Medicare tax is the smaller part of FICA. You pay 1.45% of every dollar of wages and your employer pays the same, according to the IRS. There is no wage base, so unlike Social Security it never stops during the year.
High earners pay an extra 0.9% on top. Your employer must start withholding it in the pay period in which your wages from that job pass $200,000 for the calendar year, whatever your filing status, and pays no matching share. On $260,000 of wages you pay $3,770 at the ordinary rate and $540 of the additional tax, $4,310 in all.
The withholding rule and the tax you finally owe do not always agree. On the return the threshold depends on filing status: $250,000 for a married couple filing jointly, $125,000 for married filing separately and $200,000 for everyone else, per IRS Topic 560. Two spouses each earning $150,000 have nothing withheld, yet owe 0.9% of the $50,000 by which their joint wages exceed $250,000, which is $450. One spouse earning $220,000 has $180 withheld but, if the other earns nothing, owes none of it and gets it back.
Like the rest of FICA, the tax is charged on wages from the first dollar, with no standard deduction, so it applies even when no federal income tax is due.
The take-home pay calculator charges 1.45% on all of your gross pay and adds the 0.9% above the threshold for the filing status you pick, which matches the return rather than the withholding rule. Your payslip may therefore show the extra tax starting at a different point from the result.