A fit designated as 30H7/g6 is a 

Concept:

The fundamental deviation of tolerance zone from the basic size is designated by capital letters A, B, C...ZC is provided for holes and small letters a, b, c....zc is provided for shafts.
Hole 'A' and shaft 'a' have the largest fundamental deviations, hole being positive and shaft being negative.
The fundamental deviations for both hole 'H' and shaft 'h' are zero.

• Clearance fit:
  • Hole diameter is always bigger than the shaft diameter.
  • Both maximum and minimum clearance are always positive.
• Transition fit:
  • The hole diameter is close to the shaft diameter.
  • Maximum clearance is positive and minimum clearance is negative.
• Interference fit:
  • Hole diameter is always lesser than shaft diameter. 
  • Both maximum and minimum clearances are always negative.

Calculation

Here H specifies the hole basis assembly meaning the lower deviation of the hole is zero.

g specifies the shafts whose upper deviation is below the zero line.

If the lower limit of the hole lies above the upper limit of the shaft, then the fit will be a clearance fit.