The basic data on nutrient requirements (recommended daily values and upper intake limits) for different age/sex groups is taken from the most popular source – the guidance "IOM. Dietary Reference Intakes: The Essential Guide to Nutrient Requirements. Washington, DC: The National Academies Press; 2006." You can download this book for free from HERE, for example. There are many comprehensive tables and equations to calculate your nutrient and energy needs. In particular, on pages 532–536, you can find tables with all the nutrient requirements relevant to you while on the page 82 you will find energy equations. If you want, you can compare these values and calculated results against the ones provided by the standalone calculators here, just to be sure of the website's accuracy.

At the same time, you should keep in mind that this is initial or, let's say, generic data, and there are some steps to make it more personal. Also, the website doesn't use provided by the guidance values for protein/carbohydrates/fat adhering to the macros approach instead. And the last thing to mention is related to amino acids requirements which are initially taken from the guidance but then noticeably modified.