Interest coverage
Operating profit before interest and tax, divided by interest expense. The result is a multiple: how many times the operations earned the interest bill in the period.
It is a flow test, not a stock test. Leverage ratios read the balance sheet. Coverage reads the income statement. A firm can look modestly borrowed on the sheet and still fail coverage if EBIT has fallen.
Some covenants use EBITDA instead of EBIT, which is a softer test because depreciation is added back. A zero interest line is not infinite coverage: there is nothing to cover.
The interest coverage calculator runs EBIT over interest and will not print Infinity.