie-off the spout and remove the filled bag. Connecting an empty bag consists of attaching the bag straps, pulling the spout over the inflatable collar, and pushing a spout inflator button. While filled bags are being removed and empty bags attached, a subsequent batch is being conveyed to the filter receiver and weighed, ready for another fill cycle.