When no interest is specified in the instrument, interest on the amount due thereon, shall be calculated at the rate of__________________ as per the NI Act. 

Under Section 80 of NI Act- When no rate of interest is specified in the instrument, interest on the amount due thereon shall, notwithstanding any agreement relating to interest between any parties to the instrument], be calculated at the rate of eighteen per centum per annum, from the date at which the same ought to have been paid by the party charged, until tender or realization of the amount due thereon, or until such date after the institution of a suit to recover such amount as the Court directs.