We present a carbon budget for the Arctic Ocean that is based on estimates of water mass transformation and transport. The budget is constrained by conservation of mass and salt. In the model, the Eurasian and Canadian basins have been divided into five and six boxes, respectively, based on the prevailing water masses. In addition, there are three boxes representing the shelf areas. Total dissolved inorganic (CT) and organic (TOC) carbon concentrations for the different water masses and different regions are used, together with the volume flows, to calculate the carbon transports. The carbon budget calculation shows that at present the oceanic transport into the Arctic Ocean is larger than out, that is, 3.296±0.008 Gt C yr−1 of CT transported in and 3.287±0.004 Gt C yr−1 transported out with the corresponding values for TOC being 0.134±0.009 Gt C yr−1 and 0.122±0.006 Gt C yr−1, respectively. However, the outflowing waters are older than the inflowing waters and had thus been exposed to an atmosphere with lower concentration of anthropogenic carbon dioxide when entering the Arctic Ocean. When recalculating the budget to the preindustrial scenario, assuming steady state, the CT transport changes to 3.243 Gt C yr−1 in and 3.266 Gt C yr−1 out. To balance the preindustrial transports, assuming no change in the TOC fluxes, a direct input of atmospheric carbon dioxide of 0.011±0.014 Gt C yr−1 is required. Added to this is the burial of organic matter which is calculated as 0.013±0.010 Gt C yr−1 using a recycling efficiency of 80 % [Hulth, 1995] and a new production of 0.05 Gt C yr−1 [Anderson et al., 1994]. An indirect contribution of atmospheric carbon dioxide via runoff adds 0.017±0.004 Gt C yr−1, resulting in a preindustrial total atmospheric input of 0.041 ±0.018 Gt C yr−1.