In Chapter 15 of the Fate/Grand Order Turas Realta manga, Fafnir unleashes an attack that Romani claims will blow away "the whole Orleans region". We can presume that this refers to the entire Orleans singularity as dragons in Fate have been stated capable of razing countries with their breath, and this attack from Fafnir was considered to be a "top ranking attack".
As shown on the map of the singularity, the "Orleans region" covers the distance between Paris and Marseille, which according to this website is about 660 km.
Diameter = 660 km
Radius = 330 km = 330,000 m
W = R^3 * ((27136*P+8649)^(1/2)/13568-93/13568)^2
W = (330000)^3 * ((27136*1.37895+8649)^(1/2)/13568-93/13568)^2
W = 2.8882005e+12 tons of TNT = Low 6-B, Small Country level