A critical review and rethinking of hydrogen jet flame research is carried out. Froude number only based correlations are shown to be deficient for under-expanded jet fires. The novel dimensionless flame length correlation is developed accounting for effects of Froude, Reynolds, and Mach numbers. The correlation is validated for pressures 0.1–90.0 MPa, temperatures 80–300 K, and leak diameters 0.4–51.7 mm. Three distinct jet flame regimes are identified: traditional buoyancy-controlled, momentum-dominated “plateau” for expanded jets, and momentum-dominated “slope” for under-expanded jets. The statement “calculated flame length may be obtained by substitution the concentration corresponding to the stoichiometric mixture in equation of axial concentration decay for non-reacting jet” is shown to be incorrect. The correct average value for non-premixed turbulent flames is 11% by volume of hydrogen in air (range 8%–16%) not stoichiometric 29.5%. All three conservative separation distances for jet fire are shown to be longer than separation distance for non-reacting jet.