Mathf.Log

public static float Log(float f, float p);

Description

Returns the logarithm of a specified number in a specified base.

using UnityEngine;

public class ScriptExample : MonoBehaviour
{
    void Start()
    {
        // logarithm of 6 in base 2
        // prints 2.584963
        Debug.Log(Mathf.Log(6, 2));
    }
}