Fluoroantimonic acid is the strongest superacid based on the measured value of its Hammett acidity function (H), which has been determined for various ratios of HF:SbF. The H of HF is −15.  A solution of HF containing 1 mol % of SbF is −20. The H is −21 for 10 mol%.  For > 50 mol % SbF, the H  is between −21 and −23. The lowest attained H is about -28. The following H values show that fluoroantimonic acid is stronger than other superacids. Increased acidity is indicated by smaller (in this case, more negative) values of H.
* Fluoroantimonic acid (−23 > H > −28)
* Magic acid (H  = −23)
* Carborane acid (H  < −18)
* Fluorosulfuric acid (H  = −15)
* Triflic acid (H  = −15)
* Perchloric acid (H = −13)
Of the above, only the carborane acids, whose H could not be directly determined due to their high melting points, may be stronger acids than fluoroantimonic acid.
The H value measures the protonating ability of the bulk, liquid acid, and this value has been directly determined or estimated for various compositions of the mixture.  The pK on the other hand, measures the equilibrium of proton dissociation of a discrete chemical species when dissolved in a particular solvent. Since fluoroantimonic acid is not a single chemical species, its pK value is not well-defined.
The gas-phase acidity (GPA) of individual species present in the mixture have been calculated using density functional theory methods.  (Solution-phase pKs of these species can, in principle, be estimated by taking into account solvation energies, but do not appear to be reported in the literature as of 2019.)  For example, the ion-pair  [HF]· was estimated to have a GPA of 254 kcal/mol.  For comparison, the commonly encountered superacid triflic acid, TfOH, is a substantially weaker acid by this measure, with a GPA of 299 kcal/mol.  However, certain carborane superacids have GPAs lower than that of [HF]·.  For example, H(CHBCl) has an experimentally determined GPA of 241 kcal/mol.