The default U-values for the floor are referred from:
ASHRAE Standard 90.1 (Energy standard for buildings except low-rise residential buildings) - table 5.5 when respective ASHRAE code version (2007, 2010, 2013, 2016, and 2019) is selected.
New York City Energy Conservation Code- table C402 when the respective code version (2016,2020) is selected.
Table 3-11 when respective California Title 24 code (Building energy efficiency standards for residential and nonresidential buildings) version (2016, 2019) is selected.
Division B-Part 3 when the respective National Energy Code of Canada for Buildings code version (2011, 2015) is selected.
Table C402.1.4 from the Washington State Energy Code when one selects the Washington State Amendments (2015) or Seattle Amendments (2015)
Part J1 Table J1.6 from National Construction Code, Building Code of Australia- 2019 when the respective code version is selected
Table 3 Limiting fabric parameters from United Kingdom National Calculation Methodology when the respective code is selected
Since there are different types of material assemblies for floor, cove.tool makes that assumption for users based on Pacific Northwest National Laboratory (PNNL) building prototypes. The app auto-populates the U-value based on the building type and location.
For example, if your project (Apartment) is in Atlanta, GA (ASHRAE Climate Zone 3A) and the energy code selected is ASHRAE 90.1 2013, the auto-populated value can be seen highlighted in the image attached below
For the conversion from F factor to its closest U-value, we relied on this Technical report from the US Department of Energy - https://www.osti.gov/biblio/1488936
The table in the above link will give you an equivalent u-value of the floor depending on the slab type and its F factor. There is no direct formula available because u-value and F factor are defining fundamentally different properties. But for close approximations, depending on the climate zone, you can solve for unknown values by using ratios.
As an example calculation, for climate zone 1 unheated slab f-factor of 0.70, you can calculate the equivalent U-value as (0.064*0.70)/0.73.
Ground Floor U value: It is the measure of the overall rate of heat transfer, through the ground floor of your construction. In other words, a U-value is used to measure how well or how badly a component transmits heat from the inside to the outside. The slower or more difficult it is for heat to transfer through the component, the lower the U-value.
Ground Floor Area: It is the Total area of the floor that is in contact with the ground