diff --git a/kandinsky/include/kandinsky/rect.h b/kandinsky/include/kandinsky/rect.h
index b879a98f9..e5573aff8 100644
--- a/kandinsky/include/kandinsky/rect.h
+++ b/kandinsky/include/kandinsky/rect.h
@@ -27,6 +27,9 @@ extern KDRect KDRectZero;
 bool KDRectIntersect(KDRect r1, KDRect r2);
 KDRect KDRectIntersection(KDRect r1, KDRect r2);
 
+// Returns the smallest rectangle containing r1 and r2
+KDRect KDRectUnion(KDRect r1, KDRect r2);
+
 bool KDRectContains(KDRect r, KDPoint p);
 KDRect KDRectTranslate(KDRect r, KDPoint p);
 
diff --git a/kandinsky/src/rect.c b/kandinsky/src/rect.c
index d313c2b3a..b0d803a23 100644
--- a/kandinsky/src/rect.c
+++ b/kandinsky/src/rect.c
@@ -42,6 +42,54 @@ KDRect KDRectIntersection(KDRect r1, KDRect r2) {
     return intersection;
 }
 
+static void KDRectComputeUnionBound(KDCoordinate size1, KDCoordinate size2,
+    KDCoordinate * outputMin, KDCoordinate * outputMax,
+    KDCoordinate min1, KDCoordinate min2,
+    KDCoordinate max1, KDCoordinate max2)
+{
+  if (size1 != 0) {
+    if (size2 != 0) {
+      *outputMin = min(min1, min2);
+      *outputMax = max(max1, max2);
+    } else {
+      *outputMin = min1;
+      *outputMax = max1;
+    }
+  } else {
+    if (size2 != 0) {
+      *outputMin = min2;
+      *outputMax = max2;
+    }
+  }
+}
+
+KDRect KDRectUnion(KDRect r1, KDRect r2) {
+  /* We should ignore coordinate whose size is zero
+   * For example, if r1.height is zero, just ignore r1.y and r1.height. */
+
+  KDCoordinate resultLeft = 0;
+  KDCoordinate resultTop = 0;
+  KDCoordinate resultRight = 0;
+  KDCoordinate resultBottom = 0;
+
+  KDRectComputeUnionBound(r1.width, r2.width,
+      &resultLeft, &resultRight,
+      left(r1), left(r2),
+      right(r1), right(r2));
+
+  KDRectComputeUnionBound(r1.height, r2.height,
+      &resultTop, &resultBottom,
+      top(r1), top(r2),
+      bottom(r1), bottom(r2));
+
+  return (KDRect){
+    .x = resultLeft,
+    .y = resultTop,
+    .width = resultRight-resultLeft,
+    .height = resultBottom-resultTop
+  };
+}
+
 bool KDRectContains(KDRect r, KDPoint p) {
   return (p.x >= r.x && p.x < (r.x+r.width) && p.y >= r.y && p.y < (r.y+r.height));
 }
diff --git a/kandinsky/test/rect.c b/kandinsky/test/rect.c
index ce4ad9c17..c46f69a0e 100644
--- a/kandinsky/test/rect.c
+++ b/kandinsky/test/rect.c
@@ -24,3 +24,59 @@ QUIZ_CASE(kandinsky_rect_intersect) {
   assert(c.width == 5);
   assert(c.height == 5);
 }
+
+QUIZ_CASE(kandinsky_rect_union) {
+  KDRect a = {
+    .x = -5, .y = -5,
+    .width = 10, .height = 10
+  };
+  KDRect b = {
+    .x = 0, .y = 0,
+    .width = 10, .height = 10
+  };
+
+  KDRect c = KDRectUnion(a,b);
+  assert(c.x == -5);
+  assert(c.y == -5);
+  assert(c.width == 15);
+  assert(c.height == 15);
+
+  c = KDRectUnion(a,b);
+  assert(c.x == -5);
+  assert(c.y == -5);
+  assert(c.width == 15);
+  assert(c.height == 15);
+}
+
+QUIZ_CASE(kandinsky_rect_empty_union) {
+  KDRect a = {
+    .x = 1, .y = 2,
+    .width = 3, .height = 4
+  };
+  KDRect b = {
+    .x = 5, .y = 6,
+    .width = 0, .height = 0
+  };
+  KDRect c = {
+    .x = -2, .y = -1,
+    .width = 0, .height = 1
+  };
+
+  KDRect t = KDRectUnion(a,b);
+  assert(t.x == a.x);
+  assert(t.y == a.y);
+  assert(t.width == a.width);
+  assert(t.height == a.height);
+
+  t = KDRectUnion(b,a);
+  assert(t.x == a.x);
+  assert(t.y == a.y);
+  assert(t.width == a.width);
+  assert(t.height == a.height);
+
+  t = KDRectUnion(a,c);
+  assert(t.x == a.x);
+  assert(t.y == c.y);
+  assert(t.width == a.width);
+  assert(t.height == 7);
+}