How to calculate the content-length of Form which is available in jersey api to send multipart/formdata?

Below is the code where i am framing the formData.

                  Form formData = new Form();

              formData.add("filename",f);  ----------> File Object
              formData.add("options",jobj.toString()); ----------> JSON String

How can i calculate the content length of the above formData to send it in header of my request.