Requires a+b ≠ 0.
For two positive numbers, H = 2ab / (a + b). If a + b = 0 the expression is undefined (guarded numerically). This is the pairwise harmonic mean; extensions to more values are different formulas.
This calculator uses plain decimal numbers. If a result looks wrong, check whether you used a comma instead of a dot (or vice versa) where the field expects one.