The loop officially begins and ends at I-10 east of San Antonio. It heads south from I-10 as a two-lane road and turns towards the southeast and intersects FM 1346, FM 1518, and FM 3465 while turning back towards the south and reaching a junction with US 87 in Lone Oak. Continuing to the south, Loop 1604 has a second intersection with FM 3465, an intersection with FM 3432 as the road turns towards the southwest and a junction with US 181 near Calaveras Lake. Heading southwest from US 181, the highway intersects FM 327 as it enters the city of Elmendorf. It intersects FM 327 once again as it leaves Elmendorf heading towards the southwest. At FM 1303, Loop 1604 turns towards the west before a junction with I-37.