Turbine blade and set of turbine blades

A turbine blade comprising an airfoil (11) and a fir tree root (12). The fir tree root has a lengthwise direction (I), a crosswise direction extending between two lateral sides of the fir tree root, and a span direction (s) extending from a root base (13) towards an airfoil tip. The fir tree root comprises at least one longitudinal groove arranged on each lateral side and extending along and defining the lengthwise direction. The fir tree root has a root thickness measured between the two lateral sides, said width varying along the span direction. The fir tree root comprises at least two channels (31, 32, 33) extending in the span direction, each of said channels having a channel length (I1, I2, I3) measured along the lengthwise direction, the root further comprises a web (34, 35) interposed between each pair of neighboring channels, each of said at least one webs having a web length (I4, I5) measured along the lengthwise direction. For each web, a web-to-channel ratio with each of the two neighboring channels is chosen to be larger than or equal to 0.5 and is smaller than or equal to 0.85 at least at a position where the root width is a minimum load bearing root width in a load bearing section (26) of the fir tree root. In another aspect of the disclosure an overall web-to-channel ratio defined as a ratio between the sum of all web lengths (I4, I5) and the sum of all channel lengths (I1, I2, I3) is larger than or equal to 0.3 and is smaller than or equal to 0.6 at least at a position where the root width is a minimum load bearing root width in a load bearing section (26) of the fir tree root.