If the coverage is given (eg: 0.5 kg/mtr sqr) then how to calculate the WFT (Paint wet flm thickness) or DFT ?

You indicated something like 0.5\,\frac {kg}{m^2}.

If you wish real accuracy, you need to know the density of the paint, ρ. An average density is typically thought to be 10 lbs/gal for paint.

10 pound per US gallon = 1.19826427 × 10-6 kg / (mm^3)

inverting that gives 834540 \, mm^3/kg

WFT\,=0.5\, \frac {kg}{m^2}\, \times (\frac {m}{1000\,mm})^2 \times \,\, 834540 \, \frac {mm^3}{kg}\,\approx\, 0.42\,mm

Note how the units cancel. That's how we know the units of the thickness.

or, for a general coverage in Kg/m^2

WFT\,in\,millimeters\,\approx\,Coverage\,in\,\frac {kg}{m^2}\, \times \,0.835

If you have the coverage in other units, or if you want the thickness in other units, you'll need to make the conversion.