CGST Rules – Central Goods and Services Tax Rules, 2017

Rule 31 – Residual method for determination of value of supply of goods or services or both

Where the value of supply of goods or services or both cannot be determined
under rules 27 to 30, the same shall be determined using reasonable means
consistent with the principles and the general provisions of section
15 and
the provisions of this Chapter:

Provided that in the case of supply of services, the supplier may opt for
this rule, ignoring rule 30.