Hi freaks

I have a few mcp3424(18 bit adc) lying around & recently ordered a OLED display.So i was thinking to build a precise DVM with that.As per this link,a 5.5 digit DVM is constructed with a 24 bit ADC.My question is preety simple....what is the relation between ADC resolution and DVM digits??With a 18 bit ADC,what will be the precision digits after decimal to measure??Or suppose,if i use a 20/22 bit ADC,then what will be the decimal digits for them?? Please explain anybody.................