The surface tension of pure liquid water in contact with its vapor has been given by IAPWS as
where both  and the critical temperature  = 647.096 K are expressed in kelvins. The region of validity the entire vapor–liquid saturation curve, from the triple point (0.01 °C) to the critical point. It also provides reasonable results when extrapolated to metastable (supercooled) conditions, down to at least −25 °C. This formulation was originally adopted by IAPWS in 1976 and was adjusted in 1994 to conform to the International Temperature Scale of 1990.
The uncertainty of this formulation is given over the full range of temperature by IAPWS.  For temperatures below 100 °C, the uncertainty is ±0.5%.