A new index was proposed for the carbonate precipitation rate observed for quickly growing travertines in hydrothermal settings, where degassing of carbon dioxide from high-pCO_2 water is the principal process that induces CaCO_3 precipitation. The travertine precipitation rate hardly fits to previous surface-controlled equations that often inverse with pCO_2 . Mathematical model in this study relates the precipitation rate with the CO_2 degassing rate by assuming a stable degree of supersaturation（Ω）. Here, the accelerating force by CO_2 degassing and the inhibiting force by carbonate precipitation are conceptually balanced to sustain a site-specific Ω value. The precipitation rate equals the product of the newly proposed index for degassing-induced carbonate precipitation（IDP） and an undefined hydrodynamic factor. The IDP, which can be calculated from the bulk water chemistry, is proportional to the pCO_2 excess of water from atmosphere, with secondary influence from pH and ionic strength.