diff --git a/src/System.Private.CoreLib/shared/System/Number.BigInteger.cs b/src/System.Private.CoreLib/shared/System/Number.BigInteger.cs
index 607b450686e1..07c4112da9b1 100644
--- a/src/System.Private.CoreLib/shared/System/Number.BigInteger.cs
+++ b/src/System.Private.CoreLib/shared/System/Number.BigInteger.cs
@@ -460,14 +460,7 @@ public static uint CountSignificantBits(uint value)
 
             public static uint CountSignificantBits(ulong value)
             {
-                uint upper = (uint)(value >> 32);
-
-                if (upper != 0)
-                {
-                    return 32 + CountSignificantBits(upper);
-                }
-
-                return CountSignificantBits((uint)value);
+                return 64 - (uint)BitOps.LeadingZeroCount(value);
             }
 
             public static uint CountSignificantBits(ref BigInteger value)