what is the ratio of chest compressions to ventilations when performing cpr for an adult?

The adult CPR compression-to-ventilation ratio is 30:2 : give 30 chest compressions, then 2 rescue breaths.

Adult CPR ratio

For an adult without an advanced airway, the standard ratio is 30 compressions to 2 ventilations.

Quick note

If you have an advanced airway in place, compressions are usually continuous while breaths are given separately at about 1 breath every 6 seconds.