How do you convert square feet to diameter of a circle?

The Area of a circle may be quoted in 'sq.ft'.

Hence

A = pi r^(2)

A(sq.ft) = pi * (r ft)^(2)

Hence

r ft = sqrt[(A sq.ft) / pi ]

NB When you square root 'sq.ft' it just becomes 'ft' (feet).

Since 'r ft' is the radius in feet'. Then the diameter is 2r ft = d ft. ( diamter in feet).

NB 'pi = 3.14....' is a constant of proportion and has NO units.