RaycastHit2D.fraction

Description

Fraction of the distance along the ray that the hit occurred.

If the ray's direction vector is normalized, then this value is the distance between the origin and the hit point. If the direction is not normalized then this distance is expressed as a "fraction" (which could be greater than 1) of the vector's magnitude.

Additional resources: point.

using UnityEngine;
using System.Collections;

public class ExampleClass : MonoBehaviour
{
    public Vector2 direction;

    void Update()
    {
        //Cast a ray in the direction specified in the inspector.
        RaycastHit2D hit = Physics2D.Raycast(this.gameObject.transform.position, direction, 10.0f);

        //If something was hit.
        if (hit.collider != null)
        {
            Debug.Log(hit.fraction);
        }
    }
}