Size 4 fits pelvic circum. 45.5 -51.25 (take loose measurements at pelvis)* The SacroLoc back support provides the correct support for sacroiliac joint syndrome (SI joint syndrome) loosening of the symphysis or other pain or complaints of the locomotor system in the pelvic region * It stabilizes and relieves pressure on the pelvis and symphysis and with its two-part cushion (pad) massages the sacroiliac joints * This stimulates circulation thus helping the healing process * Stabilizes and relieves pressure * Specifically targets the sacroiliac joints * Can be individually adjusted with tensioning straps