Ofcourse, the data needs to be in same unit for same kind of observation to use in any kind of calculation, doesn't matter a model or a simple arithmetic. The area should have same unit, so as with length, temperature, depth, volume etc. It is the basic rules of mathematics that same unit should for similar kind of data. (the calculations can still be done between different unit of similar kind, but you have to be very careful about the conversion among the units, for example: hectare*10^6==million hectre )
Dear Bushra, the conversion factor of one unit to another unit is available in all standard laboratory manuals/ books and back or back side of the cover pages of any standard Dairy.
@Bushra - actually you don't need any special calculator for that. Just analyze your data frame and track the variations in unit. If your find different units for the same type of data, use the conversion factor to make the data in same unit. From example above, if you wanted to convert kg/ha to tonnes/ha, you need to divide the kg data by 10^3.