Cakra-yantra

, or circle instrument was an instrument used by Vedic astronomers.

It was one of Lalla's 12 instruments of astronomy.

Lalla described it as a 360° graduated circular board which had a central upright needle. The whole thing was tilted at a certain angle. It indicated nāḍis elapsed since sunrise, or nāḍis to-elapse before sunset. Subbarayappa and Sarma note problems with the calculation method described by Lalla. They also describe a criticism made by Brahmagupta on the matter of calculating nāḍis.

Half of a Cakra is a Dhanur-yantra, a half-circle. Modifications can be made resulting in instruments like the Kapāla-yantra and Pīṭha-yantra.