diff --git a/HandmadeMath.h b/HandmadeMath.h
index 8db5f05..ba435b7 100644
--- a/HandmadeMath.h
+++ b/HandmadeMath.h
@@ -343,6 +343,24 @@ HMM_SquareRoot(float Float)
     return(Result);
 }
 
+HINLINE float
+HMM_FastInverseSquareRoot(float Number)
+{
+	long i;
+	float x2, y;
+	const float threehalfs = 1.5f;
+
+	x2 = Number * 0.5f;
+	y  = Number;
+	i  = * ( long * ) &y;          // evil floating point bit level hacking
+	i  = 0x5f3759df - ( i >> 1 );  // what the fuck? 
+	y  = * ( float * ) &i;
+    
+    y  = y * ( threehalfs - ( x2 * y * y ) );
+
+	return ( y );
+}
+
 HINLINE float
 HMM_LengthSquareRoot(hmm_vec3 A)
 {