It has been quite a while since I flew one of these, but the total useable fuel for a PA28-161 is 48 US gallons, so if you're at that tabs that should work out to 24 gallons (as I recall; Phoenix09 says otherwise, and may well be correct). An inch below, I don't know, maybe figure on 18 gallons?
Best to check the actual aircraft POH, just to be sure.
