diff --git a/src/ref_oct.c b/src/ref_oct.c
index 00663ba59..503e69de4 100644
--- a/src/ref_oct.c
+++ b/src/ref_oct.c
@@ -53,6 +53,80 @@ REF_FCN REF_STATUS ref_oct_free(REF_OCT ref_oct) {
   return REF_SUCCESS;
 }
 
+REF_FCN REF_STATUS ref_oct_child_bbox(REF_DBL *bbox, REF_INT child_index,
+                                      REF_DBL *box) {
+  switch (child_index) {
+    case 0:
+      box[0] = bbox[0];
+      box[1] = 0.5 * (bbox[0] + bbox[1]);
+      box[2] = bbox[2];
+      box[3] = 0.5 * (bbox[2] + bbox[3]);
+      box[4] = bbox[4];
+      box[5] = 0.5 * (bbox[4] + bbox[5]);
+      break;
+    case 1:
+      box[0] = 0.5 * (bbox[0] + bbox[1]);
+      box[1] = bbox[1];
+      box[2] = bbox[2];
+      box[3] = 0.5 * (bbox[2] + bbox[3]);
+      box[4] = bbox[4];
+      box[5] = 0.5 * (bbox[4] + bbox[5]);
+      break;
+    case 2:
+      box[0] = 0.5 * (bbox[0] + bbox[1]);
+      box[1] = bbox[1];
+      box[2] = 0.5 * (bbox[2] + bbox[3]);
+      box[3] = bbox[3];
+      box[4] = bbox[4];
+      box[5] = 0.5 * (bbox[4] + bbox[5]);
+      break;
+    case 3:
+      box[0] = bbox[0];
+      box[1] = 0.5 * (bbox[0] + bbox[1]);
+      box[2] = 0.5 * (bbox[2] + bbox[3]);
+      box[3] = bbox[3];
+      box[4] = bbox[4];
+      box[5] = 0.5 * (bbox[4] + bbox[5]);
+      break;
+    case 4:
+      box[0] = bbox[0];
+      box[1] = 0.5 * (bbox[0] + bbox[1]);
+      box[2] = bbox[2];
+      box[3] = 0.5 * (bbox[2] + bbox[3]);
+      box[4] = 0.5 * (bbox[4] + bbox[5]);
+      box[5] = bbox[5];
+      break;
+    case 5:
+      box[0] = 0.5 * (bbox[0] + bbox[1]);
+      box[1] = bbox[1];
+      box[2] = bbox[2];
+      box[3] = 0.5 * (bbox[2] + bbox[3]);
+      box[4] = 0.5 * (bbox[4] + bbox[5]);
+      box[5] = bbox[5];
+      break;
+    case 6:
+      box[0] = 0.5 * (bbox[0] + bbox[1]);
+      box[1] = bbox[1];
+      box[2] = 0.5 * (bbox[2] + bbox[3]);
+      box[3] = bbox[3];
+      box[4] = 0.5 * (bbox[4] + bbox[5]);
+      box[5] = bbox[5];
+      break;
+    case 7:
+      box[0] = bbox[0];
+      box[1] = 0.5 * (bbox[0] + bbox[1]);
+      box[2] = 0.5 * (bbox[2] + bbox[3]);
+      box[3] = bbox[3];
+      box[4] = 0.5 * (bbox[4] + bbox[5]);
+      box[5] = bbox[5];
+      break;
+    default:
+      THROW("not 2^3");
+  }
+
+  return REF_SUCCESS;
+}
+
 REF_FCN REF_STATUS ref_oct_split(REF_OCT ref_oct, REF_INT node) {
   REF_INT i;
   for (i = 0; i < 8; i++) {
diff --git a/src/ref_oct.h b/src/ref_oct.h
index f07345ace..665451c56 100644
--- a/src/ref_oct.h
+++ b/src/ref_oct.h
@@ -38,6 +38,9 @@ REF_FCN REF_STATUS ref_oct_create(REF_OCT *ref_oct);
 
 REF_FCN REF_STATUS ref_oct_free(REF_OCT ref_oct);
 
+REF_FCN REF_STATUS ref_oct_child_bbox(REF_DBL *parent_bbox, REF_INT child_index,
+                                      REF_DBL *child_bbox);
+
 REF_FCN REF_STATUS ref_oct_split(REF_OCT ref_oct, REF_INT node);
 
 REF_FCN REF_STATUS ref_oct_contains(REF_OCT ref_oct, REF_DBL *xyz,
diff --git a/src/ref_oct_test.c b/src/ref_oct_test.c
index 60d18bb9e..355643500 100644
--- a/src/ref_oct_test.c
+++ b/src/ref_oct_test.c
@@ -58,6 +58,20 @@ int main(int argc, char *argv[]) {
     RSS(ref_oct_free(ref_oct), "free oct");
   }
 
+  { /* child's bbox */
+    REF_DBL parent_bbox[6], child_bbox[6];
+    REF_INT child_index = 0;
+    REF_DBL tol = -1.0;
+    parent_bbox[0] = 0.0;
+    parent_bbox[1] = 1.0;
+    parent_bbox[2] = 0.0;
+    parent_bbox[3] = 1.0;
+    parent_bbox[4] = 0.0;
+    parent_bbox[5] = 1.0;
+    RSS(ref_oct_child_bbox(parent_bbox, child_index, child_bbox), "bbox");
+    RWDS(0.0, child_bbox[0], tol, "not zero");
+  }
+
   { /* split root */
     REF_OCT ref_oct;
     REF_DBL xyz[] = {0.1, 0.1, 0.1};