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The substitution of fossil energy carriers by renewable hydrocarbons is a promising measure to reduce net CO2 emissions. The production of renewable hydrocarbons requires hydrogen and carbon. Although hydrogen is easily accessible, the finding of optimal carbon sources requires an understanding of natural and anthropogenic carbon flows. However, there is no overview on the entire carbon flows in Switzerland yet. Although the potential of carbon from biomass is well known, it is unknown whether CO2 is the limiting factor for the production of renewable synthetic fuels. In a comprehensive analysis, 57 carbon paths within Switzerland and across its borders are identified. The carbon flows are quantified for 2013 and illustrated in a Sankey diagram. The diagram is universal and may be applied to other countries in this form. From this analysis, optimal carbon sources for renewable fuels are found. The question is discussed of whether CO2 for synthetic renewable fuels may also originate from nonbiogenic sources. Optimal carbon sources are either biomass or CO2 from incineration plants or cement‐manufacturing plants. If all accessible CO2 from these plants was used for the production of CH4, 43 % of Switzerland's road transportation could be powered with only a very low net CO2 emission if the electric power originates from renewable sources. The limiting factor for the production of synthetic fuel is renewable electrical energy rather than CO2. We propose that the origin of CO2 does not affect the sustainability of the synthetic fuel but the origin of the electric power does.