It is difficult to measure karst volume but important to do so to assess risk of sinkhole formation. Measurements of airflow and atmospheric pressure pattern on a water well in buried karst of the overexploited Tournai hydrogeological basin (Belgium) show that inflow and outflow are highly correlated to atmospheric pressure and the differences of external and internal air densities. The volume of natural voids in connection with the well within the vadose zone can best be estimated by a calculation based on outflow under constant atmospheric pressure, which suggests a possible cavity volume of 700 000 m3 indicating likelihood of sinkhole formation.