Two of the US employer's costs are simple matches of what the employee already pays: Social Security and Medicare. If you know an employee's FICA withholding, you already know most of the employer's matching FICA cost too, with one exception, the Additional Medicare Tax on high earners is employee-only, never matched.
FUTA and SUTA work differently, and this is where the US genuinely diverges from every other market on this site. FUTA is a single federal rate, small and predictable at 0.6% on a tiny $7,000 wage base. SUTA is the real variable, both the rate AND the wage base differ by state, sometimes dramatically, a state with a low new-employer rate but a high wage base can end up costing more than a state with the opposite combination.
This calculator deliberately doesn't pretend to know your specific state's SUTA figures, that would require maintaining 50 separate, frequently-revised tables. Instead it gives you a reasonable default and lets you override both the rate and the wage base directly, so the total is only as accurate as the numbers you put in for that one line.