Decimal precision - how to have 18 decimals?

Hello, I am inputting numbers that have 18 decimals. For example :

Default settings seem to round up this number and save it as:

I have tried to use the Format… dialog to increase precision, but it still rounds it up to 16 decimals:

So…? How do I increase precision? Thanks in advance!!!

You can’t. LibreOffice uses IEEE 754 representation of the numbers, and that just doesn’t offer that level of precision.

see also column has been defined as a "Decimal" type, the max. length is 19 characters (with 15 decimal places).

and general description of the way precision works in IEEE 754 here:

If you want to store the values in a spreadsheet, you have to do so as text (enter them by prefixing them with a single quotation mark, that will force the input to text)