Hi Moona
I have flown various PA28 and currently P28R Arrow II and on the latter the capacity to tabs is 14 imperial gallons per side and 21 i.g. to full so the difference is 7 i.g. per side or about 63 litres. I have not yet found this important detail in the pilots operating handbook or flight manual, but it is placarded next to the filler caps.
Note figs are total not usable - refer to POH for usable