In fact You can use BJH analysis to calculate incremental pore volume to the required pore width in the mesopore range.  

Moreover, for total pore volume  of mesopores, it is better to take total pore volume at p/po=0.95 to avoid any volume coming from  multilayer adsorption on open surfaces.

Dear Khalil, Thank you very much for your reply.

My concern is to differentiate material porosity from void porosity.

Is pore volume measured at p/po=0.95 represent exclusive material pore volume and not void volume ?

I had attached the isotherm. 

Dear Govind

There are two methods:

First,based on BJH analysis and it is a data evaluated directly from the electronic sorpometer if you use it

Secondly, if you manually measure the adsorption isotherm using the old volumetric system you can use alpha S method beside t-method 

From simple inspection of your isotherm it results that you have a bimodal distribution: some micropores and a lot of large mesopores. Some intravoid condensation may be present. Take total pore volume at p/po=0.95.

to know the exact mesopore size, you have carry out adsorption studies of different organic molecules having different kinetic diameter, by trail method. in normal temperature and atmospheric pressure. 

Dear Dr Govind Sethia

I am glad to send you the following recommendation, which is the best ever written in the subject. Please read it carefully, it will give you a very broad mind towards the subject. If you have seen this recommendation before, just skip. Thank you very much.

Kind Regards

Kamal

INTERNATIONAL UNION OF PURE AND APPLIED CHEMISTRY PHYSICAL CHEMISTRY DIVISION COMMISSION ON COLLOID AND SURFACE CHEMISTRY INCLUDING CATALYSIS.

REPORTING PHYSISORPTION DATA FOR GAS/SOLID SYSTEMS with Special Reference to the Determination of Surface Area and Porosity (Recommendations 1984)

I can recommend also the more recent Recommendations 1994, to be found in Pure Appl. Chem. 8 (1994) 1739.