Ezekiel 40:6-7

6Then he went over to the eastern gateway. He climbed the steps and measured the threshold of the gateway; it was 10-1/2 feet front to back.

7There were guard alcoves on each side built into the gateway passage. Each of these alcoves was 10-1/2 feet square, with a distance between them of 8-3/4 feet along the passage wall. The gateway’s inner threshold, which led to the entry room at the inner end of the gateway passage, was 10-1/2 feet front to back.