Down to which line of which statement
The arithmetic is not the hard part. Choosing what goes in the numerator and the denominator is: whether cost of revenue includes selling costs, whether the operating income on the page is before or after interest, whether inventory divides by sales or by cost. Get one of those wrong and the answer is quietly off, with nothing to signal it.
So every input carries a note saying where to take it from, and every page says what its measure cannot tell you — how EBITDA hides capital spending, how a CAGR erases what happened between its two endpoints.
A figure needs something to sit beside
An 18% operating margin is not high or low on its own. This site collects financial data for 45 listed companies from SEC EDGAR and OpenDART and places your figure in that distribution. The median operating margin across the set is currently 11.6%, the lowest company is at -13.0% and the highest at 60.4%.
That comparison works because a ratio has no unit. Samsung's revenue in won and Apple's in dollars cannot share an axis without an exchange rate, but their operating margins can. This site never converts a currency: the moment a rate is applied, the result is a number the site invented rather than one a company reported.