conversation problems

Good afternoon geeks, I’m working on a project that mmeasure the resistivity of earth.  I used the circuit voltmeter ammeter circuit posted by Ligo couple of years ago. I’m having problem with float to string conversion. The results are not accurate in term of decimal places. For example I used

char res[7] ;

float Resistance;

resistance=volt/(current/1000);

floattostr(resistance, res)

But instead of getting the decimal places the way they suppose to appear is the problem.  Such as 191.876 instead of 19.1876. Help me please