The second largest city of Ukraine, Kharkiv, stands at the confluence of the Udy with the river Lopan.

[1] The source of the river begins with springs in one of the beams Central Russian Upland near the village Bessonovka Belgorodsky District Belgorod Oblast (Russia) at an altitude of 190 m above sea level.

The river crosses the border with Russia to the northeast of the village of Okip in Bohodukhiv Raion Kharkiv Oblast.

From the source to the confluence of the river Lopan flows from north to south, and below - in a south-easterly direction.

The total height difference (from the source to the mouth is 105 m, the average river slopeis 0.64 m/km.