diff --git a/src/core/BufferGeometry.js b/src/core/BufferGeometry.js
index a857141f6df248d9c76e067217a6bcd833c9cbc5..f3a726cd3d0a2c4048adf2bb5362af1c44ea6af6 100644
--- a/src/core/BufferGeometry.js
+++ b/src/core/BufferGeometry.js
@@ -473,7 +473,7 @@ THREE.BufferGeometry.prototype = {
 				var bb = this.boundingBox;
 				bb.makeEmpty();
 
-				for ( var i = 0, il = positions.length; i < il; i ++ ) {
+				for ( var i = 0, il = positions.length; i < il; i += 3 ) {
 
 					vector.fromArray( positions, i );
 					bb.expandByPoint( vector );
@@ -520,7 +520,7 @@ THREE.BufferGeometry.prototype = {
 
 				var center = this.boundingSphere.center;
 
-				for ( var i = 0, il = positions.length; i < il; i ++ ) {
+				for ( var i = 0, il = positions.length; i < il; i += 3 ) {
 
 					vector.fromArray( positions, i );
 					box.expandByPoint( vector );
@@ -534,7 +534,7 @@ THREE.BufferGeometry.prototype = {
 
 				var maxRadiusSq = 0;
 
-				for ( var i = 0, il = positions.length; i < il; i ++ ) {
+				for ( var i = 0, il = positions.length; i < il; i += 3 ) {
 
 					vector.fromArray( positions, i );
 					maxRadiusSq = Math.max( maxRadiusSq, center.distanceToSquared( vector ) );