The top quark mass is one of the most important parameters for the Standard Model and has been measured at the LHC at a sub percent precision. The most precise kinematic extractions of the top mass rely on Monte Carlo event generators, and identifying these measurements with a Lagrangian top-mass scheme induces an additional ambiguity that is of the same order as the experimental precision. In this talk I describe a factorization approach to remove this uncertainty by measuring top jet mass of a boosted top sample with jet grooming, an observable that has a high kinematic sensitivity to the top mass and at the same time allows for hadron level predictions from QCD.