In Horroscope it is necessary to submit correct time of birth. In ancient days, hindus were using time format in Ghati. So, how to convert time stated in Ghati to IST Format ?

Namaskaar Poddar ji,
The conversion is 1 Ghathi is equal to 24 minutes.
1 vighathi is 24 seconds.

the time was reckoned from sunrise (local) and the birth time was ususally given as Istha Ghathi.
Like say for example the Istha was given as 24 gh 30 vi.
Say sunrise for the place of birth was 06:03 am (IST).
first convert the istha into hours and minutes.
24 divided by 2.5 (1 hour = 2.5 ghathi) is equal to 9hrs 36 min.
30vi divided by2.5 (1 minute = 2.5 vighathis) is equal to 12 minutes.
Adding the two we get 9 hrs and 48 minutes from Sunrise.
Sunrise was 06:03 IST
              + 09:48
                ---------
               =15:51 hrs or 03:51 pm is the birth time.  
Some regions used to give Istha from Sunrise for Day birth and from sunset for night birth.
Some regional variation was to calculate from the four Sandhi (junctions).
So it was not uniform for the whole of India and there were regional variations.
Usually birth istha was calculated from Sunrise.
vivek