diff --git a/HandmadeMath.h b/HandmadeMath.h
index 912b2ad..7f54423 100644
--- a/HandmadeMath.h
+++ b/HandmadeMath.h
@@ -223,6 +223,7 @@ HMMDEF float HMM_ToRadians(float Degrees);
 HMMDEF float HMM_Inner(hmm_vec3 A, hmm_vec3 B);
 HMMDEF float HMM_SquareRoot(float Float);
 HMMDEF float HMM_LengthSquareRoot(hmm_vec3 A);
+HMMDEF float HMM_FastInverseSquareRoot(float Number);
 HMMDEF float HMM_Length(hmm_vec3 A);    
 HMMDEF float HMM_Power(float Base, int Exponent);
 HMMDEF float HMM_Clamp(float Min, float Value, float Max);
@@ -351,6 +352,24 @@ HMM_LengthSquareRoot(hmm_vec3 A)
     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_Length(hmm_vec3 A)
 {