AbstractThe rate coefficient K2 of the physical exchange of oxygen between flowing water and the atmosphere is dependent on water temperature and the hydraulic variables: mean velocity of flow U, mean depth H, and slope S of the stream channel.36 equations from the literature predicting K2 on basis of combinations of U, H and S have been tested against 144 corresponding measurements of K2 and hydraulic variables taken in 10 reaches in 6 Danish streams with different development of the submerged macrophyte community.A statistical analysis showed that all equations except one had to be rejected. To improve predictive accuracy an equation K2 (20 °C) = 8784 U0.734H−0.420 S0.930 was developed applying multiple linear regression. This equation allows K2‐predictions for streams where the hydraulic variables U range from 0.06 to 0.52 m s−1, H from 0.12 to 1.37 m and S from 0.3 to 7.4 × 10−3 m m−1, respectively.Using this equation the lower and upper 95 % limit of confidence for a given K2‐prediction (K2p) are K2p/1.95 and K2p × 1.95, indicating that such equations at best approximate the real K2. Therefore K2 should preferably be measured.