A Brønsted-Lowry acids strength corresponds with its ability to release a hydrogen ion.  One common measure of acid strength for concentrated, superacidic liquid media is the Hammett acidity function, H.  Based on its ability to quantitatively protonate benzene, the chlorinated carborane acid  was conservatively estimated to have an H value at or below −18, leading to the common assertion that carborane acids are at least a million times stronger than 100% sulfuric acid (H = −12). However,  since the H value measures the protonating ability of a liquid medium, the crystalline and high-melting nature of these acids precludes direct measurement of this parameter. In terms of pK, a slightly different measure of acidity defined as the ability of a given solute to undergo ionization in a solvent, carborane acids are estimated to have pK values below −20, even without electron-withdrawing substituents on the boron atoms (e.g.,  is estimated to have a pK of −24), with the (yet unknown) fully fluorinated analog  having a calculated pK of −46. The known acid  with one fewer fluorine is expected to be only slightly weaker (pK' < −40).
In the gas phase,  has a computed acidity of 216 kcal/mol, compared to an experimentally determined acidity of 241 kcal/mol (in reasonable agreement with the computed value of 230 kcal/mol) for .  In contrast, HSbF (a simplified model for the proton donating species in fluoroantimonic acid) has a computed gas phase acidity of 255 kcal/mol, while the previous experimentally determined record holder was (CFSO)NH, a congener of bistriflimide, at 291 kcal/mol.  Thus,  is likely the most acidic substance so far synthesized in bulk, in terms of its gas phase acidity.  In view of its unique reactivity, it is also a strong contender for being the most acidic substance in the condensed phase (see above).  Some even more strongly acidic derivatives have been predicted, with gas phase acidities < 200 kcal/mol.
Carborane acids differ from classical superacids in being well-defined one component substances.  In contrast, classical superacids are often mixtures of a Brønsted acid and Lewis acid (e.g. HF/SbF).  Despite being the strongest acid, the boron-based carborane acids are described as being "gentle", cleanly protonating weakly basic substances without further side reactions. Whereas conventional superacids decompose fullerenes due to their strongly oxidizing Lewis acidic component, carborane acid has the ability to protonate fullerenes at room temperature to yield an isolable salt. Furthermore, the anion that forms as a result of proton transfer is nearly completely inert.  This property is what makes the carborane acids the only substances that are comparable in acidity to the mixed superacids that can also be stored in a glass bottle, as various fluoride-donating species (which attack glass) are not present or generated.